Fairchild SEMICONDUCTOR RC5051 User's Manual
Contents
1. TM 20 0mV vs M i00gs Chi 7 35 2mV INI 20 0mV M 100us Chi 7 43 2mV Attachment A Attachment B 20 APPLICATION NOTE AN50 Tek Running 500kS s Hi Res j Tek Running S00kS s Hi Res A 97 6mV GE y se A 99 2mV amp 94 8mV i 97 6mv TL 26 0mV M Toons Chi 7 35 6mv TI 20 0mV i MM 100ps Chi 7 35 6mv Attachment C Attachment E Tek Running 500kS s Hi Res Tek Running 500kS s Hi Res j T t T E RIDERE A 80 0mv i T E ud A 105 2mv j amp 61 2mV 80 4mV TT 20 0mv s M 100ps Chi 7 36 8mV TD 20 0mvis oops Chi 38 8mV Attachment D Attachment F a mmy COME HUM UU SE tonya ano E ii M PH WINES AS E EG UR Gp oq oid PET TUAE UN DE UR uu RTS TATT EGO Attachment G 21 AN50 APPLICATION NOTE Summary This application note covers many aspects of the RC5050 and RC5051 for implementation of a DC DC converter a on Pentium Pro motherboard A detailed discussion includes the processor power requirements a description of the RC5050 and RC5051 design considerations and compo nents selection layout guidelines and considerations guide lines for debugging and performance evaluations RC5050 Evaluation Board Raytheon Electronics provides an evaluation board to verify ing system level performance of the RC5050 The evaluation board serv2. FAIRCHILD SEMICONDUCTOR w www fairchildsemi com Application Note 50 Implementing the RC5050 and RC5051 DC DC Converters on Pentium Pro Motherboards Introduction This document describes how to implement a switching volt age regulator using an RC5050 or an RC5051 high speed controller a power inductor a Schottky diode appropriate capacitors and external power MOSFETS This regulator forms a step down DC DC converter that can deliver up to 14 5A of continuous load current at voltages ranging from 1 3V to 3 5V A specific application circuit design consider ations component selection PCB layout guidelines and per formance evaluations are covered in detail In the past 10 years microprocessors have evolved at such an exponential rate that a modern chip can rival the computing power of a mainframe computer Such evolution has been possible because of the increasing numbers of transistors that processors integrate Pentium CPUS for example integrate well over 5 million transistors on a single piece of silicon To integrate so many transistors on a piece of silicon their physical geometry has been reduced to the sub micron level As a result of each geometry reduction the corresponding operational voltage for each transistor has also been reduced The changing CPU voltage demands the design of a pro grammable power supply a design that is not completely re engineered with every change in CPU voltage The volt
3. Vv si ca op 1N4691 AR T O 1uF 0 1uF M1 M2 C12 IRF7413 e ms 1uF IRF7413 L1 R sense Hz oe 1 3u H RC5051 MF VREF DSi ii c7 es 1959 se e t I o MS M4 55 p I uF IRF7413 IRF7413 e e eo Cexr soopr e sZ VID4 R6 10K PWRGD nen cio ENABLE gt 0 4 uF TT 0 14F NE Figure 4 Synchronous DC DC Converter Application Schematic Using the RC5051 APPLICATION NOTE AN50 MOSFET Selection Cosiderations MOSFET Selection This application requires N channel Logic Level Enhance ment Mode Field Effect Transistors Desired characteristics are as follows Low Static Drain Source On Resistance Power package with low Thermal Resistance Drain current rating of 20A minimum Drain Source voltage 15V The on resistance Rps on is the primary parameter for MOSFET selection It determines the power dissipation within the MOSFET and therefore significantly affects the efficiency of the DC DC converter Table 5 is a selection Rps ow lt 37 mQ lower is better table for MOSFETs Low gate drive voltage Vgs lt 4 5V Table 3 MOSFET Selection Table Rps on m2 Thermal Manufacturer amp Model Conditions Typ Max Package Resistance Fuji Vas 4V Ip 17 5A Ty 25 C 25 37 TO 220 644 75 2SK1388 Tj 125 C 37 Siliconix Vas 4 5V Ip 5A Ty 2 25 C 16 5 20 SO 8 ja 50 SI4410DY T 125 C 28 34 SMD Natio
4. Figure 11 shows a design that provides flexibility with a solution to address wide tolerances In this design you have the option to choose an embedded or a discrete MnCu sense resistor To use the discrete sense resistor populate R21 with a shorting bar zero Ohm resis tor for proper Kelvin connection and add the MnCu sense resistor To use the embedded sense resistor on the other hand populate R22 with a shorting bar for Kelvin connec APPLICATION NOTE AN50 Embedded Sense Resistor IFBH O MnCu Discrete Resistor R21 R22 OOJO IFBL Output Power O C Q0 Plane Vout R Ar R e R Ar Figure 11 Short Circuit Sense Resistor Design Using a PC Trace Resistor and an Optional Discrete Sense Resistor tion The embedded sense resistor allows the user to choose a plus or a minus delta resistance tap to offset any large sheet resistivity change In this design the center tap yields 6mQ the left tap yields 6 7mQ and the right tap yields 5 3mQ RC5050 and RC5051 Short Circuit Current Characteristics The RC5050 and RC5051 short circuit current characteristic includes a hysteresis function that prevents the DC DC con verter from oscillating in the event of a short circuit Figure 12 shows the typical characteristic of the DC DC converter circuit with a 6mQ sense resistor The converter exhibits
5. M2 so that the trace length of the HIDRV pin from the RC5050 to the FET gates is minimized A long lead length on this pin would cause high amounts of ringing due to the inductance of the M1 12 13 o no mo so do 11 RC5050 10 M2 Good layout Quiet Pins trace and the large gate capacitance of the FET This noise radiates all throughout the board and because it is switching at such a high voltage and frequency it is very difficult to suppress Figure 15 shows an example of good placement for the MOSFETS in relation to the RC5050 In addition this fig ure shows an example of problematic placement for the MOSFETs Bad layout 11 RC5050 10 7 9L 1 13 8 L 1 14 7 1 15 6 16 5 1 SG 4L 1 18 3 L 1 19 p2l 1 20 my I 4 M1 M2 Figure 15 Placement of the MOSFETs 16 APPLICATION NOTE AN50 In general all of the noisy switching lines should be kept away from the quiet analog section of the RC5050 That is traces that connect to pins 12 and 13 HIDRV and VCCQP should be kept far away from the traces that con nect to pins 1 through 5 and pin 16 e Place the 0 1uF decoupling capacitors as close to the RC5050 pins as possible Extra lead length negates their
