BQ29312_15 Datasheet, PDF (10/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION

bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
FUNCTIONAL DESCRIPTION
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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 outputs. REG does not start up unless a voltage above V_(STARTUP) is supplied to the PACK
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 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.
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