BQ24075T Datasheet, PDF (17/29 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

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bq24075T
bq24079T
SLUS937 – DECEMBER 2009
Figure 27. Typical Charging Cycle
Figure 27 illustrates a normal Li-Ion charge cycle using the bq2407xT. In the pre-charge phase, the battery is
charged at with the pre-charge current (I_PRECHG). Once the battery voltage crosses the V_LOWV threshold, the
battery is charged with the fast-charge current (I_CHG). As the battery voltage reaches V_BAT(REG), the battery is held
at a constant voltage of V_BAT(REG) and the charge current tapers off as the battery approaches full charge. When
the battery current reaches I_TERM, the CHG pin indicates charging done by going high-impedance.
Note that termination detection is disabled whenever the charge rate is reduced because of the actions of the
thermal loop, the DPPM loop or the V_IN-DPM loop.
The value of the fast-charge current is set by the resistor connected from the ISET pin to VSS, and is given by
the equation
I_CHG = K_ISET / R_ISET
The charge current limit is adjustable up to 1.5A. The valid resistor range is 590Ω to 3 kΩ. Note that if I_CHG is
programmed as greater than the input current limit, the battery will not charge at the rate of I_CHG, but at the
slower rate of I_IN(MAX) (minus the load current on the OUT pin, if any). In this case, the charger timers will be
proportionately slowed down.
CHARGE CURRENT TRANSLATOR
When the charger is enabled, internal circuits generate a current proportional to the charge current at the ISET
input. The current out of ISET is 1/400 (±10%) of the charge current. This current, when applied to the external
charge current programming resistor, R_ISET, generates an analog voltage that can be monitored by an external
host to calculate the current sourced from BAT.
V_ISET = I_CHARGE / 400 × R_ISET
Copyright © 2009, Texas Instruments Incorporated
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