Z8 Encore! XP-Based NiCd Battery Charger Application Note
Contents
1. In the constant current mode fast charging occurs when the charging current equals the rated battery capacity C Fast charging requires constant moni toring of battery parameters and precise termina tion techniques It is therefore important to know when to terminate charging In the NiCd battery charger application the battery parameters are con stantly monitored and the negative AV termina tion technique is used As a result the NiCd battery charger ensures the safety of the battery For detailed information about termination tech niques associated with NiCd batteries see Appen dix D Battery Technology on page 14 Page 2 of 15 Developing the Application with the Z8 Encore XP MCU This section provides an overview of the functional architecture of the NiCd battery charger implemen tation using the Z8 Encore XP MCU Hardware Architecture Figure 1 displays a hardware block diagram for the battery charger application The Z8 Encore XP based NiCd battery charger application features the following hardware blocks e Z8 Encore XP Development Board e External power source 32 V 3 A e Step down DC DC buck converter e Feedback section analog design e NiCd battery l External i 1 PowerSource i 82V3A Power Section Step Down DC DC buck Converter Z8 Encore XP Development Board Feedback Section Output Voltage Battery Voltage Battery Current Battery Temperature Figure 1 Block Di2. also forms a part of the test setup The external DC power supply provides two differ ent voltages to the charger circuits the DC DC step down converter and the feedback attenuators The operational amplifier based feedback attenua tor circuits are fed with a 12 V supply The DC DC converter works on a 8 V to 12 V DC input for the batteries tested The control algorithm provides the necessary line regulation to sustain the voltage variation at the input AN022103 0608 Z8 Encore XP Based NiCd Battery Charger Equipment Used The equipment used to test the Z8 Encore XP based NiCd battery charger are listed in Table 1 Table 1 Battery Charger Test Equipment System Equipment Z8 Encore XP 4K Series Development Kit Z8F042A28100KIT External power supply Make Aplab Model LQ 6324 Test Equipment Oscilloscope Make Tektronix Model TDS 724D 500 MHz 1 GSps Multimeter Make Motwane Model DM3750 Batteries Used for Testing BP T16 Make Sony Type NiCd Ratings 3 6 V 270 mAh Procedure To test the Z8 Encore XP based NiCd battery charger application perform the following steps 1 Download the AN0221 SC01 zip file from www zilog com Extract its contents to a folder on your PC 2 Launch ZDS II for Z8 Encore 3 Make the hardware connections as described in Figure 3 and the schematics provided in Appendix B Schematics on page 8 4 Connect the batter
3. Application Note Charger AN022103 0608 Abstract This Application Note describes Zilog s Z8 Encore XP based Nickel Cadmium NiCd bat tery charger The battery charger application uses the internal clock of the Z8 Encore XP microcon trollers unit MCU as the system clock An inter nal reference voltage of 2 V is applied to the ADC peripheral of the Z8 Encore XP MCU gt Note The source code file associated with this Application Note AN0221 SCO1l zip is available on www zilog com Z8 Encore XP 4K Series Flash Microcontrollers Zilog s Z8 Encore products are based on the new eZ8 CPU and introduce Flash memory to Zilog s extensive line of 8 bit MCU Flash memory in circuit programming capability allows faster development time and program changes in the field The high performance register to register based architecture of the eZ8 core maintains back ward compatibility with Zilog s popular 73 MCU Z8 Encore MCUs combine a 20 MHz core with Flash memory linear register SRAM and an extensive array of on chip peripherals The Z8 Encore XP 4K Series of devices support up to 4 KB of Flash program memory and 1 KB register RAM An on chip temperature sensor allows temperature measurement over a range of 40 C to 105 C These devices include two enhanced 16 bit timer blocks featuring Pulse Width Modulation PWM Capture and Compare capabilities An on chip Internal Precision Oscilla tor 5 MHz 32
