BQ24747 Datasheet, PDF (18/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
SLUS988 – OCTOBER 2009 ............................................................................................................................................................................................. www.ti.com
ENABLE AND DISABLE CHARGING
The following conditions must be valid before charging is enabled:
• CE is HIGH;
• Adapter is detected (ACIN > 2.4 V);
• Adapter is higher than the DCIN-VFB threshold;
• 200μs delay is complete after adapter detected;
• VDDP and VREF are valid;
• Thermal Shutdown (TSHUT) is not active;
Any of the following conditions stop the charge cycle:
• CE is LOW;
• Adapter is removed;
• Adapter voltage is less than 250 mV above the battery;
• Adapter is over voltage;
• Charge output current is over programmed current;
• TSHUT IC temperature threshold is reached (145°C on rising-edge with 15°C hysteresis).
AUTOMATIC INTERNAL SOFT-START CHARGER CURRENT
The charger automatically soft-starts the charger regulation current every time the charger is enabled to ensure
that there is no overshoot or stress on the output capacitors or the power converter. The soft-start function steps
up the charge current into 8 evenly-divided steps, gradually building up to the full programmed charge current.
Each step lasts approximately 1 ms, for a typical rise time of 8 ms. No external components are needed for this
function.
CONVERTER OPERATION
The synchronous-buck PWM converter operates at a fixed frequency (300 kHz) in voltage mode with a
feed-forward control scheme. A type-III compensation network allows the use of ceramic capacitors at the output
of the converter. The input compensation 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 has a characteristic resonant frequency that ensures 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 converter duty cycle.
The ramp height is 1/15 of the input adapter voltage, always keeping it directly proportional to the input adapter
voltage. This cancels out any loop-gain variation due to an input voltage change, simplifying loop-compensation
design. The ramp is offset by 250 mV in order to allow a 0% 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 respond to a 100% duty-cycle
PWM request. The internal gate-drive logic allows a 99.98% duty cycle while ensuring that the N-channel upper
device always has enough voltage to stay fully on. If the BOOT-pin-to-PHASE-pin voltage falls below 4.5 V for
more than 3 cycles, 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 again detected falling low due to
leakage current discharging the BOOT capacitor below 4 V, and the reset pulse is reissued.
The 300-kHz fixed-frequency oscillator keeps tight control of the switching frequency under all conditions of input
voltage, battery voltage, charge current, and temperature, simplifying output-filter design and keeping it out of the
audible-noise region. The charge-current sense resistor R_SR) should be positioned with half or more of the total
output capacitance placed before R_SR, contacting both R_SR and the output inductor; and the remaining
capacitance placed after R_SR. The output capacitance should be divided and placed on either side of R_SR. A ratio
of 50:50% gives the best performance, but the node in which the output inductor and R_SR connect should have a
minimum of 50% of the total capacitance. This capacitance provides sufficient filtering to remove the switching
noise and give better accuracy. The type-III compensation provides phase boost near the crossover frequency to
provide sufficient phase margin.
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