BQ25708 datasheet(26/77 Pages) TI2

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bq25708

SLUSCU2 – NOVEMBER 2017

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8.3.8.4 System Overvoltage Protection (SYSOVP)

When the converter starts up, the bq25708 reads CELL pin configuration and sets MaxChargeVoltage() and

SYSOVP threshold (1s – 5 V, 2s – 12 V, 3s/4s – 19.5 V). Before REGx15() is written by host, the battery

configuration will change with CELL pin voltage. When SYSOVP happens, the device latches off the converter.

REG20[4] is set as 1. The user can clear the latch-off by either writing 0 to SYSOVP bit or removing and

plugging in adapter again. After the latch-off is cleared, converter starts again.

8.3.8.5

Battery Overvoltage Protection (BATOVP)

Battery over-voltage may happen when battery is removed during charging or the user plugs in a wrong battery.

The BATOVP threshold is 104% (1 s) or 102% (2 s to 4 s) of regulation voltage set in REG0x15().

8.3.8.6

Battery Short

If BAT voltage falls below SYSMIN during charging, the maximum current is limited to 384 mA.

8.3.8.7 Thermal Shutdown (TSHUT)

The WQFN package has low thermal impedance, which provides good thermal conduction from the silicon to the

ambient, to keep junction temperatures low. As added level of protection, the charger converter turns off for self-

protection whenever the junction temperature exceeds the 155°C. The charger stays off until the junction

temperature falls below 135°C. During thermal shut down, the LDO current limit is reduced to 16 mA and REGN

LDO stays off. When the temperature falls below 135°C, charge can be resumed with soft start.

8.4 Device Functional Modes

8.4.1 Forward Mode

When input source is connected to VBUS, bq25708 is in forward mode to regulate system and charge battery.

8.4.1.1 System Voltage Regulation with Narrow VDC Architecture

The bq25708 employs Narrow VDC architecture (NVDC) with BATFET separating system from battery. The

minimum system voltage is set by MinSystemVoltage(). Even with a deeply depleted battery, the system is

regulated above the minimum system voltage.

When the battery is below minimum system voltage setting, the BATFET operates in linear mode (LDO mode).

As the battery voltage rises above the minimum system voltage, BATFET is fully on when charging or in

supplement mode and the voltage difference between the system and battery is the VDS of BATFET. System

voltage is regulated 160 mV above battery voltage when BATFET is off (no charging or no supplement current).

See System Voltage Regulation for details on system voltage regulation and register programming.

8.4.1.2 Battery Charging

The bq25708 charges 1-4 cell battery in constant current (CC), and constant voltage (CV) mode. Based on

CELL_BATPREZ pin setting, the charger sets default battery voltage 4.2V/cell to ChargeVoltage(), or

REG0x15(). According to battery capacity, the host programs appropriate charge current to ChargeCurrent(), or

REG0x14(). When battery is full or battery is not in good condition to charge, host terminates charge by setting

REG0x12[0] to 1, or setting ChargeCurrent() to zero.

See Feature Description for details on register programming.

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