BQ40Z50-R1 datasheet(32/56 Pages) TI2

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bq40z50-R1

SLUSCB3 – JULY 2015

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9.2.2.1.4

Sense Resistor

As with the cell connections, the quality of the Kelvin connections at the sense resistor is critical. The sense

resistor must have a temperature coefficient no greater than 50 ppm in order to minimize current measurement

drift with temperature. Choose the value of the sense resistor to correspond to the available overcurrent and

short-circuit ranges of the bq40z50-R1. Select the smallest value possible to minimize the negative voltage

generated on the bq40z50-R1 VSS node(s) during a short circuit. This pin has an absolute minimum of –0.3 V.

Parallel resistors can be used as long as good Kelvin sensing is ensured. The device is designed to support a 1-

Ω to 3-mΩ sense resistor.

The ground scheme of bq40z50-R1 is different from the older generation devices. In previous devices, the device

ground (or low current ground) is connected to the SRN side of the Rsense resistor pad. The bq40z50-R1,

however, connects the low-current ground on the SRP side of the Rsense resistor pad, close to the battery 1N

terminal (see Lithium-Ion Cell Connections). This is because the bq40z50-R1 has one less VC pin (a ground

reference pin VC5) compared to the previous devices. The pin was removed and was internally combined to

SRP.

Figure 25. Sense Resistor

9.2.2.1.5

ESD Mitigation

A pair of series 0.1-

μF ceramic capacitors is placed across the PACK+ and PACK– terminals to help in the

mitigation of external electrostatic discharges. The two devices in series ensure continued operation of the pack

if one of the capacitors becomes shorted.

Optionally, a tranzorb such as the SMBJ2A can be placed across the terminals to further improve ESD immunity.

9.2.2.2 Gas Gauge Circuit

The Gas Gauge Circuit includes the bq40z50-R1 and its peripheral components. These components are divided

into the following groups: Differential Low-Pass Filter, PBI, System Present, SMBus Communication, FUSE

circuit, and LED.

9.2.2.2.1

Coulomb-Counting Interface

The bq40z50-R1 uses an integrating delta-sigma ADC for current measurements. Add a 100-

Ω resistor from the

sense resistor to the SRP and SRN inputs of the device. Place a 0.1-µF (C18) filter capacitor across the SRP

and SRN inputs. Optional 0.1-µF filter capacitors (C19 and C20) can be added for additional noise filtering, if

required for your circuit.

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