BQ24314 Datasheet, PDF (10/13 Pages) Texas Instruments – OVERVOLTAGE AND OVERCURRENT PROTECTION IC AND Li+ CHARGER FRONT-END PROTECTION IC

bq24314
bq24316
SLUS763 – JULY 2007
www.ti.com
APPLICATION INFORMATION (WITH REFERENCE TO FIGURE 8)
Selection of R_BAT
It is strongly recommended that the battery not be tied directly to the VBAT pin of the device, as under some
failure modes of the IC, the voltage at the IN pin may appear on the VBAT pin. This voltage can be as high as
30V, and applying 30V to the battery in case of the failure of the bq2431x can be hazardous. Connecting the
VBAT pin through R_BAT prevents a large current from flowing into the battery in case of a failure of the IC. In the
interests of safety, R_BAT should have a very high value. The problem with a large R_BAT is that the voltage drop
across this resistor because of the VBAT bias current I_VBAT causes an error in the BV_OVP threshold. This error is
over and above the tolerance on the nominal 4.35V BV_OVP threshold.
Choosing R_BAT equal to 220kΩ is a good compromise. In the case of an IC failure, the maximum current flowing
into the battery would be (30V – 3V) ÷ 220kΩ = 123μA, which is low enough to be absorbed by the bias currents
of the system components. R_BAT equal to 220kΩ would result in a worst-case voltage drop of R_BAT × I_VBAT =
4.4mV. This added to the internal tolerance of 50mV results in a total BV_OVP threshold error of less than 55mV,
which should be acceptable in most applications.
Selection of R_CE
The CE pin can be used to enable and disable the IC. If host control is not required, the CE pin can be tied to
ground or left un-connected, permanently enabling the device.
In applications where external control is required, the CE pin can be controlled by a host processor. As in the
case of the VBAT pin (see above), the CE pin should be connected to the host GPIO pin through as large a
resistor as possible. The limitation on the resistor value is that the minimum V_OH of the host GPIO pin less the
drop across the resistor should be greater than V_IH of the bq2431× CE pin. The drop across the resistor is given
by R_CE × I_IH.
FAULT Pin
The FAULT pin is an open-drain output that goes low during OV, OC, and battery-OV events. If the application
does not require monitoring of the FAULT pin, it can be left unconnected. But if the FAULT pin has to be
monitored, it should be pulled high externally through R_PU, and connected to the host through R_FAULT. R_FAULT
prevents damage to the host controller if the bq2431x fails (see above). The resistors should be of high value, in
practice values between 22kΩ and 100kΩ should be sufficient.
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