6. ability to suppress noise Each VCC and GND pin should have its own via to the appropriate plane This helps to provide isolation between pins Surround the CEXT timing capacitor with a ground trace Be sure to place a ground or power plane under the capacitor for further noise isolation to provide additional shielding to the oscillator pin 1 from the noise on the PCB In addition place this capacitor as close to the RC5050 pin I as possible Place the MOSFETS inductor and Schottky as close together as possible for the same reasons on the first bullet above Place the input bulk capacitors as close to the drains of MOSFETS as possible In addition placement of a 0 14F decoupling capacitor right on the drain of each MOSFET helps to suppress some of the high frequency switching noise on the input of the DC DC converter Qam dm e diem eum oun oom oun Er EEE mu THODE 4p 10 a Raytheon Electronics MiM3 RCMB 2C 2 3 2 A M4 cio cid C14 17 z 3 C16 R ud RB R1 TOP SILK Pis Place the output bulk capacitors as close to the CPU as possible to optimize their ability to supply instantaneous current to the load in the event of a current transient Additional space between the output capacitors and the CPU allows the parasitic resistance of the board traces to degrade the DC DC converter s performance under severe load transient conditions causing higher voltage de
7. 2 75W During a short circuit the duty cycle dramatically reduces to around 20 The forward current in the short circuit condi tion decays exponentially through the inductor The power dissipated in the diode during short circuit condition is approximately given by 1 1 5us L R 13us IXe 20Axe 5 I 7 9A F ending Ir ave 20A 7 9A 2 14A Pp Diode lg ave X Vp X 1 DutyCycle 14 x 0 45 x 0 8 5W Thus for the Schottky diode the thermal dissipation during a short circuit is greatly magnified This requires that the thermal dissipation of the diode be properly managed by an appropriate heat sink To protect the Schottky from being destroyed in the event of a short circuit you should limit the junction temperature to less than 130 C You can find the required thermal resistance using the equation for maximum junction temperature T T Pp J max Roja Assuming that the ambient temperature is 50 C R _ Tj max Fa 130 50 16 C W OJA Pp 5 Thus you need to provide a heat sink that gives the Schottky diode a thermal resistance of 16 C W or lower to protect the device during an indefinite short In summary with proper heat sink the Schottky diode is not over stressed during a short circuit condition Schottky Diode Selection The application circuit diagram of Figure 3 shows a Schottky diode DS1 In non synchronous mode DS1 is used as a fly back diode to prov
8. 253 354 5 6 7 8 9 10 11 Gate Source Voltage Vgs V Figure 7 Rps oN vs Vas for Selected MOSFETs APPLICATION NOTE AN50 Converter Efficiency Losses due to parasitic resistance in the switches coil and e gate charge losses sense resistor dominate at high load current level The major e diode conduction losses loss mechanisms under heavy loads in usual order of impor transition losses tance are Input Capacitor losses losses due to the operating supply current of the IC MOSFET PR Losses Coil Losses Sense Resistor Losses Calculation of Converter Efficiency Under Heavy Loads I xV P Efficiency OUT PIN Iour X Vour Pross OUT OUT Pross PDyosrgr PPcom PPsenser PDgATg PDpjopg PPrran PDcap PDic 7 2 h Vour t Vp where PDyosret lout x Rps on X DutyCycle Where DutyCycle UNSERE CERES Vew 2 PDcon lour X Rcom 2 PDsgxser lour XRsense PDgate doare Xf X5V where qgArg is the gate charge and f is the switching frequency PDpjopg VrxIp 1 Dutycycle 2 Vin X Crss X ILoap Xf PDqgAN Where Cass is the reverse transfer capacitance of the high side MOSFET IDRIVE PDcap Irys XESR PDic Vec X lec Example 3 3 0 5 DutyCycle 5705 03 0 73 PDyosret 107 x0 030x0 73 2 19W 2 PDcon 107x0 010 1W PDsgnser 107 x0 0065 0 65W PDgare CVXfx5V 1 75nf x 9 1 Vx 285Khzx 5V 0 019W PDpiopg 05x10 1 0 73 1 35W 2 5 x 400pf
9. 6 0 3 2606 7 0 3 2611 8 0 3 2613 9 0 3 2611 10 0 3 2607 11 0 3 2599 12 0 3 2596 12 4 3 2596 Load Regulation 0 5A 12 4A 0 64 18 APPLICATION NOTE AN50 VID load A Vou V 11010 0 5 2 505 1 0 2 504 2 0 2 501 3 0 2 496 4 0 2 493 5 0 2 493 6 0 2 492 7 0 2 492 8 0 2 491 9 0 2 490 10 0 2 989 11 0 2 488 12 0 2 486 13 0 2 485 13 9 2 484 Load Regulation 0 5A 13 9A 0 8496 Note Load regulation is expected to be typically around 0 896 The load regulation performance for this device under evaluation is excellent Output Voltage Load Transients Due to Load Current Step This test is performed using Intel P6 0 P6S P6T Voltage Transient Tester Note Low to High 0 5A 9 9A 76 0mV Refer to Current Step Attachment A for Scope Picture High to Low 9 9A 0 5A 70mV Refer to Current Step Attachment B for Scope Picture Low to High 0 5A 12 4A 97 6mV Refer to Current Step Attachment C for Scope Picture High to Low 12 4A 0 5A 80 0mV Refer to Current Step Attachment D for Scope Picture Low to High 0 5A 13 9A 99 2mV Refer to Current Step Attachment E for Scope Picture High to Low 13 9A 0 5A 105 2mV Refer to Current Step Attachment F for Scope Picture Transient voltage is recommended to be less than 4 of the output voltage The performance of the device under evalua tion i
10. Diagram The HIDRV driver has a power supply VCCQP supplied from a 12V source as illustrated in Figure 2 The resulting voltage is sufficient to provide the gate to source voltage to the external MOSFET that is required to achieve a low Rps ow Since the low side synchronous FET is referenced to ground there is no need to boost the gate drive voltage and its VCCP power pin can be tied to VCC Internal Voltage Reference The reference included in the RC5050 and RC5051 is a pre cision band gap voltage reference The internal resistors are precisely trimmed to provide a near zero temperature coeffi cient TC Added to the reference input is the resulting out put from an integrated 5 bit DAC provided in accordance to the Pentium Pro specification guidelines These guidelines require the DC DC converter output to be directly program mable via a 4 bit voltage identification VID code This code scales the reference voltage from 2 0V no CPU to 3 5V in 100mV increments To target future generations of low voltage processors the RC5050 and RC5051 incorpo rate a VID4 pin to allow additional programmability between 1 3V and 2 05V For guaranteed stable operation under all operating conditions a 0 1uF of decoupling capacitance should be connected to the VREF pin No load should be imposed on this pin Power Good PWRGD The RC5050 and RC5051 Power Good function is designed in accordance with the Pentium Pro DC DC converter speci fica
