BQ40Z50-R1 Datasheet, PDF (32/55 Pages) Texas Instruments – Li-Ion Battery Pack Manager

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 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 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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