BQ25708 datasheet(21/77 Pages) TI1

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bq25708

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SLUSCU2 – NOVEMBER 2017

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8.3 Feature Description

8.3.1

Power-Up from Battery Without DC Source

If only battery is present and the voltage is above VVBAT_UVLOZ, the BATFET turns on and connects battery to

system. By default, the charger is in low power mode (REG0x12[15] = 1) with lowest quiescent current. The LDO

stays off. When device moves to performance mode (REG0x12[15] = 0), The host enables IBAT buffer through

SMBus to monitor discharge current. For PSYS, PROCHOT or independent comparator, REGN LDO is enabled

for an accurate reference.

8.3.2 Power-Up From DC Source

When an input source plugs in, the charger checks the input source voltage to turn on LDO and all the bias

circuits. It sets the input current limit before the converter starts.

The power-up sequence from DC source is as follows:

1. 50 ms after VBUS above VVBUS_CONVEN, enable 6V LDO and CHRG_OK goes HIGH

2. Input voltage and current limit setup

3. Battery CELL configuration

4. 150 ms after VBUS above VVBUS_CONVEN, converter powers up.

8.3.2.1 CHRG_OK Indicator

CHRG_OK is an active HIGH open drain indicator. It indicates the charger is in normal operation when the

following conditions are valid:

•

VBUS is above VVBUS_CONVEN

•

VBUS is below VACOV

•

No MOSFET/inductor fault

8.3.2.2

Input Voltage and Current Limit Setup

When CHRG_OK goes LOW, the charger sets default input current limit in REG0x3F() to 3.30A. The actual input

current limit is the lower setting of REG0x3F() and ILIM_HIZ pin.

Charger initiates a VBUS voltage measurement without any load (VBUS at no load condition). The default

VINDPM threshold is VBUS@noLoad-1.28 V.

After input current and voltage limits are set, the charger device is ready to power up. The host can always

update input current and voltage limit based on input source type.

8.3.2.3

Battery Cell Configuration

CELL_BATPRESZ pin is biased with resistors from REGN to CELL_BATPRESZ to GND. After VDDA LDO is

activated, the device detects the battery configuration through CELL_BATPRESZ pin bias voltage. Refer to

Electrical Characteristics for CELL setting thresholds.

Table 1. Battery Cell Configuration

CELL COUNT

PIN VOLTAGE w.r.t. VDDA

BATTERY VOLTAGE (REG0x15)

SYSOVP

75%

16.800V

19.5V

55%

12.592V

19.5V

40%

8.400V

12V

25%

4.192V

8.3.2.4 Device Hi-Z State

The charger enters Hi-Z mode when ILIM_HIZ pin voltage is below 0.4 V or REG0x32[15] is set to 1. During Hi-Z

mode, the input source is present, and the charger is in the low quiescent current mode with REGN LDO

enabled.

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