4. KHz is used as a trimmable clock source requiring no external components The Z8 J Z8 Encore XP Based NiCd Battery Z8 amp Encore xr Flash Microcontrollers Encore XP devices include 128 bytes of Non Volatile Data Storage NVDS memory where indi vidual bytes are written or read The full duplex UART provides serial communications Infrared Data Association IrDA encoding and decoding capability and supports multidrop address process ing in hardware The on chip peripherals make the Z8 Encore XP MCUs suitable for a variety of applications includ ing motor control security systems home appli ances personal electronic devices and sensors Discussion This section discusses the functionality of the Z8 Encore XP based battery charger application in detail For detailed information about NiCd battery technology see Appendix D Battery Technology on page 14 Theory of Operation At the core of a battery charger is the DC DC con verter also known as a buck converter that acts as a regulated power source The charger hardware is capable of regulating the charger output in a num ber of modes such as constant voltage constant current or constant voltage with a current limit The charger is a control system in itself The type and capacity of the battery determines the mode of operation of the battery controller namely a con stant current source or a constant voltage source The voltage V sgr and curre
5. MBR360 C1 gt gt V_batt D3 MBR360 NOTE For Testing the VP30V is obtained from External Power Source Figure 6 Schematic Displaying the DC DC Step Down Buck Converter ANO022103 0608 Page 10 of 15 Z8 Encore XP Based NiCd Battery Charger The schematic diagram in Figure 7 displays the feedback circuits for the battery charger application 0 1uF 0 1uF VCC C7 ET V_batt gt 10K V_batt gt gt gt PBO ANAO PB1 ANA1 m R15 3 Battery Temperature V_batt gt gt Battery Voltage Thermistor 10K J6 R28 R16 1K 1K AVDD 12V AVDD12V AVDD 12V 0 1uF 0 1uF C7 c11 9 c10 C7 100uF 0 1uF 0 1uF 1K LM324 UIC 1K 10K 10K _out gt R34 V_out gt R23 10 gt PB2 ANA2 R19 PB3 ANA3 Lout gt gt Battery Current v ouw gt gt Converter Output Voltage 1K 10K R20 R24 cs 1K 1K AVDD 12V 10uF AVDD 12V NOTE For Testing the AVDD12V is obtained from External Power Source Figure 7 Schematic Displaying the Feedback Circuits AN022103 0608 Page 11 of 15 Z8 Encore XP Based NiCd Battery Charger Zilog Appendix C Flowcharts This appendix provides flowcharts for the battery charger application described in this document Figure 8 explains a flowchart of the main routine for the battery charger application The main routine involves calculation of safety limits thresholds duty cycle reading of f
6. agram of Battery Charger Hardware The battery charger application uses Port B on the Z8 Encore XP MCU as ADC inputs Timer 1 is used in PWM mode and the output is tapped at the PC1 Timer 1 output pin The system clock is AN022103 0608 Z8 Encore XP Based NiCd Battery Charger derived from the internal precision oscillator of the Z8 Encore XP MCU The reference voltage required for the ADC is generated internally by the Z8 Encore XP MCU hence the external compo nent requirement and the Bill of Material BOM cost is reduced The step down DC DC buck converter provides a voltage or current appropriate to the NiCd bat tery The buck converter modulates a higher volt age from the external source with a varying pulse width PWM method to generate a lower voltage The pulse width is controlled by the control algo rithm based on the values obtained from the feed back section The feedback section consists of four differential amplifiers attenuators The parameters controlled by the first three amplifiers are the converter volt age Vour the battery voltage Vpgarr and the battery current Ig arr The battery current and the converter current are same The fourth differential amplifier is used for temperature measurement in the case of batteries featuring built in temperature sensors For schematic diagrams associated with the battery charger application see Appendix B Schematics on page 8 Software Implemen
7. eedback values for battery voltage charging current and converter voltage ANO022103 0608 Initialize peripherals Calculate safety limits and thresholds for charging and termination Read feedback values for battery voltage charging current and converter voltage Within safety limits No Is the battery charged Terminate No Calculate the duty cycle Figure 8 The Main Routine Page 12 of 15 Z8 Encore XP Based NiCd Battery Charger Zilog Figure 9 presents a flowchart of the Interrupt Service Routine ISR for reloading the PWM value and updating the charge termination data Start ISR Reload PWM Value Update charge termination data every 10 seconds Return from ISR Figure 9 The ISR Return Routine ANO022103 0608 Page 13 of 15 Appendix D Battery Technology The four mainstream battery chemistries NiCd NiMH SLA and Li Ion feature different charging and discharging characteristics Long term battery life and performance are critically dependent up on how batteries are charged Therefore it is impor tant to charge batteries with a mechanism specific to their requirements It is also important to know when to terminate charging because overcharging of a battery invari ably results in poor performance and can damage the battery in extreme cases Moreover different battery types function differently near full charge condition and therefore require specific charge ter minati
