BQ40Z50-R2 Datasheet, PDF (31/51 Pages) Texas Instruments – 1-Series, 2-Series, 3-Series, and 4-Series Li-Ion Battery Pack Manager

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bq40z50-R2
SLUSCS4 – JUNE 2017
Figure 24. Li-Ion Cell Connections
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-R2 device. Select the smallest value possible to minimize the negative
voltage generated on the bq40z50-R2 V_SS 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-mΩ to 3-mΩ sense resistor.
The bq40z50-R2 ground scheme is different from that of the older generation devices. In previous devices, the
device ground (or low current ground) is connected to the SRN side of the R_SENSE resistor pad. In the bq40z50-
R2 device, however, connects the low-current ground on the SRP side of the R_SENSE resistor pad, close to the
battery 1N terminal (see Li-Ion Cell Connections). This is because the bq40z50-R2 device 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-R2 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-R2 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.
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