BQ27742-G1 Datasheet, PDF (34/46 Pages) Texas Instruments – Single-Cell Li-Ion Battery Fuel Gauge with Programmable Hardware Protection

bq27742-G1
SLUSBV9C – MARCH 2014 – REVISED FEBRUARY 2016
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8.2.3.4 TS Temperature Sense Input
Similar to the BAT pin, a ceramic decoupling capacitor for the TS pin is used to bypass AC voltage ripple away
from the high-impedance ADC input, minimizing measurement error. Another helpful advantage is that the
capacitor provides additional ESD protection since most thermistors are handled and manually soldered to the
PCB as a separate step in the factory production flow. As before, it should be placed as close as possible to the
respective input pin for optimal filtering performance.
8.2.3.5 Thermistor Selection
The fuel gauge temperature sensing circuitry is designed to work with a negative temperature coefficient-type
(NTC) thermistor with a characteristic 10-kΩ resistance at room temperature (25°C). The default curve-fitting
coefficients configured in the fuel gauge specifically assume a 103AT-2 type thermistor profile and so that is the
default recommendation for thermistor selection purposes. Moving to a separate thermistor resistance profile (for
example, JT-2 or others) requires an update to the default thermistor coefficients in data flash to ensure highest
accuracy temperature measurement performance.
8.2.3.6 VPWR Power Supply Input Filtering
A ceramic capacitor is placed at the input to the fuel gauge's internal LDO in order to increase power supply
rejection (PSR) and improve effective line regulation. It ensures that voltage ripple is rejected to ground instead
of coupling into the device's internal supply rails.
8.2.3.7 REG25 LDO Output Filtering
A ceramic capacitor is also needed at the output of the internal LDO in order to provide a current reservoir for
fuel gauge load peaks during high peripheral utilization. It acts to stabilize the regulator output and reduce core
voltage ripple inside of the device.
8.2.3.8 Communication Interface Lines
A protection network composed of resistors and zener diodes is recommended on each of the serial
communication inputs to protect the fuel gauge from dangerous ESD transients. The Zener should be selected to
break down at a voltage larger than the typical pullup voltage for these lines but less than the internal diode
clamp breakdown voltage of the device inputs (~6 V). A zener voltage of 5.6 V is typically recommended. The
series resistors are used to limit the current into the Zener diode and prevent component destruction due to
thermal strain once it goes into breakdown. 100 Ω is typically recommended for these resistance values.
For HDQ-based designs, a pullup resistor is normally designed in on the battery pack PCB and can be
connected to the RC2 input since a 1.8-V pullup voltage is readily available and provided by the fuel gauge.
8.2.3.9 PACKP Voltage Sense Input
Inclusion of a 2-kΩ series resistor on the PACKP input allows it to tolerate a charger overvoltage event up to 28
V without device damage. The resistor also protects the device in the event of a reverse polarity charger input,
since the substrate diode will be forward biased and attempt to conduct charger current through the fuel gauge
(as well as the high FETs). An external reverse charger input FET clamp can be added to short the DSG FET
gate to its source terminal, forcing the conduction channel off when negative voltage is present at PACK+ input
to the battery pack and preventing large battery discharge currents. A ceramic capacitor connected at the
PACKP pin helps to filter voltage into the comparator sense lines used for checking charger and load presence.
In addition, in the Low Voltage Charging State, the minimal circuit elements that are operational are powered
from this input pin and require a stable supply.
8.2.3.10 CHG and DSG Charge Pump Voltage Outputs
The series resistors used at the DSG and CHG output pins serve to protect them from damaging ESD events or
breakdown conditions, allowing the resistors to be sacrificed in place of the fuel gauge itself. An added bonus is
that they also help to limit in-rush currents due to use of FETs with large gate capacitance, allowing smooth ramp
of power-path connection turn-on to the system.
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