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FUNCTIONAL DESCRIPTION

Low-Dropout Regulator (REG)

Initialization

Overload Detection

Short-Circuit Detection

Overload and Short-Circuit Delay

Overload and Short-Circuit Response

bq29312A

SLUS629A – JANUARY 2005 – REVISED AUGUST 2005

The inputs for this regulator can be derived from the battery cell stack (BAT) or the pack positive terminal

(PACK). The output is typically 3.3 V with the minimum output capacitance for stable operation is 4.7 µF and is

also internally current limited. During normal operation, the regulator limits output current to typically 50 mA.

The bq29312A internal control circuit is powered by the REG voltage, which it also monitors. When the voltage at

REG falls below 2.3 V, the internal circuit turns off the FETs and disables all controllable functions, including the

terminal. After the regulator has started, based on PACK voltage, it keeps operating through the BAT input, even

if the PACK voltage is removed. If the BAT input is below the minimum operating range, then the bq29312A does

not operate if the supply to the PACK input is removed. After start up, when the REG voltage is above 2.4 V, the

bq29312A is in Normal mode.

The initial state of the CHG output depends on the PMS input. If PMS = PACK, then CHG = ON, however, if

PMS = GND, then CHG = OFF.

The overload detection is used to detect abnormal currents in the discharge direction. This feature is used to

protect the pass FETs, cells and any other inline components from excessive current conditions. The detection

circuit also incorporates a blanking delay before driving the control for the pass FETs to the OFF state. The

overload sense voltage is set in the OLV register, and delay time is set in the OLT register. The overload

threshold can be programmed from 50 mV to 205 mV in 5-mV steps with the default being 50 mV and hysteresis

of 10 mV.

The short current circuit detection is used to detect abnormal current in either the charge or discharge direction.

This safety feature is used to protect the pass FETs, cells, and any other inline components from excessive

current conditions. The detection circuit also incorporates a blanking delay before driving the control for the pass

FETs to the OFF state. The short-circuit thresholds and delay time are set in the SCC and SCD registers,

respectively, where SCC is for charging and SCD is for discharge. The short-circuit threshold can be

programmed from 100 mV to 475 mV in 25-mV steps with the default being 100 mV and hysteresis of 50 mV.

The overload delay (default =1 ms) allows the system to momentarily accept a high current condition without

disconnecting the supply to the load. The delay time can be increased via the OLT register, which can be

programmed for a range of 1 ms to 31 ms with steps of 2 ms.

The short-circuit delay (default = 0 µs) is programmable in the SCC and SCD registers. This register can be

programmed from 0 µs to 915 µs with steps of 61 µs.

When an overload or short-circuit fault is detected, the FETs are turned off. The STATUS (b0

…b2) register

reports the details of short-circuit (charge), short-circuit (discharge), and overload. The respective STATUS

(b0

…b2) bits are set to 1 and the XALERT output is triggered. This condition is latched until the CONTROL (b0)

is set and then reset. If a FET is turned on via resetting CONTROL (b0) and the error condition is still present on

the system, then the device reenters the protection response state.

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