BQ24745_14 Datasheet, PDF (25/39 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery

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bq24745
SLUS761D – DECEMBER 2007 – REVISED OCTOBER 2011
REFRESH BTST CAPACITOR
If the BOOT pin to PHASE pin voltage falls below 4 V for more than three cycles, then the high-side n-channel
power MOSFET is turned off and the low-side n-channel power MOSFET is turned on for 40 ns to pull the
PHASE node down and recharge the BOOT capacitor. The 40-ns low-side MOSFET on-time is required protect
from ringing noise, and to ensure the bootstrap capacitor is always recharged and able to keep the high-side
power MOSFET on during the next cycle.
UCP (CHARGE UNDERCURRENT), USING SENSE RESISTOR
In the bq24745, the cycle-by-cycle UCP allows using very small inductors seamlessly, even if they have large
ripple current. Every cycle when the low-side MOSFET turns-on, if the CSOP-CSON voltage falls below 10 mV
(inductor current falls below 1 A if using a 10-mΩ sense resistor), the low-side MOSFET is latched off until the
next cycle begins and resets the latch.
The converter automatically detects when to turn off the low-side MOSFET every cycle. The converter goes into
discontinuous conduction mode (DCM) when the current falls below 1/2 the inductor peak-to-peak current ripple.
The inductor current ripple is given by
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 = DCIN voltage
V_VFB: output voltage = VFB voltage
f_S: switching frequency = 300 kHz
L_OUT: output inductor
For proper cycle-by-cycle UCP sensing, the output filter capacitor should sit on CSON. Only a 0.1-µF capacitor is
on CSOP, close to the device input.
AVERAGE CHARGE OVERCURRENT, USING SENSE AMPLIFIER
The charger has average overcurrent 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 the programmed
regulated charge current. If the charge current limit falls below 3.3 A (on 10 mΩ), the overcurrent limit is fixed at
5 A. The high-side gate drive turns off when the overcurrent is detected, and automatically resumes when the
current falls below the overcurrent threshold. There is an internal 160-kHz filter pole, to filter the switching
frequency and prevent false tripping. This adds a small delay, depending on the amount of overdrive over the
threshold.
BATTERY OVERVOLTAGE PROTECTION, USING REMOTE SENSING VFB
The converter does 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 overvoltage condition, such as occurs when the load is removed or the battery is
disconnected. A current sink from CSOP and CSON to GND is on only during charging and allows discharging
the stored output inductor energy that is transferred to the output capacitors.
BATTERY TRICKLE CHARGING
The bq24745 automatically reduces the charge current limit to a fixed 220 mA to trickle-charge the battery when
the voltage on the VFB pin falls below 2.5 V. The charge current returns to the value programmed on the
ChargeCurrent(0x14) register when the VFB pin voltage rises above 2.7 V.
This function provides a safe trickle charge to close deeply discharged open packs.
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