11. Figure 12 shows how the protection works The power dis sipated in the MOSFET at normal operation for a load cur rent of 14 5A is given by DES 1452 Pp I xRoyxDutyCycle 7 x 037x 62 12W for each MOSFET The power dissipated in the MOSFET at short circuit condition for a peak short current of 20A is given by 2 Pp 2 x 037 x 2 0 74W for each MOSFET These calculations show that the MOSFET is not being over stressed during a short circuit condition 13 AN50 APPLICATION NOTE Tek Run amamari Sainple chi Coupilng T impedance B KE TY ae eran dug a EX 50 Figure 13A Vccop Output Waveform for Normal Operation Condition with Vout 3 3V 10A TS bal T ch Coupling mpedance a gt DO cu a lt I i I i l i l J I T i ix n i I E i 10 i GNDA EA 8 e Esc po i 3 1 aai vaia aia EREE H i B I i R i M32 00ps Chis ov el Ca Probe Offset ov initialized Figure 13B Vccap Output Waveform for Output Shorted to Ground Power dissipation on the Schottky diode during a short cir cuit condition must also be considered During normal oper ation the Schottky diode dissipates power while the power MOSFET is off The power dissipated in the diode during normal operation is given by Pp Diode Ip X Vp X 1 DutyCycle 14 5 x 0 5V x 1 0 62
12. Next check the oscillator pin You should see a saw tooth wave at the frequency set by the external capacitor When the VREF and CEXT pins are checked and correct and the output voltage is incorrect look at the waveform at VCCQP This pin should be swinging from ground to 12V in the 12V application and from slightly below 5V to about 10V charge pump appli cation If the VCCQP pin is noisy with ripples over shoots riding on it this may make the converter not to function correctly 10 Next look at HIDRV pin This pin directly drives the gate of the FET It should provide a gate drive Vgs of about 5V when turning the FET on A careful study of the layout is recommended Refer to the PCB Layout Guidelines section Past experience shows that the most frequent errors are incorrect components improper connections and poor layout Performance Evaluation This section shows a sample evaluation results as a reference guide for evaluating a DC DC Converter using the RC5050 on a Pentium Pro motherboard Load Regulation VID load A Vout V 10100 0 5 3 0904 1 0 3 0825 2 0 3 0786 3 0 3 0730 4 0 3 0695 5 0 3 0693 6 0 3 0695 7 0 3 0695 8 0 3 0694 9 0 3 0694 9 9 3 0691 Load Regulation 0 5A 9 9A 0 70 VID load A Vout V 10010 0 5 3 2805 1 0 3 2741 2 0 3 2701 3 0 3 2642 4 0 3 2595 5 0 3 2597
13. Vy is the forward voltage of diode DS1 Then the inductor value can be calculated using the relationship Vin a Vsw i Vo vel JU PK MIN Where Vsw Rps ow X Io is the drain to source voltage of M1 when it is switched on Implementing Short Circuit Protection Intel currently requires all power supply manufacturers to provide continuous protection against short circuit condi tions that may damage the CPU To address this requirement Raytheon Electronics has implemented a current sense meth odology to limit the power delivered to the load in the event of overcurrent The voltage drop created by the output cur rent across a sense resistor is presented to one terminal of an internal comparator with hysterisis The other comparator terminal has the threshold voltage nominally of 120mV Table 6 states the limits for the comparator threshold of the Switching Regulator Table 6 RC5050 Short Circuit Comparator Threshold Voltage Short Circuit Comparator Vihreshold mV Typical 120 Minimum 100 Maximum 140 When designing the external current sense circuitry pay careful attention to the output limitations during normal operation and during a fault condition If the short circuit protection threshold current is set too low the DC DC con verter may not be able to continuously deliver the maximum CPU load current If the threshold level is too high the out put driver may not be disabled at a safe limit and the resul
14. and reverses bias when the VCCQP goes to 10V The charge pump capacitor CP needs to be a high Q high fre quency capacitor A 1uF ceramic capacitor capacitor is recommended here Control I t I I I PWM PFM I I i x DSi FOB l 65 AP50 01 Figure 5 Charge Pump Configuration Method 2 12V Gate Bias Figure 6 illustrates how a 12V source can be used to bias the VCCQP A 47 Q resistor is used to limit the transient current into the VCCQP pin and a 1 UF capacitor filter is used to filter the VCCQP supply This method provides a higher gate bias voltage VGs to the MOSFET and there fore reduces the Rps on of the MOSFET and reduces the power loss due to the MOSFET Figure 7 shows how Rps on reduces dramatically with Vgs increases A 6 2V Zener diode D1 is placed to clamp the voltage at VCCQP to a maximum of 12V and ensure that the absolute maxi mum voltage of the IC will not be exceeded 470 Poco EE C CENE 4 D1 I I 6 2V VCCQP I M1 I t ju HIDRV 1 oo L i tur L1 RS PWM PFM I f TON Co Control I l X DSI CB I I I 1 e e E EP E ee S l i 65 AP50 02 Figure 6 12V Gate Bias Configuration 0 1 x 0 094 R D 0 08 DS Fuji 0071 R DS 7060 E 0 06 k R DS 706A 9 0 051 Xx R DS 706AEL d 0 044 0 03 X 0 02 dus MN 0 014 X 0 m m H m 152
15. effect of ringing caused by long trace lengths The ESR rating of a capacitor is a difficult number to quantify ESR is defined as the resonant impedance of the capacitor Since the capacitor is actually a complex imped ance device having resistance inductance and capacitance it is natural for this device to have a resonant frequency As a rule the lower the ESR the better suited the capacitor is for use in switching power supply applications Many capacitor manufacturers do not supply ESR data A useful estimate of the ESR can be obtained using the following equation DF ESR 5af where DF is the dissipation factor of the capacitor f is the operating frequency and C is the capacitance in farads With this in mind correct calculation of the output capaci tance is crucial to the performance of the DC DC converter The output capacitor determines the overall loop stability output voltage ripple and load transient response The calcu lation is as follows Io X AT CQ AV IGXESR where AV is the maximum voltage deviation due to load transients AT is the reaction time of the power source loop response time for the RC5050 and RC5051 isapproximately 2us and Ip is the output load current For Io 12 2A 0 13A load step and AV 100mV the bulk capacitance required can be approximated as follows Io x AT 12 2A x 2ps CCHF AV x ESR 100mV 122A x 75mQ 2870uF Because the control loop response of the cont
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18. 5V current draw 0 0 0 1 1 1 90V Processor Voltage Identification 0 0 0 1 0 1 95V There are four voltage identification Pins VID3 VIDO on 0 0 0 0 1 2 00V the Pentium Pro processor package which can be used to 0 0 0 0 0 2 05V support automatic selection of the power supply voltage 1 1 1 1 1 No CPU These pins are internally unconnected or are shorted to 1 1 1 1 0 24V ground Vss The logic status of the VID pins defines the 1 1 1 0 1 2 2N voltage required by the processor In order to address future 1 1 1 0 0 2 3V low voltage microprocessors the RC5050 and RC5051 include a VID4 input bit to extend the output voltage range 1 1 0 1 1 2 4V as low as 1 3V The output voltage programming codes are 1 1 0 1 0 2 5V presented in Table 3 A 1 refers to an open pin and a 0 1 1 0 0 1 2 6V refers to a short to ground 1 1 0 0 0 27V 1 0 1 1 1 2 8V Table 3 Output Voltage Programming 1 0 1 1 0 2 9V Codes 1 0 1 0 1 3 0V VIDA VID3 VID2 VID1 VIDO Voy to CPU 1 0 1 0 0 3 1V 0 1 1 1 1 1 30V 1 0 0 1 1 3 2V 0 1 1 1 0 1 35V 1 0 0 1 0 3 3V 0 1 1 0 1 1 40V 1 0 0 0 1 3 4V 0 1 1 0 0 1 45V 1 0 0 0 0 3 5V 0 1 0 1 1 1 50V Note 0 1 0 1 0 1 55V 1 0 processor pin is tied to GND 1 processor pin is open APPLICATION NOTE AN50 I O Controls In addition to the Voltage Identification there are several sig nals that control the DC DC converter or provide feedback from the DC DC converter to the CPU They are Po
