BQ24751RHDRG4 Datasheet, PDF (20/38 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger

bq24751
SLUS734D – DECEMBER 2006 – REVISED MARCH 2009............................................................................................................................................... www.ti.com
I _ADAPTER
+
V_ACSET
V_VDAC
0.10
R_AC
(3)
The input voltage range of ACSET is between 0 and V_DAC, up to 3.6 V.
The ACP and ACN pins are used to sense R_AC with a default value of 10 mΩ. However, resistors of other values
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.
Adapter Detect and Power Up
An external resistor voltage divider attenuates the adapter voltage to the ACDET pin. 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 4 V, the device is disabled.
If ACDET is below 0.6 V but PVCC is above 4 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 4 V, all the bias circuits are enabled and the REGN output
rises to 6 V and VREF rises 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 is pulled high through the external pull-up resistor to the host digital voltage rail;
• ACFET is allowed to 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)
Enable and Disable Charging
The following conditions must be valid before the charge function is enabled:
• CHGEN is LOW
• PVCC > UVLO
• Adapter is detected
• Adapter voltage is higher than BAT + 185 mV
• Adapter is not over voltage (ACOV)
• 700 ms delay is complete after the adapter is detected plus 10 ms ACOC time
• Thermal Shut (TSHUT) is not valid
• TS is within the temperature qualification window
• VDAC > 2.4 V
• LEARN is low
System Power Selector
The bq24751 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
dissipation, with its low R_dson, compared to a Schottky diode. The other p-channel FET connected to ACP
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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