8. igure 4 displays the Z8 Encore XP MCU pin diagram VDD RESET gt DBG PWM lt ANO022103 0608 RESET PDO DBG PAO TOIN TOOUT XIN PA1 TOOUT XOUT PA2 DE0O PA3 CTSO PA4 RXDO PA5 TXDO PA6 T1OUT T1IN PA7 T1OUT Z8F042A VDD PCO ANA4 CINP LED PC1 ANA5 CINN LED PC2 ANA6 LED PC3 COUT LED PC4 LED PC5 LED PC6 LED PC7 LED PBO ANAO AMPOUT PB1 ANA1 AMPINN g PB2 ANA2 AMPINP PB3 CLKIN ANA3 PB4 ANA7 PB5 VREF PB6 AVDD PB7 AVSS R10 560E D7 LED ee lt PBO ANAO lt PB1 ANA1 lt PB2 ANA2 lt PB3 ANA3 lt ADD_VREF_2V5 lt AVDD lt AVSS Figure 4 Schematic Displaying the Z8 Encore XP MCU Pin Diagram Page 8 of 15 Z8 Encore XP Based NiCd Battery Charger The schematic diagram in Figure 5 displays the external circuitry for the battery charger application VDD CONN JACK PWR RXE050 SENSE RESET C12 C16 100uF 10V C15 C14 J8 0 1uF 0 1uF LT1129 3 3 DD 100uF 10V NOTE SENSE should be sense d as close to the processor as possible VDD RESET C27 lt DBG 0 1uF Figure 5 Schematic Displaying the External Circuitry for the Battery Charger Application ANO022103 0608 Page 9 of 15 Z8 Encore XP Based NiCd Battery Charger The schematic diagram in Figure 6 displays the DC DC step down buck converter R3 1K IRF9540 Q1 L1 Q2 PWM 2N2222
9. ive AV charge termination technique is not recommended for NiMH batteries NiMH batteries exhibit plateau characteristics after a minimal drop in the voltage This flat region of the battery characteristics represents the full charge condition Therefore the charge termination technique used in NiMH batteries is termed as zero AV NiMH batteries do not exhibit memory effect these batteries are used in consumer durables such as cell phones NiMH batteries are more expensive compared to NiCd batteries as they are lighter in weight and are not prone to memory effect Page 14 of 15 Z8 Encore XP Based NiCd Battery Charger 2 a IA AN Z l l l A L A EE Y amp y AN Warning DO NOT USE IN LIFE SUPPORT LIFE SUPPORT POLICY ZILOG S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF ZILOG CORPORATION As used herein Life support devices or systems are devices which a are intended for surgical implant into the body or b support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or sy
10. nt Igy set points are also determined by the type and capacity of the battery The parameters current and voltage are controlled using the PWM technique In the PWM technique the frequency of the signal is maintained constant Copyright 2008 by Zilog Inc All rights reserved www zilog com and the width of the pulse or the duty cycle of the signal is varied This variation is reflected as a change in voltage and or current at the output The switching regulator reads the parameters through a feedback circuit and the battery controller operates based on the control algorithm The PWM output is obtained by comparing the actual value of the parameter under control with the corresponding set point In the constant voltage mode the converter voltage is compared with the voltage set point In contrast in the constant cur rent mode the voltage developed by the charging current across a sense resistor is compared with the current set point The feedback loop maintains the converter voltage or the converter current constant depending on the selected mode of operation Controllers are differentiated based on the method of regulation of parameters in accordance with the corresponding set points For detailed information about battery controllers see the related document provided under the Electronics topic in References on page 6 In a proportional controller the actual value and the set value are compared and the resulting erro
11. on techniques During charging all batteries exhibit a marked rise in voltage above the rated battery voltage The two major rechargeable battery types NiCd and NiMH are briefly discussed below For more information see References on page 6 Nickel Cadmium NiCd batteries are used in camcorders walkmans and other portable consumer electronic equipment The voltage cell ratio for a NiCd battery is 1 2 V NiCd batteries are charged using the constant current charging method If the voltage crosses the full charge point during charging the charging current drops to 15 mV cell This voltage drop represents the full charge condition Charging is terminated when the battery is in full charge condition NiCd batteries use the negative AV charge termination technique During charging the battery voltage rises to 1 65 V cell Therefore the battery must be discharged periodically to ensure that the battery functions efficiently This is memory effect ANO022103 0608 Z8 Encore XP Based NiCd Battery Charger rif Z ILN Nickel Metal Hydride NiMH batteries exhibit high power density com pared to NiCd batteries The voltage cell ratio for a NiMH battery is 1 2 V NiMH batteries are charged using the constant current charging method If the voltage crosses the full charge point during charg ing the charging current drops but this drop in charging current is not as low as that in the case of NiCd batteries Therefore the negat