19. E AN50 Short Circuit Protection A current sense methodology is implemented to disable the output drive signal to the MOSFET s when an over current condition is detected The voltage drop created by the output current flowing across a sense resistor is presented to an internal comparator When the voltage developed across the sense resistor exceeds the comparator threshold voltage the chip reduces the output drive signal to the MOSFET s The DC DC converter returns to normal operation after the fault has been removed for either an over voltage or a short circuit condition Oscillator The RC5050 and RC5051 oscillator section uses a fixed cur rent capacitor charging configuration An external capacitor Cgxn is used to preset the oscillator frequency between 200KHz and 1MHz This scheme allows maximum flexibil ity in setting the switching frequency and in choosing exter nal components 12V L2 In general a lower operating frequency decreases the peak ripple current flowing in the output inductor thus allowing the use of a smaller inductor value Unfortunately operation at lower frequencies increases the amount of energy storage that must be provided by the bulk output capacitors during load transients due to slower loop response of the controller In addition the efficiency losses due to switching of the MOSFETs increase as the operating frequency is increased Thus efficiency is optimized at lower operating frequ
20. a normal load regulation characteristic until the voltage across the resistor exceeds the internal short circuit threshold of 120mV At this point the internal comparator trips and signals the controller to turn off the gate drive to the power MOSFET This causes a drastic reduction in output voltage as the load regulation collapses into the short circuit control mode The output voltage does not return to its nominal value the output current is reduced to a value within the safe range for the DC DC converter 3 5 3 0 2 5 2 0 1 5 1 0 Output Voltage 0 5 0 0 5 10 15 25 Output Current 20 Figure 12 RC5050 Short Circuit Characteristic Power Dissipation Consideration During a Short Circuit Condition The RC5050 and RC5051 controllers respond to an output short circuit by drastically changing the duty cycle of the gate drive signal to the power MOSFET In doing this the power MOSFET is protected from stress and from eventual failure Figure 13A shows the gate drive signal of a typical RC5050 operating in continuous mode with a load current of 10A The duty cycle is set by the ratio of the input voltage to the output voltage If the input voltage is 5V and the output voltage is 3 1V the ratio of Vout Vin is 62 Figure 13B shows the result of a RC5050 going into its short circuit mode with a duty cycle approximately of 2096 Calculating the power in the MOSFET at each condition on the graph
21. age range of the CPU has shown a downwards trend for the past 5 years from 3 3V for the Pentium to 3 1V for the Pentium Pro and to 1 8V for future processors With this trend in mind Raytheon Electronics has designed the RC5050 and RC5051 controllers These controllers integrate the necessary programmability to address the changing power supply requirements of lower voltage CPUs Previous generations of DC DC converter controllers were designed with fixed output voltages adjustable only with a set of external resistors In a high volume production envi ronment such as with personal computers however a CPU voltage change requires a CPU board re design to accommo date the new voltage requirement The 5 bit DAC in the RC5050 and the RC5051 reads the voltage ID code that is programmed into modern processors and provides the appro priate CPU voltage In this manner the PC board does not have to be re designed each time the CPU voltage changes The CPU can thus automatically configure its own required supply voltage Intel Pentium Pro Processor Power Requirements Refer to Intel s AP 523 Application Note Pentium Pro Processor Power Distribution Guidelines November 1995 order number 242764 001 as a basic reference The speci fications contained in this document have been modified slightly from the original Intel document to include updated specifications for more recent processors Please contact Intel Corporation for specific det
22. ails Input Voltages Available inputs are 12V 5 and 5V 5 Either one or both of these inputs can be used by the DC DC converter The input voltage requirements for Raytheon s RC5050 and RC5051 DC DC converters are listed in Table 1 Table 1 Input Voltage Requirements MOSFET MOSFET Vcc for IC Drain Gate Bias 12V 5 or 5V 45 RC5050 5V 5 5V 5 RC5051 Pentium Pro DC Power Requirements Refer to Table 2 Intel Pentium Pro and OverDrive Proces sor Power Specifications For a motherboard designs without a standard VRM Voltage Regulator Module socket the on board DC DC converter must supply a minimum of 13 9A of current 92 5V and 12 4A of current 3 3V For a Flexible Motherboard design the on board DC DC con verter must supply 14 5A maximum Ic cF DC Voltage Regulation As indicated in Table 2 the voltage level supplied to the CPU must be within 5 of its nominal setting Voltage reg ulation limits must include Output load ranges specified in Table 2 Output ripple noise DC output initial voltage set point Temperature and warm up drift Ambient 10 C to 50 C at full load with a maximum rate of change of 5 C per 10 minutes minimum but no more than 10 C per hour Output load transient with Slew rate gt 30A us at converter pins Range 0 3A IccP Max as defined in Table 2 Rev 1 1 0 AN50 APPLICATION NOTE Table 2 Intel Pentium Pro and OverDrive Processor Power Spe
