STBCFG01 final - Arrow Electronics
Contents
1. IBAT IBUS A Constant Current Fast charge Pre charge During the constant current phase the charging current is at its maximum value and the battery voltage increases When the battery voltage reaches the target voltage Vrroar it is kept constant while the current tapers down until it reaches the termination threshold With reference to Figure 10 the decreasing trend of the charging current is due to the battery pack s internal impedance R and the parasitic resistance from the battery terminals to positive and negative sensing terminals of the charging device Rp and Rp3 While the charging device keeps the voltage between battery voltage sensing terminals constant Vsens the internal battery pack s voltage Vcr keeps increasing during the constant voltage phase This makes the drop across the battery s internal impedance and parasitic resistance R Rp2 Rp3 decrease and therefore the charging current also decreases Figure 10 Charging system parasitic effects Battery Pack As already mentioned in this article the voltage drop over Rj Rp and Rp also makes the charger enter the CV constant voltage phase earlier than expected before Vcg reaches Vproar generating an increase of charging time at high charging current level In order to mitigate this phenomenon and extend the duration of the constant current phase the STBCFGO01 enters the CV phase when the sensed battery voltage is slightly2. Figure 12 Open Circuit Voltage vs State of Charge The STBCFGO1 uses the OCV curve to detect the battery s SOC when the battery is first plugged in to the application and then continuously monitors the battery voltage to determine the evolution of the SOC The starting point of the SOC is evaluated once again each and every time a battery disconnection is detected combining the information of battery voltage and RID input A generic OCV curve is stored in a LUT inside the STBCFGO1 but in order to reach accuracy the battery to be used in the application must be characterized to extract OCV curve and adjustment parameters It can be observed in Figure 13 that an accurate reading of the OCV is mandatory to achieve an acceptable error in the initial SOC evaluation the curve shows a very low slope portion where a small error in the OCV reading AV translates into a big error in the SOC ASOC For this reason the STBCFGO1 integrates a high accuracy 14bits delta sigma modulator into the fuel gauge block STMicroelectronics must be contacted for details about battery characterization and LUT update routine Figure 14 and Figure 15 show typical accuracy performance extracted from real charging and discharging cycles using the STBCFGO1 in a mobile phone application The error curve is obtained by comparing the SOC calculated by the STBCFGO1 and the ideal SOC The latter is calculated using a high precision amperometer to measure the current fl
3. yj life augmented STBCFG01 ST s new highly integrated switch mode Li Ion battery charger Agatino Alessandro Adele Castorina Giorgio Catanzaro Barbaro Marano Federico Musarra ABSTRACT Today s portable applications are integrating ever more multimedia functions each with different power needs Battery charge indicators or fuel gauges have become essential for managing devices such as smartphones laptops or digital cameras Accurate time remaining predictions enhance the user s experience and can be critical in certain types of portable electronics such as medical devices ST has combined a powerful and configurable switching battery charger with an accurate Voltage mode Fuel Gauge to simplify charging and battery monitoring INTRODUCTION First in the world the STBCFGO1 switch mode battery charger for single cell Li Ion batteries integrates a highly accurate voltage mode fuel gauge to monitor the battery s state of charge The device also provides a 5 V output to supply USB OTG On The Go bus powered devices in addition to an LDO linear voltage regulator to support system boot in dead battery conditions STBCFGO1 uses accurate measurements of the battery voltage which allows the estimation of the battery s state of charge SOC without a current sensing resistor The switching charger works together with the fuel gauge to simplify monitoring features and to save current consumption when the device is not c
