BQ35100 Datasheet, PDF (19/26 Pages) Texas Instruments – Lithium Primary Battery Fuel Gauge

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bq35100
SLUSCM6A – JUNE 2016 – REVISED JULY 2016
Layout Guidelines (continued)
10.1.2 Power Supply Decoupling Capacitor
Power supply decoupling from V_CC to ground is important for optimal operation of the gas gauge. To keep the
loop area small, place this capacitor next to the IC and use the shortest possible traces. A large loop area
renders the capacitor useless and forms a small-loop antenna for noise pickup. Ideally, the traces on each side
of the capacitor should be the same length and run in the same direction to avoid differential noise during ESD. If
possible, place a via near the V_SS pin to a ground plane layer.
10.1.3 Capacitors
Power supply decoupling for the gas gauge requires a pair of 0.1-μF ceramic capacitors for (PBAT) and (V_CC)
pins. These should be placed reasonably close to the IC without using long traces back to V_SS. The LDO voltage
regulator, whether external or internal to the main IC, requires a 0.47-μF ceramic capacitor to be placed fairly
close to the regulation output pin. This capacitor is for amplifier loop stabilization and as an energy well for the
2.5-V supply.
10.1.4 Communication Line Protection Components
The 5.6-V Zener diodes used to protect the communication pins of the gas gauge from ESD should be located as
close as possible to the pack connector. The grounded end of these Zener diodes should be returned to the
Pack(–) node rather than to the low-current digital ground system. This way, ESD is diverted away from the
sensitive electronics as much as possible.
10.2 Layout Example
10.2.1 Ground System
The fuel gauge requires a low-current ground system separate from the high-current PACK(–) path. ESD ground
is defined along the high-current path from the Pack(–) terminal to the sense resistor. It is important that the low-
current ground systems only connect to the PACK(–) path at the sense resistor Kelvin pick-off point. It is
recommended to use an optional inner layer ground plane for the low-current ground system.
In Figure 18, the green is an example of using the low-current ground as a shield for the gas gauge circuit.
Notice how it is kept separate from the high-current ground, which is shown in red. The high-current path is
joined with the low-current path only at one point, shown with the small blue connection between the two planes.
Figure 18. Differential Filter Component with Symmetrical Layout
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