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 .

2 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. 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.

3 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.

4 Opening thisProtector FETsRegulatorGas GaugeADCC oulomb CounterNon-volatile MemTime BaseCPURAMP rogram MemDataClockPack+Pack Three-Terminal FuseSecondaryProtectorLEDS witch andDisplayLevelTranslationSenseResistorP ackThermistorPrimary Li-IonProtectorFig. 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.

5 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.

6 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.

7 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). The bestpractice is to connect the sense resistor into thecircuitry at the location that includes the leastamount of copper trace in the current path.

8 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.

9 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. 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.

10 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. 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.