BQ24765_15 Datasheet, PDF (24/37 Pages) Texas Instruments – SMBus-Controlled Multi-Chemistry Battery Charger With Integrated Power MOSFETs

bq24765
SLUS999 – NOVEMBER 2009
www.ti.com
AUTOMATIC INTERNAL SOFT-START CHARGER CURRENT
The charger automatically soft-starts the output regulation current every time the charger is enabled to ensure
there is no overshoot or stress on the output capacitors or the power converter. The soft-start consists of
stepping-up the charge regulation current into 8 evenly divided steps up to the programmed charge current. Each
step lasts around 1.6ms, for a typical rise time of 10ms. No external components are needed for this function.
The regulation limits can be changed in the middle of charging without soft start.
SWITCHING FREQUENCY
The bq24765 switching frequency is 700kHz. A high switching frequency allows a smaller inductor to give the
same ripple current, or can be used to reduce the ripple current for the same inductor. A smaller inductor value
may allow using a smaller inductor physical size, for a smaller board footprint area.
Table 6. Output LC Filter Component Selection Table
bq24765 (Fs = 700 kHz)
Vin
Vout
Iout
Lout
Cout
1.5 A
4.7 µH
10 µF
19.5 V
3s/4s
12.6 V/16.8 V
3A
4.5 A
4.7 µH
3.3 µH
10 µF, 20 µF
20 µF
6A
3.3 µH
20 µF, 30 µF
1.5 A
5.6 µH
10 µF
12 V
2s
8.4 V
3A
4.5 A
4.7 µH
3.3 µH
10 µF, 20 µF
20 µF
6A
2.2 µH
20 µF, 30 µF
• Shaded areas are the most likely applications.
• External compensation can be recalculated if need other values.
• Lower current applications can use the inductance used at higher currents, but would operate in DCM more
often.
CONVERTER OPERATION
The synchronous buck PWM converter uses a fixed frequency (700kHz) voltage mode with feed-forward control
scheme. A type III compensation network allows using ceramic capacitors at the output of the converter. The
compensation input stage is connected between the feedback output (FBO) and the error amplifier input (EAI).
The feedback compensation stage is connected between the error amplifier input (EAI) and error amplifier output
(EAO). The LC output filter selected gives a characteristic resonant frequency that is used to determine the
compensation to ensure there is sufficient phase margin for the target bandwidth.
The
resonant
frequency,
f_o,
is
given
by:
f_o
+
2p
1
Ǹ_LoCo
An internal saw-tooth ramp is compared to the internal EAO error control signal to vary the duty-cycle of the
converter. The ramp height is one-fifteenth of the input adapter voltage making it always directly proportional to
the input adapter voltage. This cancels out any loop gain variation due to a change in input voltage, and
simplifies the loop compensation. The ramp is offset by 200mV in order to allow zero percent duty-cycle, when
the EAO signal is below the ramp. The EAO signal is also allowed to exceed the saw-tooth ramp signal in order
to get a 100% duty-cycle PWM request. Internal gate drive logic allows achieving 99.98% duty-cycle while
ensuring the N-channel upper device always has enough voltage to stay fully on. If the BOOT pin to PHASE pin
voltage falls below 4V for more than 3 cycles, then the high-side n-channel power MOSFET is turned off and the
low-side n-channel power MOSFET is turned on to pull the PHASE node down and recharge the BOOT
capacitor. Then the high-side driver returns to 100% duty-cycle operation until the (BOOT-PHASE) voltage is
detected to fall low again due to leakage current discharging the BOOT capacitor below the 4V, and the recharge
pulse is reissued.
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