BQ24314EVM Datasheet, PDF (13/26 Pages) Texas Instruments – OVERVOLTAGE AND OVERCURRENT PROTECTION IC

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bq24314
bq24316
SLUS763C – JULY 2007 – REVISED OCTOBER 2007
APPLICATION INFORMATION (WITH REFERENCE TO FIGURE 23)
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 in the range 100kΩ to 470kΩ is a good compromise. In the case of an IC failure, with R_BAT equal
to 100kΩ, the maximum current flowing into the battery would be (30V – 3V) ÷ 100kΩ = 246μA, which is low
enough to be absorbed by the bias currents of the system components. R_BAT equal to 100kΩ would result in a
worst-case voltage drop of R_BAT × I_VBAT = 1mV. This is negligible to compared to the internal tolerance of 50mV
on BV_OVP threshold.
If the Bat-OVP function is not required, the VBAT pin should be connected to VSS.
Selection of R_CE, R_FAULT, and R_PU
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.
The FAULT pin is an open-drain output that goes low during OV, OC, battery-OV, and OT 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.
Selection of Input and Output Bypass Capacitors
The input capacitor C_IN in Figure 23 is for decoupling, and serves an important purpose. Whenever there is a
step change downwards in the system load current, the inductance of the input cable causes the input voltage to
spike up. C_IN prevents the input voltage from overshooting to dangerous levels. It is strongly recommended that a
ceramic capacitor of at least 1μF be used at the input of the device. It should be located in close proximity to the
IN pin.
C_OUT in Figure 23 is also important: If a very fast (< 1μs rise time) overvoltage transient occurs at the input, the
current that charges C_OUT causes the device’s current-limiting loop to kick in, reducing the gate-drive to FET Q1.
This results in improved performance for input overvoltage protection. C_OUT should also be a ceramic capacitor of
at least 1μF, located close to the OUT pin. C_OUT also serves as the input decoupling capacitor for the charging
circuit downstream of the protection IC.
Powering Accessories
In some applications, the equipment that the protection IC resides in may be required to provide power to an
accessory (e.g. a cellphone may power a headset or an external memory card) through the same connector pins
that are used by the adapter for charging. Figure 25 and Figure 26 illustrate typical charging and
accessory-powering scenarios:
Copyright © 2007, Texas Instruments Incorporated
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