BQ4050 Datasheet, PDF (33/53 Pages) Texas Instruments – CEDV Gas Gauge and Protection Solution

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bq4050
SLUSC67A – MARCH 2016 – REVISED MARCH 2016
8.2.2.1.4 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 bq4050. Select the smallest value possible to minimize the negative voltage generated
on the bq4050 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 bq4050 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 bq4050, 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 bq4050 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
8.2.2.1.5 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.
8.2.2.2 Gas Gauge Circuit
The gas gauge circuit includes the bq4050 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.
8.2.2.2.1 Coulomb-Counting Interface
The bq4050 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 a circuit.
Copyright © 2016, Texas Instruments Incorporated
Figure 26. Differential Filter
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