BQ24751B Datasheet, PDF(24/38 Page) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger with Low Iq and System Power Selector

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Not Recommended For New Designs

bq24751B

SLUS835A â JULY 2008 â REVISED MARCH 2009 .......................................................................................................................................................... www.ti.com

is detected (ACDET pin > 2.4V), there is a 700-ms delay before ACGOOD is asserted low, and Q3 (BATFET) is

turned-off. Then Q1/Q2 (ACFET) are turned on by the ACDRV pin. When Q1/Q2 (ACFET) are turned on, the

ACFET allows operation in linear-regulation mode to limit the maximum input current, ACOC, to a safe level. The

ACOC current limit is 1.5 times the programmed DPM input current limit set by the ratio of SRSET/VDAC. The

maximum allowable current limit is 100 mV across ACP â ACN (10 A for a 10-mâ¦ sense resistor).

The first 2 ms after the ACDRV signal begins to turn on, ACOC may limit the current; but the controller is not

allowed to latch off in order to allow a reasonable time for the system voltage to rise.

After 2 ms, ACOP is enabled. ACOP allows the ACFET to latch off before the ACFET can be damaged by

excessive thermal dissipation. The controller only latches if the ACOP pin voltage exceeds 2 V with respect to

AGND. In ACOP, a current source begins to charge the ACOP capacitor when the input current is being limited

by ACOC. This current source is proportional to the voltage across the source-drain of the ACFET (VPVCC-ACP) by

an 18-ÂµA/V ratio. This dependency allows faster capacitor charging if the voltage is larger (more power

dissipation). It allows the time to be programmed by the ACOP capacitor selected. If the controller is not limiting

current, a fixed 5-ÂµA sink current into the ACOP pin to discharge the ACOP capacitor. This charge and discharge

effect depends on whether there is a current-limit condition, and has a memory effect that averages the power

over time, protecting the system from potentially hazardous repetitive faults. Whenever the ACOP threshold is

exceeded, the charge is disabled and the adapter is disconnected from the system to protect the ACFET and the

whole system. If the ACFET is latched off, the BATFET is turned on to connect the battery to the system.

The capacitor provides a predictable time to limit the power dissipation of the ACFET. Since the input current is

constant at the ACOC current limit, the designer can calculate the power dissipation on the ACFET.

The ACOC current Limit threshold is equal to Power = Id Ã Vsd = IACOC _ LIM Ã V(PVCC - ACP) .

The time it takes to charge to 2V can be calculated from

Dt = CACOP Ã DVACOP =

CACOP Ã 2V

iACOP

18mA/V Ã V(PVCC - ACP)

(5)

An ACOP fault latch off can only be cleared by bringing the ACDET pin voltage below 2.4 V, then above 2.4 V

(i.e. remove adapter and reinsert), or by reducing the PVCC voltage below the UVLO threshold and raising it.

Conditions for ACOP Latch Off:

702ms after ACDET (adapter detected), and

a. ACOP voltage > 2V. The ACOP pin charges the ceramic capacitor when in an ACOC current-limit condition.

The ACOP pin discharges the capacitor when not in ACOC current-limit.

b. ACOP protects from a single-pulse ACOC condition depending on duration and source-drain voltage of

ACFET. Larger voltage across ACFET creates more power dissipation so latch-off protection occurs faster,

by increasing the current source out of ACOP pin.

c. Memory effect (capacitor charging and discharging) allows protection from repetitive ACOC conditions,

depending on duration and frequency. (Figure 35)

d. In short conditions when the system is shorted to ground (ACN < 2.4 V) after the initial 2-ms ACDET.

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