Battery Circuit Architecture - TI.com

1 Workbook 2-1 WorkbookPresentationApplication ReportsBattery Circuit ArchitectureBill JacksonABSTRACTB attery-pack requirements have gone through a major evolution in the past several years, and today sdesigns have considerable electronic content. The requirements for these batteries include high dischargerates, low insertion loss from components in series with the cells, high-precision measurements,redundant safety protection, and no upset with very high electrostatic discharge (ESD) all Li-ion protector circuits for one- and two-cell applications have protector FETs in the low(negative) side of the Battery . Key issues particular to a low-side Li-ion protector Circuit are transients produced when the Li-ion protector opens during a momentary short or when the Battery isunplugged while under load may exceed the voltage rating of semiconductors in the Battery pack. Thistopic describes a number of design issues and proposes solutions to resolve or improve them.

2 Resolutionof these issues requires attention to both the Circuit design and the printed Circuit board (PCB) TYPICALBATTERYCIRCUITRYFOR ALI-IONBATTERYPACKFig. 1 is a block diagram of circuitry in atypical Li-ion Battery pack. It shows an example ofa safety protection Circuit for the Li-ion cells and agas gauge (capacity measuring device). The safetycircuitry includes a Li-ion protector that controlsback-to-back FET switches. These switches can beopened to protect the pack against fault conditionssuch as overvoltage, undervoltage, and over-current. The diagram also includes a temperature-sensitive three-terminal fuse that will open due toprolonged overcurrent or overtemperature, or itcan be forced to open by redundant protectioncircuitry in case there is a fault where the primaryprotection circuitry fails to respond. Opening thisProtector FETsRegulatorGas GaugeADCC oulomb CounterNon-volatile MemTime BaseCPURAMP rogram MemDataClockPack+Pack Three-Terminal FuseSecondaryProtectorLEDS witch andDisplayLevelTranslationSenseResistorP ackThermistorPrimary Li-IonProtectorFig.

3 1. Block diagram of circuitry in a typical Li-ion Battery 2-2 WorkbookPresentationApplication Reportsfuse is a last resort, as it will render the packpermanently disabled. The gas-gauge circuitrymeasures the charge and discharge current bymeasuring the voltage across a low-value senseresistor with low-offset measurement current measurement is integrated to deter-mine the change in coulometric capacity. Inaddition, the gauge measures temperature andvoltage, evaluates gas-gauging algorithms to deter-mine the available capacity in the Battery , andcomputes time-to-empty and other values requiredby the host. The available capacity as well as othermeasurements and computational results are com-municated to the host over a serial communicationline. A visual indication of available capacity canbe displayed by the LEDs when activated by apush-button OPTIMIZING THEDESIGN FORMEASUREMENTACCURACYM easurement accuracy requires an accuratemeasurement data acquisition system and measure-mentsensor and also requires a careful boardlayout.

4 If the designer does not pay attention to allthese items, the resulting system performancemay not deliver the expected sense resistor and how it is connected tothe data acquisition system are critical designdecisions. The sense resistor may see changes intemperature that are much larger than the ambientvariations of the Battery pack due to power dissi-pation in the resistor. Use of a low-temperaturecoefficient resistor will improve available capacityand current-measurement accuracy. The effectivesense resistance seen by the measurement circuitrymay depend on how the printed Circuit board(PCB) etch is connected to the sense resistor. Ifthe sense resistor is connected in such a manner asto include some of the PCB etch resistance in theactual sense resistance seen by the measurementcircuitry, the effective resistance is increased. Inaddition to having a larger resistance thanintended, the portion of the effective sense resist-ance that is due to copper etch will have a veryhigh temperature coefficient ( C).

5 The bestpractice is to connect the sense resistor into thecircuitry at the location that includes the leastamount of copper trace in the current path. If thismethod is followed, there will be very little errordue to the voltage drop across the connection, andthe connection resistance will not add to theeffective sense resistor single-ended measurements are made on thevoltage across the sense resistor, it is critical thatthe VSSof the measuring device be connected tothe sense resistor with much care. In the single-ended system, the measurement system groundprovides one of the inputs for the value that is measured is the differencebetween the single-ended input and the measuringsystem ground. If the measuring system groundpath has a voltage difference between the on-chipground and the ground end of the voltagemeasurement desired, this difference will createan error.

