12.1 Layout Guidelines

• A capacitor, of value at least 0.47 µF, is connected between the V_DD pin and V_SS. The capacitor should be placed close to the gauge IC and have short traces to both the V_DD pin and V_SS.
• It is recommend to have a capacitor, at least 1.0 µF, connect between the BAT pin and V_SS if the connection between the battery pack and the gauge BAT pin has the potential to pick up noise. The capacitor should be placed close to the gauge IC and have short traces to both the V_DD pin and V_SS.
• If the external pullup resistors on the SCL and SDA lines will be disconnected from the host during low-power operation, it is recommend to use external 1-MΩ pulldown resistors to V_SS to avoid floating inputs to the I²C engine.
• The value of the SCL and SDA pullup resistors should take into consideration the pullup voltage and the bus capacitance. Some recommended values, assuming a bus capacitance of 10 pF, can be seen in Table 4.

Table 4. Recommended Values for SCL and SDA Pullup Resistors

VPU	1.8 V		3.3 V
RPU	Range	Typical	Range	Typical
	400 Ω ≤ RPU ≤ 37.6 kΩ	10 kΩ	900 Ω ≤ RPU ≤ 29.2 kΩ	5.1 kΩ

• If the GPOUT pin is not used by the host, the pin should still be pulled up to V_DD with a 4.7-kΩ or 10-kΩ resistor.
• If the battery pack thermistor is not connected to the BIN pin, the BIN pin should be pulled down to V_SS with a 10-kΩ resistor.
• The BIN pin should not be shorted directly to V_DD or V_SS.
• The actual device ground is the center pin (B2). The C1 pin is floating internally and can be used as a bridge to connect the board ground plane to the device ground (B2).

12.2 Layout Example

Figure 2. bq27421-G1 Board Layout