11.1 Layout Guidelines

The predominant layout concern for the bq3055 is related to the coulomb counter measurement. The external components and PCB layout surrounding the SRP and SRN pins should be carefully considered.

11.2 Layout Example

As shown in Figure 24, a differential filter must precede the current sense inputs of the gas gauge. This filter eliminates the effect of unwanted digital noise, which can cause offset in the measured current. Even the best differential amplifier has less common-mode rejection at high frequencies. Without a filter, the amplifier input stage may rectify a strong RF signal, which then may appear as a DC-offset error.

Five percent tolerance of the components is adequate, because capacitor C15 shunts C12 and C13 and reduces AC common mode arising from a component mismatch. It is important to locate C15 as close as possible to the gas gauge pins. The other components also must be relatively close to the IC. The ground connection of C12 and C13 must be close to the IC. It is also proven to reduce offset and noise error by maintaining a symmetrical placement pattern and adding ground shielding for the differential filter network.

Figure 24. PCB Layout Example