23. ching loss IL Rps on where Vow The RC5050 and RC5051 Controllers The RC5050 is a programmable non synchronous DC DC controller IC The RC5051 is a synchronous version of the RC5050 When designed around the appropriate external components either of these devices can be configured to deliver more than 14 5A of output current The RC5050 and RC5051 utilize both current mode and voltage mode PWM control to create an integrated step down voltage regulator The key differences between the RC5050 and RC5051 are listed in Table 4 Table 4 RC5050 and RC5051 Differences RC5051 RC5050 Operation Synchronous Non Synchronous Package 20 SOIC 20 SOIC Output Enable Yes Yes Disable Main Control Loop Refer to the RC5051 Block Diagram illustrated in Figure 2 The control loop of the regulator contains two main sections the analog control block and the digital control block The analog section consists of signal conditioning amplifiers feeding into a set of comparators which provide the inputs to the digital control block The signal conditioning section accepts inputs from the IFB current feedback and VFB voltage feedback pins and sets up two controlling signal paths The voltage control path amplifies the VFB signal and presents the output to one of the summing amplifier inputs The current control path takes the difference between the IFB and VFB pins and presents the resulting signal to another input of the summing ampl
24. cifications Voltage Maximum Maximum Thermal Specification Current Design power CPU Model Features VecP VDC IccP A W 150MHz 256K L2 Cache 3 1 5 9 9 29 2 166MHz 512K L2 Cache 3 3 5 11 2 35 0 180MHz 256K L2 Cache 3 3 5 10 1 31 7 200MHz 256K L2 Cache 3 3 5 11 2 35 0 200MHz 512K L2 Cache 3 3 5 12 4 37 9 OverDrive Processors 150Mhz 2 5 5 11 2 26 7 180Mhz 12 5 29 7 200Mhz 13 9 32 9 Flexible Motherboard 2 4 3 5 5 14 5 45 0 Notes 1 Maximum power values are measured at typical VccP to take into account the thermal time constant of the CPU package 2 Flexible motherboard specifications are recommendations only Actual specifications are subject to change Output Ripple and Noise Table 3 Output Voltage Programming Ripple and noise are defined as periodic or random signals Codes continued over the frequency band of 20Mhz at the output pins Output J ripple and noise requirements of 13mV must be met VID4 VID3 VID2 VID1 VIDO Vour to CPU throughout the full load range and under all specified input 0 1 0 0 1 1 60V voltage conditions 0 1 0 0 0 1 65V 5r 0 0 1 1 1 1 70V Efficiency 0 0 1 1 0 1 75V The efficiency of the DC DC converter must be greater than 0 0 1 0 1 1 80V 80 at maximum output current and greater than 40 at low 0 0 1 0 0 1 8
25. d vg t Thickness mils For loz copper t 1 35 mils p 717 86 uQ mil 1 L 1 W 1 Square 11 For example you can layout a 5 30mQ embedded sense resistor using the equations above I 10 W 2 200mil 0 05 0 05 Omis RxWxt _ 0 00530 x 200 x 1 35 i L 2000mils p 717 86 Omils L W 100 Therefore to model 5 30mQ embedded sense resistor you need W 200 mils and L 2000 mils Refer to Figure 9 W 200 mils L 2000 Figure 9 5 30mQ Sense Resistor 10 _ You can also implement the sense resistor in the following manner Each corner square is counted as 0 6 square since current flowing through the corner square does not flow uniformly and it is concentrated towards the inside edge as shown in Figure 10 Figure 10 5 30mQ Sense Resistor 10 A Design Example Combining an Embedded Resistor and a Discrete Resistor For low cost implementation the embedded PC trace resistor is most desirable However its wide tolerance 29 pre sents a challenge In addition requirements for the CPU change frequently and thus the maximum load current may be subject to change Combining embedded resistors with discrete resistors may be a desirable option
26. ectrolytic ESR lt 0 047 Q LXF16VB102M capacitor 10mm x 20mm 4 C13 C14 Sanyo 1500uF 6 3V electrolytic ESR lt 0 047 Q C15 C16 6MV1500GX capacitor 10mm x 20mm 1 DS1 Motorola Shottky diode 15A Vf lt 0 52V Ij 10A note 1 MBR2015CT 1 D1 Motorola 1N4691 6 2V Zener Diode 15 AN50 APPLICATION NOTE Table 11 Bill of Materials for a 13A Pentium Pro Klamath Application continued Refer to Appendix A for Directory of component suppliers Notes Quantity Reference Manufacturer Part Description Requirements and Order Comments 1 L1 Pulse Engineering 1 3uH inductor PE 53680 1 L2 Pulse Engineering 2 5uH inductor Optional helps PE 53681 reduce ripple on 5v line 2 4 M1 M4 International Rectifier N Channel Logic Level Rps on lt 18mQ note 2 IRF7413 Enhancement Mode MOSFET Vas 4 5V lp 5A 1 Rsense Coppel 6 mQ 1W CuNi Wire resistor 1 R5 Panasonic 47Q 5 resistors ERJ 6GEYO050Y 1 R6 Panasonic 10KQ 5 resistor ERJ 6ENF10 0KY U1 Raytheon Programmable DC DC RC5050M or RC5051M converter 1 When used in synchronous mode a 1A schottky diode such as the 1N5817 should be substituted for the MBR2015CT 2 A target Rps on value of 10mQ should be used for each output driver switch Refer to Table 3 for alternative MOSFETs PCB Layout Guidelines and Considerations PCB Layout Guidelines Placement of the MOSFETs relative to the RC5050 is critical Place the MOSFETs M1 amp
27. encies An operating frequency of 300 kHz was chosen to optimize efficiency while maintaining excellent regulation and tran sient performance under all operating conditions Design Considerations and Component Selection Figure 3 shows a typical non synchronous application using the RC5050 Figure 4 illustrates the synchronous applica tion using the RC5051 lt av gt BERG 2 5uH C4 T C1 c2 is tg TS 0 1uF e 1000 uF 1000 uF mor One R5 47 NZ e A4 D1 C8 C9 1N4691 AR T 0 1uF 0 1 uF M1 M2 c12 Rei IRF7413 1uF IRF7413 R I E SENSE 8 7 lt vo gt 15 6 p p 66 4 1 34 H 6mQ E FI E Te RC5050 Ge 8 8 18 8 ix VREF Fi psi cz MBR2015CTL T 18 3 o e o lo b j 5 pr 19 2 E i e e o Cor AR 100pF e hd NA VIDA R6 10K PWRGD zen NABLE gt cio I OE ES 0 1uF NA NA Figure 3 Non Synchronous DC DC Converter Application Schematic Using the RC5050 AN50 APPLICATION NOTE 412V L2 Ru CY Y Ye 2 5uH C4 C1 c2 c3 C5 7 a A O 1uF 1000 uF 00 oor 01uF R5 47
28. es as a guide to performance expectations when using the supplied external components and PCB layout Call Raytheon Electronics local Sales Office or the Market ing department at 415 966 7819 for an evaluation board 22 APPLICATION NOTE AN50 Appendix A Directory of Component Suppliers Dale Electronics Inc E Hwy 50 PO Box 180 Yankton SD 57078 0180 PH 605 665 9301 Fuji Electric Collmer Semiconductor Inc 14368 Proton Rd Dallas Texas 75244 PH 214 233 1589 General Instrument Power Semiconductor Division 10 Melville Park Road Melville NY 11747 PH 516 847 3000 Hoskins Manufacturing Co Copel Resistor Wire 10776 Hall Road Hamburg MI 48139 0218 PH 313 231 1900 Intel Corp 5200 NE Elam Young Pkwy Hillsboro OR 97123 PH 800 843 4481 Tech Support for Power Validator International Rectifier 233 Kansas St El Segundo CA 90245 PH 310 322 3331 IRC Inc PO Box 1860 Boone NC 28607 PH 704 264 8861 Motorola Semiconductors PO Box 20912 Phoenix Arizona 85036 PH 602 897 5056 National Semiconductor 2900 Semiconductor Drive Santa Clara CA 95052 8090 PH 800 272 9959 Nihon Inter Electronics Corp Quantum Marketing Int l Inc 12900 Rolling Oaks Rd Caliente CA 93518 PH 805 867 2555 Panasonic Industrial Co 6550 Katella Avenue Cypress CA 90630 PH 714 373 7366 Pulse Engineering 12220 World Trade Drive San Diego CA 92128 PH 619 674 8
