A simplified application schematic for a 16-series battery pack is shown in Figure 8-1, using the BQ76972 together with an external secondary protector, a host microcontroller, and a communications transceiver. This configuration uses CHG and DSG FETs in series, together with high-side PFET devices used to implement precharge and predischarge functionality. Several points to consider in an implementation are included below:

• The external NPN BJT used for the REGIN preregulator can be configured with its collector routed either to the cell battery stack or the middle of the protection FETs.
• A diode is recommended in the drain circuit of the external NPN BJT, which avoids reverse current flow from the BREG pin through the BJT base to collector in the event of a pack short circuit. This diode can be a Schottky diode if low voltage pack operation is needed, or a conventional diode can be used otherwise.
• A series diode is recommended at the BAT pin, together with a capacitor from the pin to VSS. These components allow the device to continue operating for a short time when a pack short circuit occurs, which may cause the PACK+ and top-of-stack voltages to drop to approximately 0 V. In this case, the diode prevents the BAT pin from being pulled low with the stack, and the device will continue to operate, drawing current from the capacitor. Generally operation is only required for a short time, until the device detects the short circuit event and disables the DSG FET. A Schottky diode can be used if low voltage pack operation is needed, or a conventional diode can be used otherwise.
• The diode in the BAT connection and the diode in the BJT collector should not be shared, since then the REG0 circuit might discharge the capacitor on BAT too quickly during a short circuit event.
• The recommended voltage range on the VC0 to VC4 pins extends to –0.2 V. This can be used, for example, to measure a differential voltage that extends slightly below ground, such as the voltage across a second sense resistor in parallel with that connected to the SRP and SRN pins.
• If a system does not use high-side protection FETs, then the PACK pin can be connected through a series 10-kΩ resistor to the top of stack. The LD pin can be connected to VSS. In this case, the LD pin can also be controlled separately, in order to wake the device from SHUTDOWN mode, such as through external circuitry which holds the LD pin at the voltage of VSS while the device stays in SHUTDOWN, and to be driven above a voltage of V_WAKEONLD in order to wake from SHUTDOWN.
• TI recommends using 100 Ω resistors in series with the SRP and SRN pins, and a 100 nF with optional 100 pF differential filter capacitance between the pins for filtering. The routing of these components, together with the sense resistor, to the pins should be minimized and fully symmetric, with all components recommended to stay on the same side of the PCB with the device. Optional 0.1-μF filter capacitors can be added for additional noise filtering at each sense input pin to VSS.
• Due to thermistors often being attached to cells and possibly needing long wires to connect back to the device, it may be helpful to add a capacitor from the thermistor pin to the device VSS. However, it is important to not use too large of a value of capacitor, since this will affect the settling time when the thermistor is biased and measured periodically. A rule of thumb is to keep the time constant of the circuit < 5% of the measurement time. When Settings:Configuration:Power Config[FASTADC] = 0, the measurement time is approximately 3 ms, and with [FASTADC] = 1 the measurement time is halved to approximately 1.5 ms. When using the 18 kΩ pullup resistor with the thermistor, the time constant will generally be less than (18 kΩ) × C, so a capacitor less than 4 nF is recommended. When using the 180-kΩ pullup resistor, the capacitor should be less than 400 pF.
• The integrated charge pump generates a voltage on the CP1 capacitor, requiring approximately 60 ms to charge up to approximately 11 V when first enabled, when using the recommended 470 nF capacitor value. When the CHG or DSG drivers are enabled, charge redistribution occurs from the CP1 capacitor to the CHG and DSG capacitive FET loads. This will generally result in a brief drop in the voltage on CP1, which is then replenished by the charge pump. If the FET capacitive loading is large, such that at FET turn-on the voltage on CP1 drops below an acceptable level for the application, then the value of the CP1 capacitor can be increased. This has the drawback of requiring a longer startup time for the voltage on CP1 when the charge pump is first powered on, and so should be evaluated to ensure it is acceptable in the system. For example, if the CHG and DSG FETs are enabled simultaneously and their combined gate capacitance is approximately 400 nF, then changing CP1 to a value of 2200 nF will result in the 11-V charge pump level dropping to approximately 9 V, before being restored to the 11-V level by the charge pump.

Figure 8-1 BQ76972 16-Series Cell Typical Implementation (Simplified Schematic)

A full schematic of a basic monitor circuit based on the BQ76972 for a 16-series battery pack is shown below. Section 8.8.2 shows the board layout for this design.

Figure 8-2 BQ76972 16-Series Cell Schematic Diagram—Monitor

Figure 8-3 BQ76972 16-Series Cell Schematic Diagram—Additional Circuitry