bq29312

SLUS546D − MARCH 2003 − REVISED SEPTEMBER 2004

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FUNCTIONAL DESCRIPTION

Low-Dropout Regulator (REG)

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.

Initialization

The bq29312 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 REG and TOUT

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 bq29312 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 bq29312 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.

Overload Detection

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.

Short-Circuit Detection

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.

Overload and Short-Circuit Delay

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.

Overload and Short-Circuit Response

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.

Cell Voltage

The cell voltage is translated to allow a system host to measure individual series elements of the battery. The series element

voltage is translated to a GND-based voltage equal to 0.15

±0.002 of the series element voltage. This provides a range

from 0 V to 4.5 V. The translation output is inversely proportional to the input using the following equation.

Programming CELL_SEL (b1, b0) selects the individual series element. The CELL_SEL (b3, b2) selects the voltage

monitor mode, cell monitor, offset etc.