A simplified application schematic for a 5-series battery pack is shown in BQ76905 5-Series Cell Typical Implementation (Simplified Schematic), using the BQ76905 together with an external secondary protector, a host microcontroller, and a communications transceiver. This configuration uses low-side CHG and DSG FETs in series. Several points to consider in an implementation are included below:

• Use a series diode 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 top-of-stack voltage 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 continues 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 FET CHG and DSG drivers use the REGSRC pin for their supply, so the user may also prefer to include a diode between the top of stack and the REGSRC pin, similar to that used for the BAT pin. If any resistance (> 1 Ω) is included in series between the top of stack and the REGSRC pin, it is recommended to include a 1-μF capacitor at the REGSRC pin to VSS. The REGSRC pin can be shorted to the BAT pin and a single diode used, but this may result in the BAT pin voltage dropping more rapidly during a short circuit event due to the increased loading of the REGOUT regulator drawing from the REGSRC pin.
• The recommended minimum voltage on the VC0 to VC3A pins extends down to –0.2 V, while the recommended minimum voltage on the VC4A, VC4B, and VC5 pins is limited to 2.0 V, relative to VSS. This restriction exists to ensure the specified cell voltage measurement accuracy.
• 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. Capacitors connected from the pins to VSS can provide filtering of common mode transients from reaching the pins, but they may also have a slight impact on current measurement performance.
• The filter network connected between the sense resistor and the SRP and SRN pins introduces an analog filter delay that can be important when fast current protections are required, such as in determining the short circuit in discharge (SCD) time until FETs are disabled. If the delay introduced by this network is too long, the resistance and capacitance values can be reduced. Thishas a tradeoff of providing less analog filtering of high frequency components.
• 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, because this affects 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:DA Config[IADCSPEED1:0] = 0x0, the measurement time is approximately 3 ms. When using this speed setting, the time constant should generally be less than (20 kΩ) × C, so a capacitor less than 7.5 nF is recommended. When using faster speed settings, the capacitor value should be reduced accordingly.

Figure 8-1 BQ76905 5-Series Cell Typical Implementation (Simplified Schematic)

A full schematic of a basic monitor circuit based on the BQ76905 for a 5-series battery pack is shown below. Figure 8-9 shows the board layout.

Figure 8-2 BQ76905 5-Series Cell Schematic Diagram—Monitor

Figure 8-3 BQ76905 5-Series Cell Schematic Diagram—Additional Circuitry