12. r value is used The drawback of a proportional controller is the possibility of a steady state error Adding an integral component to the control algorithm eliminates this error The equation for a proportional plus integral PI controller is x t k1 x e t k2 x fedo To be useful for a microcontroller based discrete system the integral is approximated by a running sum of the error signal Therefore an equation in the differential form is expressed as follows in Equation 1 u k c x e k C2 x Seti j 0 Where C1 and C2 are constants ANO022103 0608 Z8 Encore XP Based NiCd Battery Charger Equation is the position algorithm A better rep resentation of Equation 1 is explained in Equation 2 as follows k 2 U k 1 G xe k 1 C2x gt i j 0 Subtracting Equation 2 from Equation 1 and rear ranging the terms yields Equation 3 as follows u k u k 1 Kp x e k Ki x e k 1 Where Kp and Ki are the proportional and integral constants respectively Equation 3 is the velocity algorithm and is a con venient expression as only the incremental change in the manipulated variable is calculated The charging algorithms are designed based on the type of battery and the current state of charge for that battery The basic charging methods are constant current and constant voltage charging The NiCd and Nickel Metal Hydride NiMH batteries are charged using the constant current method
13. re ment is significant from the safety point of view After the actual values Vout Vaart and Igarr are determined they are checked for safety limit compliance The safety routine is responsible for the overall safety features associated with the bat tery charger The charger ensures safety by com paring the actual converter voltage the battery voltage and the battery temperature with the calcu lated thresholds Crossing these thresholds switches off the PWM output which turns off the converter output and terminates charging func tions Such termination protects the batteries in the case of a device failure If all the actual values are within limits the battery is tested for full charge NiCd batteries are consid ered to be completely charged if the voltage level as indicated by Figure 2 produces a distinctive rise over a period of time then a decrease ANO022103 0608 Z8 Encore XP Based NiCd Battery Charger Z Il Voltage Time gt Figure 2 Voltage Levels as a Function of Time The dashed line in Figure 2 represents where a decrease in voltage occurs At this point maximum charge has been reached charging then terminates If however the battery is not completely charged the duty cycle required for maintaining the set points at the converter output is calculated by a control algorithm This control algorithm implements PI control to derive a PWM output based on the equations p
14. re sented in the Theory of Operation on page 1 The timer ISR is invoked every 5 ms The PWM value computed by the control algorithm is loaded into the PWM generators to be transmitted via the out put pin The 16 bit timer PWM mode offers a pro grammable switching frequency based on the reload value This flexibility allows you to trade off between accuracy and frequency of the PWM switching signal A lower frequency results in a higher reload value and a higher resolution in the pulse width variation The reverse is also true The timer ISR also updates the charge termination vari ables every 10 s For flowcharts related to the battery charger appli cation see Appendix C Flowcharts on page 12 Page 4 of 15 Testing This section discusses the setup equipment used and procedure followed to test the Z8 Encore XP based NiCd battery charger application Setup The test setup for the Z8 Encore XP based NiCd battery charger application is described in Figure 3 External DC Power Supply Ht DC DC DC Step Down Converter Oscilloscope Z8 Encore XP Development Board Charger HardwarelExternal Circuitry Figure 3 Battery Charger Test Setup The test setup comprises of a Z8 Encore XP Z8F042A Development Board a NiCd battery that must be charged an oscilloscope an external DC power supply and a DC DC step down buck con verter A feedback circuit comprised of differential amplifiers or attenuators