6 The measurement system ground shouldtie to a low-current ground etch. The low-currentground should be separated from the high-currentground, and the ground end of the sense resistorshould be the tie point where the low-currentground is tied to the high-current ground. Thiswill also provide a more robust design for electro-static discharge (ESD), as discussed measurements of the Battery stack arealso affected by PCB layout and connection Battery -pack designs may use nickel strapsfrom the PCB connection to the Battery is used because it is easy to weld to thebattery cells, but its resistance is five times asmuch as that of copper. When current flowsthrough these straps, the voltage measured by thecircuitry on the PCB connected to these straps willnot measure the true cell voltage. To measure thetrue cell voltage, separate voltage measurementconnections should be made with wiring that doesnot carry the load current.

7 This drop may not besignificant, but if the circuitry measures each cellvoltage, such drops will cause the top and/orbottom cells to measure lower or higher (withdischarge or charge currents) than the other same issue exists for copper etch routing onthe PCB. A high-current-carrying conductor onthe PCB will have a voltage drop across it. Ingeneral, the voltage measurement connectionsneed to be made so that these connections havevery little current flowing through them betweenthe desired measurement point and the input to theWorkbook 2-3 WorkbookPresentationApplication Reportsmeasurement IC. If the gas-gauge algorithm usesthe lowest cell voltage for determining when thebattery is empty, a small voltage drop may repre-sent a significant capacity error and cause the gaugeto report a smaller available capacity from thebattery. If single-ended measurements are made ofthe voltage across the sense resistor and also ofthe voltage, accuracy considerations dictate thatthe sense resistor ground connection should bevery close to the cell stack ground THERMALISSUEST here can be a lot of power dissipation in thebattery pack.

8 There will be some temperature risedue to power dissipation in the cells. Highcurrents can also produce appreciable heat fromthe protector FETs, sense resistor, and even etchand wiring resistance. Make sure that intendedtemperature measurements are not elevated due toproximity of the thermal sensor to various heatsources. For example, if the substrate temperatureof an integrated Circuit (IC) mounted to the PCB isbeing measured, a narrow etch that runs under theIC may raise the reported temperature by at least6 with as little as 2 A flowing through the is disastrous if the design is a NiMH or NiCdpack and the temperature measurement is used todetermine full charge by a dT/dt charge termi-nation. An increase of charge current can cause afalse detection of a fully charged condition due toan increased dT/dt value. Excessive temperaturesmay also degrade the measurement accuracy ofthe gas gauge by causing drift in the on-chipreference.

9 A good design practice is to avoidplacing any measurement component close to theheat sources for example, the protector FETsand sense TRANSIENTPROTECTIONMost designers will recognize the need to addtransient protection across the pack outputterminals if the Battery is used to drive a motor orhighly inductive load. However, many designswith non-inductive loads do not have anytransient-limiting devices and depend only onsome small capacitors to attenuate the cells have a relatively high inductance fortheir size due to construction techniques. If abattery pack is removed from the system whileunder load, there is an opportunity for a damagingtransient to occur. The Battery pack should havesufficient capacitance to reduce transients or havesomething to clamp them. An even greater dangerexists if there is a momentary short across thebattery pack. The Li-ion safety protector mayopen to protect the cells from this short.

10 If the FETswitch in the protector opens quickly, the L dI/dttransient may be very large. Capacitance ortransient-limiting devices on the output of thepack will not see this transient. The transient willappear on the cell side of the protector. There is apotential to damage any components tied to thecell side of the protector if there is not sufficientcapacitance or other means to attenuate thetransient across the cell LOW-SIDEPROTECTORISSUESV irtually all one- and two-cell Li-ion pro-tectors are low-side protectors, where theprotector FETs are located between the negativelead of the Battery cell stack and the batterynegative terminal. There are several issues thatcan result depending on where the designerchooses to insert the protector FETs with respectto the Battery -pack Li-ion protectors typically use p-channelFETs for high-side protectors and n-channel FETsfor low-side protectors.