29. ide a constant current path for the inductor when M1 is turned off Table 10 shows the characteristics of several Schottky diodes Note that MBR2015CTL has a very low forward voltage drop This diode is ideal for applications where the output voltage is required to be less than 2 8V Table 10 Schottky Diode Selection Table Manufacturer Forward Voltage Model Conditions Ve Philips lc 20A T 25 C 0 84v PBYR1035 le 20A T 125 C 0 72v Motorola l 20A T 25 C 0 84v MBR2035CT I 20A T 125 C 0 72v Motorola l 15A T 25 C 0 84v MBR1545CT I c 15A T 125 C 0 72v Motorola l 20A T 25 C 0 58v MBR2015CTL I 20A T 150 C lt 0 48v Output Filter Capacitors Output ripple performance and transient response are func tions of the filter capacitors Since the SV supply of a PC motherboard may be located several inches away from the DC DC converter the input capacitance may play an impor tant role in the load transient response of the RC5050 and RC5051 The higher input capacitance the more charge stor age is available for improving current transfer through the 14 APPLICATION NOTE AN50 FET Low Equivalent Series Resistance ESR capacitors are best suited for this type of application Incorrect selection can hinder the converter s overall performance The input capacitor should be placed as close to the drain of the FET as possible to reduce the
30. ifier These two signals are then summed together with the slope compensation input from the oscillator This output is then presented to a comparator which provides the main PWM control signal to the digital control block The additional comparators in the analog control section set the point at which the current limit comparator disables the output drive signals to the external power MOSFETs The digital control block takes the comparator inputs and the main clock signal from the oscillator to provide the appropri ate pulses to the HIDRV and LODRV output pins These pins control the external power MOSFETS The digital sec tion utilizes high speed Schottky transistor logic allowing the RC5050 and the RC5051 to operate at clock speeds as high as IMHz High Current Output Drivers The RC5051 contains two identical high current output drivers that utilize high speed bipolar transistors in a push pull configuration Each driver is capable of delivering 1A of current in less than 100ns Each driver s power and ground are separated from the chip s power and ground for additional switching noise immunity AN50 APPLICATION NOTE 12V RC5051 5V le d DIGITAL CONTROL VO g Ir TES 1 24v REFERENCE VIDO VID2 RSEL VID1 VID3 65 5051 01 Figure 2 RC5051 Block
31. ly 1 15 mil to 1 35 mil Therefore error due to sheet resistiv ity is 1 35 1 15 1 25 16 Mismatch due to L W Percent error in L W is dictated by geometry and the power dissipation capability of the sense resistor The sense resistor must be able to handle the load current and therefore requires a minimum width which is calculated as follows I 0 05 where W minimum width required for proper power dissipation mils and Ij Load Current in Amps For 15A of load current minimum width required is 300mils which reflects a 1 L W error Thermal Consideration Due to PR power losses the surface temperature of the resistor will increase leading to a higher value In addition ambient temperature variation will add the change in resistor value R Roll 9 T 20 where Ryg is the resistance at 20 C 059 0 00393 C T is the operating temperature and R is the desired value For temperature T 50 C the R change 12 Table 9 is the summary of the tolerance for the Embedded PC Trace Resistor Table 9 Summary PC Trace Resistor Tolerance Tolerance due to Sheet Resistivity variation 16 Tolerance due to L W error 1 Tolerance due to temperature variation 12 Total Tolerance for PC Trace Resistor 29 Design Rules for Using an Embedded Resistor The basic equation for laying an embedded resistor is where p Resistivity uQ mil L Length mils L W Width mils an
32. nal Semiconductor Vas 5V Ip 40A Tj 2 25 C 13 15 TO 220 ja 62 5 NDP706AL jc 1 5 NDP706AEL Ty 125 C 20 24 National Semiconductor Vas 4 5V Ip 10A Ty 25 C 31 40 TO 220 yy 62 5 NDP603AL Ty 125 C 42 54 jo 2 5 National Semiconductor Vas 5V Ip 24A Ty 25 C 22 25 TO 220 j4 62 5 NDP606AL Ty 125 C 33 40 c 1 5 Motorola Vas 5V Ip 37 5A Ty 25 C 6 9 TO 263 ja 62 5 MTB75NO3HDL Ty 125 C 9 3 14 D PAK jo 1 0 Int Rectifier Vas 5V Ip 31A Tj 25 28 TO 220 Pya 62 5 IRLZ44 Ty 125 C 46 c 1 0 Int Rectifier Vas 4 5V lp 28A Ty 25 C 19 TO 220 Oj 62 5 IRL3103S Ty 125 C 31 c 1 0 Intl Rectifier Vas 4 5V Ta 25 C 18 SO 8 ja 50 IRF7413 ipio A SMD Note 1 Rps on values at Tj 125 C for most devices were extrapolated from the typical operating curves supplied by the manufacturers and are approximations only AN50 APPLICATION NOTE Two MOSFETs in parallel We recommend two MOSFETs used in parallel instead of one single MOSFET The following significant advantages are realized using two MOSFETs in parallel Significant reduction of Power dissipation Maximum current of 14A with one MOSFET Pmosret P Rps ow Duty Cycle 14 0 050 3 3 0 4 5 0 4 0 35 72W With two MOSFETS in parallel Pyosrgr P Rps on Duty Cycle 14 2 0 037 3 3 0 4 5 0 4 0 35 1 3W FET Note Rps on inc
33. nductor Ipk is found to be approximately 15 5A px B Lmin I Troad max 2 145 2 165A SC 2 linductor Therefore the short circuit detection threshold must be at least 16 5A Table 7 Comparison of Sense Resistors The next step is to determine the value of the sense resistor Including sense resistor tolerance the sense resistor value can be approximated as follows th min Vi th min zs NICO SET R x 1 TF Isc SENSE T Load max Where TF Tolerance Factor for the sense resistor Table 7 describes tolerance size power capability tempera ture coefficient and cost of various type of sense resistors Discrete Metal Discrete Discrete Iron Strip Surface Discrete MnCu CuNi Alloy Motherboard Alloy Mount Resistor Alloy Wire Wire Resistor Description Trace Resistor Resistor IRC Dale Resistor Copel Tolerance 29 5 1 10 10 Factor TF 1 available Size 2 x 0 2 x 0 001 0 45 x 0 065 x 0 25 x 0 125 x 0 200 x 0 04 x 0 200 x 0 04 x LxWxH 1 oz Cu trace 0 200 0 025 0 160 0 100 Power capability gt 50A in 1 watt 1 watt 1 watt 1 watt 3W and 5W available Temperature 4 000 ppm 30 ppm 75 ppm 30 ppm 20 ppm Coefficient Cost Low included in 0 31 0 47 0 09 0 09 810 000 piece motherboard Refer to Appendix A for Directory of component suppliers Based on the Tolerance Factor in the above table for an embedded PC trace re