15. s Product Specifi cation PS0228 e Z8 Encore XP F042A Series Development Kit User Manual UM0166 e Power Electronics Design Handbook Low Power Components and Applications Author Nihal Kularatna ISBN 0 7506 7073 8 Publisher Oxford Newnes 1998 e High Frequency Switching Power Supplies The ory and Design Author George Chryssis ISBN 0 07 010949 4 Publisher McGraw Hill Book Company ANO022103 0608 Z8 Encore XP Based NiCd Battery Charger Digital Control Systems Volume 1 Fundamen tals Deterministic Control Author Rolf Iser mann ISBN 0 387 50266 1 Publisher Springer Verlag e Zilog Developer Studio II Z8 Encore User Manual UM0130 Page 6 of 15 Z8 Encore XP Based NiCd Battery Charger Zilog Appendix A Glossary Definitions for terms and abbreviations used in this Application Note that are not commonly used are listed in Table 2 Table 2 Glossary Term Abbreviation Definition 1c A charging current rate equal to the A hr rating of the battery ADC Analog to Digital Converter ISR Interrupt Service Routine Li lon Lithium lon mAh milli Ampere hour the unit of battery capacity NiCd Nickel Cadmium NiMH Nickel Metal Hydride Pl Proportional and Integral PWM Pulse Width Modulation SLA Sealed Lead Acid ANO022103 0608 Page 7 of 15 Appendix B Schematics Z8 Encore XP Based NiCd Battery Charger The schematics in F
16. stem or to affect its safety or effectiveness Document Disclaimer 2008 by Zilog Inc All rights reserved Information in this publication concerning the devices applications or technology described is intended to suggest possible uses and may be superseded ZILOG INC DOES NOT ASSUME LIABILITY FOR OR PROVIDE A REPRESENTATION OF ACCURACY OF THE INFORMATION DEVICES OR TECHNOLOGY DESCRIBED IN THIS DOCUMENT ZILOG ALSO DOES NOT ASSUME LIABILITY FOR INTELLECTUAL PROPERTY INFRINGEMENT RELATED IN ANY MANNER TO USE OF INFORMATION DEVICES OR TECHNOLOGY DESCRIBED HEREIN OR OTHERWISE The information contained within this document has been verified according to the general principles of electrical and mechanical engineering eZ8 Z8 Z8 Encore Z8 Encore XP and eZ80 are trademarks or registered trademarks of Zilog Inc All other product or service names are the property of their respective owners ANO022103 0608 Page 15 of 15
17. tation All Z8 Encore XP peripherals are initialized from their power on state to the required mode of opera tion After initialization the battery parameters are loaded into the variables These battery parameters are defined in the charger h header file The safety and termination thresholds are calcu lated based on the battery parameters The set points for the DC DC step down buck converter voltage the current and the current limit are calcu lated After these one time calculations are com plete the charger software enters into an infinite loop which is broken only by a successful charge completion or a safety error Page 3 of 15 Inside the infinite loop the ADC reads the actual values for the converter output voltage the battery voltage the current and the temperature tempera ture is measured only if the battery features a tem perature sensor The ADC measures the output voltage and the output current of the DC DC con verter as feedback to the controller The ADC also measures the voltage at the battery terminals as an input to determine the charge termination Mea surement of the output voltage the output current and the battery voltage are the basic measurements The current across the battery terminals is same as the measured converter output current For batter ies featuring built in temperature sensors the charger reads the battery temperature in addition to the basic measurements The temperature measu
18. y to be charged across the provided battery terminals see Appendix B Schematics on page 8 5 Apply the required voltages to the appropriate circuits as described in the section Setup on page 5 6 Download the battery charger code to the Z8 Encore XP Flash memory using ZDS IJ IDE 7 Execute the battery charger code Page 5 of 15 8 Observe the PWM waveforms on the oscillo scope Results The charging of the battery began in the constant current mode with the charging current equal to C The charging is terminated when the battery voltage increases to a peak and then decreases negative AV termination technique Summary This Application Note discusses a NiCd battery charger implementation using the Z8 Encore XP MCU The battery charger software provides fast charging algorithms Fast recharge is possible due to the accurate monitoring of the charging rendered by the 10 bit accuracy of the ADC Monitoring the charging mechanism facilitates the accurate termination of charging Therefore overcharging is prevented resulting in a longer battery life Addi tionally the PWM technique facilitates an accurate DC DC buck step down converter implementa tion with excellent line load regulation References The documents associated with Z8 Encore XP ez8 and ZDS II available on www zilog com and electronics references are provided below e eZ8 CPU User Manual UM0128 Z8 Encore XP F082A Serie
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