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I

V

ACSET

V

VDAC

0.10

R

AC

(3)

Adapter Detect and Power Up

Enable and Disable Charging

System Power Selector

bq24750

SLUS735 – DECEMBER 2006

can also be used. A larger sense-resistor value yields a larger sense voltage, and a higher regulation accuracy.

However, this is at the expense of a higher conduction loss.

An external resistor voltage divider attenuates the adapter voltage before it goes to ACDET. The adapter-detect

threshold should typically be programmed to a value greater than the maximum battery voltage, and lower than

the minimum allowed adapter voltage. The ACDET divider should be placed before the ACFET in order to sense

the true adapter input voltage whether the ACFET is on or off. Before the adapter is detected, BATFET stays on

and ACFET turns off.

If PVCC is below 5 V, the device is disabled, and both ACFET and BATFET turn off.

If ACDET is below 0.6 V but PVCC is above 5 V, part of the bias is enabled, including a crude bandgap

reference, ACFET drive and BATFET drive. IADAPT is disabled and pulled down to GND. The total quiescent

current is less than 10

µA.

When ACDET rises above 0.6 V and PVCC is above 5 V, all the bias circuits are enabled and the REGN output

goes to 6 V and VREF goes to 3.3 V. IADAPT becomes valid to proportionally reflect the adapter current.

When ACDET keeps rising and passes 2.4 V, a valid AC adapter is present. 700 ms later, the following occurs:

• ACGOOD goes low through external pull-up resistor to the host digital voltage rail;

• ACFET can turn on and BATFET turns off consequently; (refer to System Power Selector)

• Charging begins if all the conditions are satisfied. (refer to Enable and Disable Charging)

The following conditions must be valid before charge is enabled:

• CHGEN is LOW

• PVCC > UVLO, UVLO = 4 V

• Adapter is detected

• Adapter voltage is higher than BAT + 250 mV

• Adapter is not over voltage (ACOV)

• 700 ms delay is complete after the adapter is detected plus 10 ms ACOC time

• Regulators are at 80% of final voltage

• Thermal Shut (TSHUT) is not valid

• TS is within the temperature qualification window

• VDAC > 2.4 V

The bq24750 automatically switches between connecting the adapter or battery power to the system load. By

default, the battery is connected to the system during power up or when a valid adapter is not present. When the

adapter is detected, the battery is first disconnected from the system, then the adapter is connected. An

automatic break-before-make algorithm prevents shoot-through currents when the selector transistors switch.

The ACDRV signal drives a pair of back-to-back p-channel power MOSFETs (with sources connected together

and to PVCC) connected between the adapter and ACP. The FET connected to the adapter prevents reverse

discharge from the battery to the adapter when it is turned off. The p-channel FET with the drain connected to

the adapter input provides reverse battery discharge protection when off; and also minimizes system power

separates the battery from the adapter, and provides both ACOC current limit and ACOP power limit to the

system. The BATDRV signal controls a p-channel power MOSFET placed between BAT and the system.

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