34. reases with temperature Assume Rpg on 25mQ at 25 C Rps ow can easily increase to 50mQ at high temperature when using a single MOSFET When using two MOSFETs in parallel the temperature effects should not cause the Rpg on to rise above the listed maximum value of 37mQ Less heat sink required With power dissipation down to around one watt and with MOSFETs mounted flat on the motherboard considerable less heat sink is required The junction to case thermal resistance for the MOSFET package TO 220 is typically at 2 C W and the motherboard serves as an excellent heat sink Higher current capability With thermal management under control this on board DC DC converter is able to deliver load currents up to 14 5A with no performance or reliability concerns MOSFET Gate Bias MOSFET can be biased by one of two methods Charge Pump and 12V Gate Bias Method 1 Charge pump or Boostrap method Figure 5 employs a charge pump to provide gate bias Capacitor CP is the charge pump deployed to boost the voltage of the RC5050 output driver When the MOSFET switches off the source of the MOSFET is at 0 6V VCCQP is charged through the Schottky diode to 4 5V Thus the capacitor CP is charged to 5V When the MOS FET turns on the source of the MOSFET voltage is equal to 5V The capacitor voltage follows and hence provides a voltage at VCCQP equal to 10V The Schottky diode is required to provide the charge path when the MOSFET is off
35. roller is not instantaneous the initial load transient must be supplied entirely by the output capacitors The initial voltage deviation is determined by the total ESR of the capacitors used and the parasitic resistance of the output traces For a detailed analysis of capacitor requirements in a high end microprocessor system please refer to Application Bulletin 5 Input Filter The DC DC converter should include an input inductor between the system 5V supply and the converter input as described below This inductor serves to isolate the 4 5 V supply from the noise in the switching portion of the DC DC converter and to limit the inrush current into the input capacitors during power up A value of 2 5uH is rec ommended as illustrated in Figure 14 2 5uH 5V Vin O IVY o 1000uF 10V ES T Electrolytic O e o L 65 AP42 17 Figure 14 Input Filter Bill of Material Table 11 is the Bill of Material for the Application Circuits of Figure 3 and Figure 4 Table 11 Bill of Materials for a 13A Pentium Pro Klamath Application Quantity Reference Manufacturer Part Description Requirements and Order Comments 7 C4 C5 C7 Panasonic 0 1uF 50V capacitor C8 C9 C10 ECU V1H104ZFX C11 1 C6 Panasonic 4 7uF 16V capacitor ECSH1CY475R 1 Cext Panasonic 120pF capacitor ECU V1H121JCG 1 C12 Panasonic 1uF 16V capacitor ECSH1CY105R 3 C1 C2 C3 United Chemi con 1000uF 6 3V el
36. s significantly better than a typical VRM 19 AN50 APPLICATION NOTE Input Ripple and Power on Input Rush Current Power on Input Rush Current was not measured on the moth erboard because we did not want to cut the 5V trace and insert a current probe in series with the supply However with the input filter design the Input Rush Current is well within specification load 9 9A Refer to Attach ment G for Scope Picture Input Ripple Voltage 15mV Note Excellent input ripple voltage Input ripple voltage is recom mended to be less than 5 of the output voltage Component Case Temperature Case Temperature Case Temperature Case Temperature load 9 9A lioaa 12 4A load 13 9A Device Description C C C Q3A MOSFET 57 63 66 3 K1388 Q3B MOSFET 58 64 66 6 K1388 L1 Inductor 53 56 61 2 Unknown Q2 Schottky Diode 66 70 87 2048CT IC Raytheon s RC5050 52 54 58 Cin Input Cap 1000uF 38 2 36 8 39 Cout Output Cap 35 34 8 38 2 1500uF Note The values for case temperatures are within guidelines That is case temperatures for all components should be below 105 C 25 C Ambient Evaluation Summary The on board DC DC converter is fully functional It has excellent load regulation transient response and input volt age ripple Tek Running 500kS s Hi Res E Hi Res Tek Running 500kS s JA 76 0mV 74 0mv A 70 0mV 50 4mv
37. sistor and for load max 14 5A V th min R x 1 TF SENSE gt 2 0A Ti oad max 100mV dG mV xq 43mQ 20A 14 5A QR g For a discrete resistor and for Ijyq max 14 5A V th min EE RSENSE E 2 0A Ij cad max i T 9 7 100mV SVT ee For user convenience Table 8 lists the recommended values for sense resistors for various load currents using embedded PC trace resistors and discrete resistors Table 8 Rgense for Various Load Currents RsENsE RsENsE IL oad max PC Trace Discrete Resistor mQ Resistor mQ 10 0 5 9 7 9 11 2 5 4 7 2 12 4 4 9 6 6 13 9 4 5 6 0 14 0 4 4 5 9 14 5 4 3 5 8 Discrete Sense Resistor Discrete Iron Alloy resistors come in variety of tolerances and power ratings and are most ideal for precision imple mentation MnCu Alloy wire resistors or CuNi Alloy wire resistors are ideal for low cost implementations 11 AN50 APPLICATION NOTE Embedded Sense Resistor PC Trace Resistor Embedded PC trace resistors have the advantage of near zero cost implementation However the value of the PC trace resistor has large variations Embedded resistors have 3 major error sources the sheet resistivity of the inner layer the mismatch due to L W and the temperature variation of the resistor All three error sources must be considered for laying out embedded sense resistors Sheet resistivity For 1 ounce copper the thickness variation is typical
38. t ing power dissipation within the MOSFET s may rise to destructive levels The following is the design equation used to set the short cir cuit threshold limit V RsgENsE I where Igsc Output short circuit current Ii m Imin Load max 2 Isc 2 linduetor I Where Ipk and Imin are peak ripple current and Toad max Maximum output load current You must also take into account the current lyk Imin or the ripple current flowing through the inductor under normal operation Figure 8 illustrates the inductor current waveform for the RC5050 DC DC converter at maximum load Ipk LOAD MAX Figure 8 Typical DC DC Converter Inductor Current Waveform The calculation of this ripple current is as follows Vour Vp Vix 7 Vsw Vp Ij Imin Vix 7 Vsw Your 2 L where Vin input voltage to converter Vsw voltage across switcher MOSFET Ij oap X Rps ow Vp Forward Voltage of the Schottky diode T the switching period of the converter 1 fs and fy switching frequency For an input voltage of 5V output voltage of 3 3V L equals 1 3uH and a switching frequency of 285KHz using Cext 100pF the inductor current can be calculated at approximately 1A Cpr Imin _ 5 0 14 5x 0 037 3 3 2 1 3 x 10 3 3 0 5 l 5 0 14 5 x 0 037 05 235 x 107 2A 10 APPLICATION NOTE AN50 Therefore for load current of 14 5A the peak current through the i