4. the regulated battery floating voltage can be programmed from 3 52 V to 4 78 V The device s system architecture allows reverse use of the switch mode buck converter resulting in a 500mA USB on the go OTG boost regulator and providing power for USB peripherals The device can work up to a 100 duty cycle and blocks reverse current from battery to input when the input voltage is disconnected or is lower than battery voltage The charger uses an innovative architecture where the analog comparators needed to manage the charging cycle have been implemented in a digital way using the fuel gauge s ADC converter ensuring a high accuracy and reducing the device dimensions The usage of the fuel gauge as an accurate voltmeter allows a voltage loop accuracy up to 0 5 through an auto adjustment strategy Figure 1 Architecture of STBCFGO1 LDO output 4 I System Charging Cycle To protect the battery and maximize mobile device usage time the STBCFGO1 implements the charging cycle in 5 phases see Figure 2 tickle charge pre charge fast charge constant voltage CV and end of charge e Trickle charge when the battery is deeply discharged lt 2V the device is in trickle charge mode and charges the battery in linear mode with a low current around 45mA up to the trickle charge threshold This mode is used to wake up batteries in dead battery conditions the internal battery pack s
5. E GEOANA fe seseg Hep seine ip Seach Doves STUSEGAO deicerstion GAOR 130 STATUS E Application 7 FUEL GAUGE RES CHARGER REC HT LOG eb WJ life augmented Figure 7 STBCFGO1 GUI Fuel Gauge Reg tab EF STBCKGON Gut Vi3 26 03 2018 le Setting Help USB Sane VOLT Low Battery Volage Alarm OCV_LOOP_DIS OCV measurement vop Figure 8 STBCFGO1 GUI Charger Reg tab FF STBCFGOI GUI V13 26 03 2014 Application 7 FUEL GAUGE REG r H 106 12 aia 95 El REGISTERS 96 94 PR tood Ba save fo faea a Tee os Toa Tos Toa Tor ea Bo o o ooo ool Q TERE Preche tt on FAST Font Chapetet en THERMLLOOP en Penor Peis ents WO Watchdog Timer Function En Qoo mom arae LDO _ert LDO Out Enable AID_STATUS RID Fade Finally the LOG tab displays a list of all the operations executed and the version of the USB GPIO Interface board s firmware A complete GUI user manual is available for further details STBCFG01 s charging performance Figure 9 shows a typical charging profile where VBUS is the charger s input voltage IBUS is the charger s input current and VBAT IBAT are battery voltage current Figure 9 Typical Li Ion battery charging cycle VBAT VBUS V
6. O Interface board JE USB SWire STA 09 2012 The STBCFG01 GUI is arranged in four different tabs The Application tab Figure 6 provides a quick overview of device status battery voltage and SOC A typical application diagram is displayed as well This tab also includes the enable button of the Auto Read function which continuously reads the whole register set to keep up to date the data displayed by the interface The Fuel Gauge Reg tab Figure 7 gives access to the fuel gauge registers From this tab the user can enable disable the fuel gauge function read fuel gauge data Battery voltage SOC and Open Circuit Voltage and fine tune the fuel gauge algorithm through the VM_CNF parameter Full control over battery voltage and SOC alarms is also provided alarm enable threshold setting and clear command Status of alarm bits and battery connection detector is displayed The OTG mode enable control bit is included as well Similarly the Charger Reg tab Figure 8 contains all the registers needed to control and configure the battery charger grouped into two sub tabs reg addresses 0x90 to 0x95 and 0x96 to 0x9A All the charging parameters charging current charging voltage input current limit and special charging functions can be set up from this tab Charger status information is also provided along with full interrupts configuration Figure 6 STBCFG01 GUI Application tab Fmicien OU VIS
7. e 2 is anticipated The VFLOAT threshold target charging voltage is reached when the actual battery pack voltage is lower than expected measured voltage Vbat Vdrop generating an increase of the charging time that can be significant when the charging current is high From the fuel gauge point of view the voltage drop during both charging and discharging generates an offset that can significantly impact the accuracy of the SOC evaluation The battery header connector also contains the battery detection pin RID used to detect the battery connection disconnection This function is very important as it allows the fuel gauge to provide an accurate starting point for the SOC evaluation algorithm when the battery is inserted or in case of battery swap The RID pin must be connected to the battery s identification resistor contact or to the battery s NTC thermistor contact STBCFGO01 s evaluation software The STEVAL ISB033V1 can be used to evaluate the device s performance directly in the customer s application but on request the USB GPIO Interface board and the STBCFGO01 GUI control software can be shipped This combination provides a user friendly interface to access all the device s configuration registers and monitor their status The interface board Figure 5 uses the USB interface to connect to the PC running the control software while the I C bus is used to control the STBCFGO1 evaluation board Figure 5 USB GPI
