BQ24765_15 Datasheet, PDF (26/37 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs

bq24765
SLUS999 – NOVEMBER 2009
www.ti.com
I_DCM
<
I_RIPPLE
2
and
I_RIPPLE
=
(V_IN
-
V_BAT )
´
æ
ç
è
V_BAT
V_IN
ö
÷
ø
L_out
´
æ 1ö
ç
è
f_S
÷
ø
(2)
Where:
V_IN: adapter voltage
V_VFB: Output Voltage = VFB voltage
f_S: switching frequency = 700kHz
L_OUT: output inductor
For proper cycle-by-cycle UCP sensing, the output filter capacitor should sit on CSON. Only 0.1µF capacitor is
on CSOP, close to the device input.
CYCLE-BY-CYCLE CHARGE OVER-CURRENT PROTECTION, USING HIGH-SIDE SENSE-FET
The charger has a cycle-to-cycle over-current protection to protect from exceeding the maximum current
capability of the integrated power MOSFETs. It monitors the drain current of the high-side power MOSFET using
a sense-FET, and prevents the current from exceeding 10A peak. The integrated high-side power MOSFET turns
off when the over-current is detected, and latches off until the following cycle.
AVERAGE CHARGE OVER-CURRENT PROTECTION, USING SENSE RESISTOR
The charger has an average over-current protection using the V(CSON-CSOP) voltage across the charge current
sense resistor. It monitors the charge current, and prevents the current from exceeding 145% of programmed
regulated charge current. If the charge current limit falls below 3.3A (on 10mΩ), the over current limit is fixed at
5A. The high-side gate drive turns off when the over-current is detected, and automatically resumes when the
current falls below the over-current threshold. There is an internal 160kHz filter pole, to filter the switching
frequency and prevent false tripping. This will add a small delay depending on the amount of overdrive over the
threshold.
BATTERY OVER-VOLTAGE PROTECTION, USING REMOTE SENSING VFB
The converter will not allow switching when the battery voltage at VFB exceeds 104% of the regulation voltage
set-point. Once the VFB voltage returns below 102% of the regulation voltage, switching resumes. This allows
quick response to an over-voltage condition – such as occurs when the load is removed or the battery is
disconnected. A 10mA current sink from CSOP and CSON to AGND is on only during charging and allows
discharging the stored output inductor energy that is transferred to the output capacitors.
BATTERY SHORT PROTECTION
The bq24765 has a BAT LOWV comparator monitoring the output battery VFB voltage. If the voltage falls below
3.6V, the battery short status is detected. Once the short status is detected, charger immediately stops for 2ms
to avoid inductor peak current surge. After 2ms, the charger will soft-start again. If the battery voltage is still
below 2.5V, a 220mA trickle charge current is applied. Otherwise, the charge current limit is set by
ChargeCurrent() (refer to the CHARGE CURRENT REGULATION section).
BATTERY TRICKLE CHARGING
The bq24765 automatically reduces the charge current limit to a fixed 220mA to trickle charge the battery, when
the voltage on the VFB pin falls below 2.5V. The charge current returns to the value programmed on the
ChargeCurrent(0x14) register, when the VFB pin voltage rises above 2.7V. This function provides a safe trickle
charge to close deeply discharged open packs.
26
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