39. th current limiting at the power supply This ensures that no cata strophic shorts are present If proper voltage is not achieved go to Procedures below When you have proper voltage increase the current lim iting of the power supply to 16A Apply load at 1A increments An active load HP6060B or equivalent is suggested In case of poor regulation refer to Procedures below Procedures 1 If there is no voltage at the output and the circuit is not drawing current look for openings in the connections check the circuitry versus schematic and check the power supply pins at the device to make sure that volt age s are applied If there is no voltage at the output and the circuit is drawing excessive current gt 100mA with no load check for possible shorts Determine the path of the excessive current and which devise is drawing it this current may be drawn by peripheral components If the output voltage comes close to the expected value check the VID inputs at the device pins The part is fac tory set to correspond to the VID inputs Premature shut down can be caused by an inappropriate value of the sense resistor See the Sense Resistor sec tion Poor load regulation can be due to many causes Check the voltages and signals at the critical pins The VREF pin should be at the voltage set by the VID pins If the power supply pins and the VID pins are correct the VREF should have the correct voltage
40. tion to provide a constant voltage monitor on the VFB pin The circuit compares the VFB signal to the VREF volt age and outputs an active low interrupt signal to the CPU when the power supply voltage exceeds 12 of nominal The Power Good flag provides no other control function to the RC5050 or the RC5051 Output Enable OUTEN The DC DC converter accepts an open collector signal for controlling the output voltage The low state disables the out put voltage When disabled the PWRGD output is in the low state Upgrade Present UP Intel specifications state that the DC DC converter should accept an open collector signal used to indicate the presence of an upgrade processor The typical state is high that is a standard processor is in the system When in the low or ground state an OverDrive processor is present the output voltage must be disabled unless the converter can supply the requirements of the OverDrive processor When disabled the PWRGD output must be in the low state Because the RC5050 and RC5051 can supply the requirements of the OverDrive processor the UP signal is not required Over Voltage Protection The RC5050 and RC5051 constantly monitor the output voltage for protection against over voltage conditions If the voltage at the VFB pin exceeds 20 of the selected program voltage an over voltage condition is assumed and the chip disables the output drive signal to the external MOSFET s APPLICATION NOT
41. viation For more detailed information regarding capacitor placement refer to Application Bulletin AB 5 The traces that run from the RC5050 IFB pin 4 and VFB pin 5 pins should be run next to each other and Kelvin connected to the sense resistor Running these lines together prevents some of the common mode noise that is presented to the RC5050 feedback input Try as much as possible to run the noisy switching signals HIDRV amp VCCQP on one layer but use the inner layers for power and ground only If the top layer is being used to route all of the noisy switching signals use the bottom layer to route the analog sensing signals VFB and IFB Example of a PC Motherboard Layout and Gerber File This section shows a reference design for motherboard implementation of the RC5050 along with the Layout Gerber File and Silk Screen The actual PCAD Gerber File can be obtained from Raytheon Electronics local Sales Office or from the Semiconductor Division Marketing department at 415 966 7819 GND x 3 Ex od y f ota T o x x x a N Xx P 5 xx X xx x Y x x Ja B H X x x x x 17 AN50 APPLICATION NOTE Guidelines for Debugging and Performance Evaluations Debugging Your First Design Implementation 1 6 Note the setting of the VID pins to know what voltage is to be expected Do not connect any load to the circuit While monitoring the output voltage apply power to the part wi
42. wer Good PWRGD Output Enable OUTEN and Upgrade Present UP These signals will be discussed later RC5050 and RC5051 Description Simple Step Down Converter S1 L1 oo Vine Di AC1i R Vout 65 5050 06 Figure 1 Simple Buck DC DC Converter Figure 1 illustrates a step down DC DC converter with no feedback control The derivation of the basic step down con verter is the basis for the design equations for the RC5050 and RC5051 Referring to Figure 1 the basic operation begins by closing the switch 1 When S1 is closed the input voltage Vyn is impressed across inductor L1 The current flowing in this inductor is given by the following equation _ Vin Vour Ton L L1 where Toy is the duty cycle the time when S1 is closed When S1 opens the diode D1 conducts the inductor current and the output current is delivered to the load accord ing to the following equation _ Your Ts Ton L L1 where Ts is the overall switching period and Ts Ton is the time during which S1 is open By solving these two equations we can arrive at the basic relationship for the output voltage of a step down converter Ton Vour vil In order to obtain a more accurate approximation for Vout we must also include the forward voltage Vy across diode D1 and the switching loss Vow After taking into account these factors the new relationship becomes Voss Vint Vn Ve EN y out Vint Vp sw D MOSFET swit
43. x 10 x 285khz PDqgAN OA 0 010W PDcap 75 2 5 x0 015 0 37W PDic 0 2W PD oss 2 19W 1 0W 0 65W 0 019W 1 35W 0 010W 0 37W 0 2W 5 789W Efficiency ve 85 AN50 APPLICATION NOTE Selecting the Inductor The inductor is one of the most critical components to be selected for a DC DC converter application The critical parameters are inductance L maximum DC current Ip and DC coil resistance Rj The inductor core material is a crucial factor in determining the amount of current the inductor is able to withstand As with all engineering designs tradeoffs exist between various types of core materi als In general Ferrites are popular due to low cost low EMI properties and high frequency gt 500KHz characteristics Molypermalloy powder MPP materials exhibit good satu ration characteristics low EMI and low hysteresis losses but tend to be expensive and more effectively utilized at operating frequencies below 400KHz Another critical parameter is the DC winding resistance of the inductor This value should typically be reduced as much as possible as the power loss in the DC resistance degrades the efficiency of the converter by the relationship Pj I x Ry The value of the inductor is a function of the oscillator duty cycle Ton and the maximum inductor current Ipx Ipg can be calculated from the relationship Vin Ysw Vp Ipk Imn Efron Where Ton is the maximum duty cycle and
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