BQ24450 Datasheet, PDF (7/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
www.ti.com ..................................................................................................................................................................................................... SLUS929 – APRIL 2009
DETAILED FUNCTIONAL DESCRIPTION
The bq24450 contains all the necessary circuitry to optimally control the charging of sealed lead-acid batteries.
The IC controls the charging current as well as the charging voltage to safely and efficiently charge the battery,
maximizing battery capacity and life. Depending on the application, the IC can be configured in various ways:
examples are a constant-voltage float charger, a dual-voltage float-cum-boost charger or a dual step current
charger.
Only an external pass transistor and minimum number of external passive components are required along with
the IC to implement a charger for sealed lead-acid batteries. The IC's internal driver transistor Q1 (see Figure 1)
supports NPN as well as PNP pass transistors, and provides enough drive current (25mA specified) to support a
wide range of charging rates.
The driver transistor is controlled by a voltage regulating loop and a current limiting-limiting loop (see Figure 1).
The current-limiting loop reduces drive when the voltage between the IN pin and the IFB pin increases towards
V_ILIM (250mV typical). The voltage regulating loop tries to maintain the voltage on the VFB pin at V_REF. Together,
these two loops constitute a current-limited precision constant-voltage system, which is the heart of any lead-acid
charger. The voltage regulating amplifier needs an external compensation circuit which depends on the type of
external pass transistor (see Application Information section).
An important feature of the bq24450 is the precision reference voltage. The reference voltage is specially
temperature compensated to track the temperature characteristics of lead-acid cells. Additionally, the IC operates
with low supply current, only 1.6mA, minimizing on-chip dissipation and permitting the accurate sensing of the
operating environmental temperature by avoiding self-heating effects. To take full advantage of the
temperature-compensated reference, the IC should be in the same thermal environment as the battery.
An undervoltage lock-out circuit is also provided (see Figure 1). This circuit disables the driver transistor as long
as the input voltage is below UVLO (4.5V typical). The UVLO circuit also drives an open-collector output
PGOOD.
Voltage-sense and current-sense comparators are available in the IC. The current-sense comparator is
uncommitted. Its open-collector output is OFF when the difference between the ISNSP and ISNSM pins is less
than V_ISNS (25mV typical), and ON when the difference is more than V_ISNS. Depending on the application, this
comparator may be used to switch to float charging after the boost phase is over. The voltage sense comparator
can be used to sense the voltage level of the battery to initiate a new charge cycle.
Latches L1 and L2 constitute a state-machine to control the charging sequence. The internal inputs to the
state-machine come from the UVLO circuit and the voltage-sense comparator. One external input is provided,
the BSTOP pin. The outputs of the L1 and L2 latches are available at the STAT1 and STAT2 pins. The BSTOP
pin is internally pulled up through a 10µA current source. The states of the state-machine are:
Q(L1)
LOW
LOW
HIGH
Q(L2)
HIGH
LOW
HIGH
STAT1
ON
ON
OFF
STAT2
OFF
ON
OFF
Condition
Bulk Charge
Boost Mode
Float Mode
State #
State 1
State 2
State 3
A small bias current source is available at the PRE-CHG pin to provide pre-charge to deeply discharged
batteries. The PRE-CHG pin sources current when the voltage at the CE pin is below VREF. Driver transistor Q1
is turned OFF when the PRE-CHG current is ON.
Copyright © 2009, Texas Instruments Incorporated
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