Datasheet

bq24278

SLUSB04 –JUNE 2012

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Battery Discharge FET (BGATE)

The bq24278 contains a MOSFET driver to drive an external P-Channel MOSFET between the battery and the

system output. This external FET provides a low impedance path for supply the system from the battery. Connect

BGATE to the gate of the external discharge MOSFET. BGATE is on under the following conditions:

1. No valid input supply connected.

2. CD=high (High-Impedance Mode)

This FET is optional and runs in parallel with the internal charge FET during discharge. Note that this FET is not

protected by the short circuit current limit.

Safety Timer

At the beginning of charging process, the bq24278 starts the 6 hour safety timer. This timer is active during the

entire charging process. If charging has not terminated before the safety timer expires, the charge cycle is

terminated and the battery FET is turned off. A new charge cycle must be entered using CE or High Impedance

mode or input power must be toggled in order to clear the safety timer fault.

During the fast charge phase, several events increase the timer durations.

1. The system load current reduces the available charging current

2. The input current is reduced because the V

INDPM

loop is preventing the supply from crashing.

3. The device has entered thermal regulation because the IC junction temperature has exceed T

J(REG)

During these events, the timer is slowed by half to extend the timer and prevent any false timer faults. Starting a

new charge cycle by toggling the input, toggling the CE pin to disable/enable charge, resets the safety timer.

Additionally, thermal shutdown events cause the safety timer to reset.

LDO Output (DRV)

The bq24278 contains a linear regulator (DRV) that is used to supply the internal MOSFET drivers and other

circuitry. Additionally, DRV supplies up to 10mA external loads to power the STAT LED or other external circuitry.

The LDO is on whenever a supply is connected to the input of the. The DRV is disabled under the following

conditions:

1. V

< UVLO

2. V

< V

BAT

+ V

SLP

3. Thermal Shutdown

External NTC Monitoring (TS)

The I

C interface allows the user to easily implement the JEITA standard for systems where the battery pack

thermistor is monitored by the host. Additionally, the bq24278 provides a flexible, voltage-based TS input for

monitoring the battery pack NTC thermistor, Figure 4. The voltage at TS is monitored to determine that the

battery is at a safe temperature during charging. The bq24278 enables the user to easily implement the JEITA

standard for charging temperature. The JEITA specification is shown in Figure 5.

Product Folder Link(s) :bq24278