8. harging The battery charger features a smart input current limit the maximum input current can be selected through the I C interface and if the input voltage drops below a programmable threshold even if the selected maximum current limit current has not been reached yet the dynamic input current limit function is activated preventing the input current from increasing further The dynamic input current limit function can be disabled if necessary An automatic input pre bias load makes the device suitable for applications using voltage sources requiring a minimum external load for the correct regulation A user level I C interface lets a microcontroller configure all the functions of the device easily Figure shows the device s simplified block diagram All of this is packed into a space saving 2 3 x 2 2 mm2 CSP that makes the STBCFGO1 an ideal fit for all portable and wearable applications where size efficiency and cost are of major concern STBCFG01 s FEATURES Charger Architecture The STBCFGO1 employs a high efficiency DC DC synchronous buck converter operating at 2MHz or 3MHz capable of supplying 1 25 A The switching frequency can be selected through the PC interface The device integrates several control loops regulating the output current and voltage the input current and voltage and silicon temperature through dedicated sensing structures Additionally to accommodate the evolution of lithium battery technologies
9. he OTG mode also features a programmable input average current limit Battery Fuel Gauge The voltage mode fuel gauge provides an accurate evaluation of the Lithium Ion battery s state of charge At power up the fuel gauge algorithm uses the voltage reading to provide a first evaluation of the SOC based on battery modeling data The evolution of voltage is then used to track the changes of the SOC while cycling the battery The external software driver performs the temperature compensation The fuel gauge block can be adapted to different batteries Programmable parameters are used to tailor the algorithm to each battery model In order to keep the optimal performance and avoid losing information learned during battery cycling the user is supposed to save data contained in the device s volatile memory when power is removed The same data has to be restored at power up The STBCFGO1 also provides programmable alarms to notify low battery voltage and low SOC conditions To enhance fuel gauging accuracy with an error of 0 5 and to reduce current consumption 25uA a 14 bit Delta Sigma modulator using switched capacitors SC technique with fully differential input output structure is implemented The ADC also reduces errors through state of art techniques such as nested choppers and a digital moving average filter The device is fully programmable to be adapted to different batteries STBCFG01 s EVALUATION TOOLS STBCFG01 s evaluat
10. higher than the target voltage Once the CV phase has started the target voltage is automatically set down to the nominal battery charging voltage This can be seen in the charging profile in Figure 11 When the CV phase starts the charging voltage red curve decreases to the final value The charging current drops accordingly Figure 11 STBCFGO01 charging profile VBAT VBUS V IBAT IBUS A Fast charge Constant Current 1S vj life augmented STBCFG01 s fuel gauge performance An accurate evaluation of a Lithium Ion battery s state of charge SOC is very important in mobile applications It provides the end user with an estimation of remaining battery life which is critical information when the state of charge starts approaching the dead battery threshold Accurate SOC evaluation helps avoid sudden and unexpected device shut down and allows the device s operating system to store important data before the system cut off voltage is reached In a relaxed Lithium Ion battery there is a univocal relationship between the SOC and the open circuit voltage OCV A battery is considered to be relaxed when a specific time period trx has passed since the last high current charge discharge operation I gt Irrx Ir x and tgx along with the relationship between SOC and OCV are parameters linked to the specific battery chemistry Figure 12 shows a typical SOC OCV curve for a high capacity 4 35V Li Ion battery
