BQ25708 datasheet(51/77 Pages) TI2

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ILIM _ HIZ

ACP

ACN

DPM

1 40 I

= +

bq25708

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SLUSCU2 – NOVEMBER 2017

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8.6.5.1 System Voltage Regulation

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

minimum system voltage is set by REG0x3E(). Even with a deeply depleted battery, the system is regulated

above the minimum system voltage with BATFET.

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

and the system is regulated above the minimum system voltage setting. 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 160mV above battery

voltage when BATFET is off (no charging or no supplement current).

When BATFET is removed, the system node VSYS is shorted to SRP. Before the converter starts operation,

LDO mode needs to be disabled. The following sequence is required to configure charger without BATFET.

1. Before adapter plugs in, put the charger into HIZ mode. (either pull pin 6 ILIM_HIZ to ground, or set

REG0x32[15] to 1)

2. Set 0x12[2] to 0 to disable LDO mode.

3. Set 0x30[0] to 0 to disable auto-wakeup mode.

4. Check if battery voltage is properly programmed (REG0x15)

5. Set pre-charge/charge current (REG0x14)

6. Put the device out of HIZ mode. (Release ILIM_HIZ from ground and set REG0x32[15]=0).

In order to prevent any accidental SW mistakes, the host sets low input current limit (a few hundred milliamps)

when device is out of HIZ.

8.6.6 Input Current and Input Voltage Registers for Dynamic Power Management

The charger supports Dynamic Power Management (DPM). Normally, the input power source provides power for

the system load or to charge the battery. When the input current exceeds the input current setting, or the input

voltage falls below the input voltage setting, the charger decreases the charge current to provide priority to the

system load. As the system current rises, the available charge current drops accordingly towards zero. If the

system load keeps increasing after the charge current drops down to zero, the system voltage starts to drop. As

the system voltage drops below the battery voltage, the battery will discharge to supply the heavy system load.

8.6.6.1 Input Current Registers

To set the maximum input current limit, write a 16-bit IIN_HOST register command (REG0x3F()) using the data

format listed in Figure 25. When using a 10-mΩ sense resistor, the charger provides an input-current limit range

of 50 mA to 6400 mA, with 50-mA resolution. The default current limit is 3.3 A. Due to the USB current setting

requirement, the register setting specifies the maximum current instead of the typical current. Upon adapter

removal, the input current limit is reset to the default value of 3.3 A. The register setting is clamped at 50mA.

With code 00h and 01h, the input current limit settings are both 50mA.

The ACP and ACN pins are used to sense RAC with the default value of 10 mΩ. For a 20 mΩ sense resistor, a

larger sense voltage is given and a higher regulation accuracy, but at the expense of higher conduction loss.

Instead of using the internal DPM loop, the user can build up an external input current regulation loop and have

the feedback signal on the ILIM_HIZ pin.

(2)

In order to disable ILIM_HIZ pin, the host can write to 0x31[7] to disable ILIM_HIZ pin, or pull ILIM_HIZ pin above

4 V.

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