10 Layout

10.1 Layout Guidelines

10.1.1 Introduction

Attention to layout is critical to the success of any battery management circuit board. The mixture of high-current

paths with an ultralow-current microcontroller creates the potential for design issues that are not always trivial to

solve. Some of the key areas of concern are described in the following sections, and can help to enable success.

10.1.2 Power Supply Decoupling Capacitor

Power supply decoupling from VCC 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 VSS pin to a ground plane layer.

10.1.3 Capacitors

Power supply decoupling for the gas gauges requires a pair of 0.1-µF ceramic capacitors for (BAT) and (VCC)

pins. These should be placed reasonably close to the IC without using long traces back to VSS. 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.

In some applications, it is sometimes necessary to cause transitions on the communication lines to trigger events

that manage the gas gauge power modes. An example of one of these transitions is detecting a sustained low

logic level on the communication lines to detect that a pack has been removed. Given that most of the gas

gauges do not have internal pulldown networks, it is necessary to add a weak pulldown resistor to accomplish

this when there's an absence of a strong pullup resistor on the system side. If the weak pulldown resistor is

used, it may take less board space to use a small capacitor in parallel instead of the Zener diode to absorb any

ESD transients that are received through communication lines.

10.2 Layout Example

10.2.1 Ground System

The gas 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 low-side protector FETs (if present) or the

sense resistor. It is important that the low-current ground systems only connect to the BAT(–) 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 10-1, 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.

BQ34Z100-R2

SLUSF37 – DECEMBER 2022

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