11. ion board The STEVAL ISB033V1 Figure 3 provides full access to STBCFGO01 s functions and allows the user to quickly set up an evaluation bench to test the device performance in a real world application Figure 3 STEVAL ISB033V1 The board includes all passive components needed for proper operation and provides several test points to monitor the device s yj life augmented voltage levels Header connectors give access to the PC interface and digital IOs and allow connection to the power supplies A micro USB receptacle can be used to supply the board from a USB port or wall charger Figure 4 STEVAL ISB033V1 schematic diagram In order to get optimal charging and fuel gauging performance the battery header connector J9 in Figure 4 has two voltage sensing pins used to connect the battery voltage sensing lines as close as possible to the battery pack s positive and negative terminals This allows the voltage drops generated by the charge discharge current over PCB tracks and connecting wires to be removed from the battery voltage evaluation The effect of voltage drops is twofold when charging if voltage drops are included in the battery voltage measurement the start of the charging cycle s constant voltage phase CV see Figur
12. owing into the battery charging or out of the battery discharging The error is well below 5 a very good performance for a voltage mode fuel gauge Figure 13 OCV to SOC error Figure 14 Fuel Gauge accuracy during charging Charge Lk yj life augmented Figure 15 Fuel Gauge accuracy during discharging CONCLUSIONS In a single chip the STBCFGO1 combines a high efficiency switching battery charger and a voltage mode fuel gauge The single chip solution saves PCB space and a number of external components Without needing any external sensing resistors the voltage mode fuel gauge provides good accuracy with its high precision ADC converter and high performance fuel gauging algorithm The high number of programmable charging parameters in a compact package together with a fuel gauge function make the STBCFGO1 a very cost effective solution for middle end smartphone platforms and medium battery capacity mobile applications REFERENCES 1 STBCFGO1 Datasheet Switch mode single cell Li battery charger with OTG boost voltage mode fuel gauge and LDO June 2014 http www st com st web ui static active en resource technical document datasheet DM0012 1014 pdf 2 STBCFGO1 GUI Windows Based GUI to Debug by I2C Bus the STBCFGO1 Device Programmable Li Po Battery Charger March 2013
13. protection switch is open e Pre charge phase as soon as the battery voltage enters the pre charge range 2V lt Vgar lt 3V the device starts the switch mode charging and increases the charging current up to the pre charge current level 100mA or yj life augmented 450mA selectable to make the system voltage rise quickly up to a level that allows the system to wake up e Fast charge phase When the battery voltage is above the pre charge threshold Vga gt 3V the STBCFGO1 enters the fast charge mode and increases the charging current up to the Ipasr value which can be programmed up to 1 25A e Constant voltage phase when the battery voltage reaches the programmable floating voltage threshold Vetoat 3 60V to 4 70V in 20mV steps the battery voltage is kept constant and as a consequence the charging current starts to decrease e End of charge during the CV phase when the charging current reaches the termination current threshold I7pgpm programmable from 50mA to 300mA in 25mA steps the charging process is stopped All the transitions between each phase are managed in a smooth way Figure 2 STBCFGO1 charging cycle CC Constant Current CV Constant Voltage TRK Trickle charge linear PRE Pre charge FAST Fast charge TAPER Taper charge Linear charging va ea Plot NOT to scale Other charger functions performed by the device are battery detection and automatic recharge when batter
14. y voltage falls below a threshold after the end of charge Battery charger temperature and charging state are fully monitored for fault conditions In the event of battery over voltage charger timers expiration battery failure and the condition of battery voltage higher than input voltage the charging process is stopped and an interrupt signal can be generated The charger is stopped also in case of input under voltage input over voltage and silicon over temperature OTG Architecture The STBCFGO1 features a bidirectional switching power manager that can power an application and charge the battery using the USB connector s voltage as input source Operating in reverse mode the same switching regulator can take power from the battery to generate 5V on the USB connector and deliver up to 500mA for USB OTG applications without any additional components The boost converter that implements the OTG function is a peak current mode control with slope compensation The compensation network works at 2MHz or 3MHz A controlling technique of pulse width modulation PWM mode and pulse frequency modulation PFM mode keeps the high efficiency within width range of loading The Over Voltage Protection OVP circuit protects the USB port when the IC is providing power in boost mode Output short circuit protection and coil s peak current protections are also implemented In order to avoid excessive battery voltage drop in boost operation t
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