Datasheet

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bq24630
SLUS894A –JANUARY 2010– REVISED OCTOBER 2011 www.ti.com
AUTOMATIC INTERNAL SOFT-START CHARGER CURRENT
The charger automatically soft-starts the charger regulation current every time the charger goes into fast-charge
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 in eight evenly divided steps up to the programmed charge current.
Each step lasts around 1.6 ms, for a typical rise time of 12.8 ms. No external components are needed for this
function.
CONVERTER OPERATION
The synchronous buck PWM converter uses a fixed-frequency 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 internally 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 is selected to give a resonant frequency of 10 kHz to 15 kHz for the bq24630, where
the resonant frequency, f
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, is given by Equation 6:
(6)
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 7% 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 300 mV 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 99.95% duty cycle while ensuring the
N-channel upper device always has enough voltage to stay fully on. If the BTST pin to PH pin voltage falls below
4.2 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 to pull the PH node down and recharge the BTST capacitor. Then the
high-side driver returns to 100% duty-cycle operation until the BTST-to-PH voltage is detected to fall low again
due to leakage current discharging the BTST capacitor below 4.2 V, and the reset pulse is reissued.
The 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. Also see Application Information for how to select the inductor, capacitor, and MOSFET.
Synchronous and Non-Synchronous Operation
The charger operates in synchronous mode when the SRP-SRN voltage is above 5 mV (0.5-A inductor current
for a 10-mΩ sense resistor). During synchronous mode, the internal gate-drive logic ensures there is
break-before-make complementary switching to prevent shoot-through currents. During the 30-ns dead time
where both FETs are off, the body diode of the low-side power MOSFET conducts the inductor current. Having
the low-side FET turn on keeps the power dissipation low, and allows safely charging at high currents. During
synchronous mode, the inductor current is always flowing and the converter operates in continuous-conduction
mode (CCM), creating a fixed two-pole system.
The charger operates in non-synchronous mode when the SRP-SRN voltage is below 5 mV (0.5-A inductor
current for a 10-mΩ sense resistor). The charger is forced into non-synchronous mode when battery voltage is
lower than 2 V or when the average SRP-SRN voltage is lower than 1.25 mV.
During non-synchronous operation, the body -diode of the low-side MOSFET can conduct the positive inductor
current after the high-side n-channel power MOSFET turns off. When the load current decreases and the
inductor current drops to zero, the body diode is be naturally turned off and the inductor current becomes
discontinuous. This mode is called discontinuous-conduction mode (DCM). During DCM, the low-side n-channel
power MOSFET turns on for around 80 ns when the bootstrap capacitor voltage drops below 4.2 V; then the
low-side power MOSFET turns off and stays off until the beginning of the next cycle, where the high-side power
MOSFET is turned on again. The 80-ns low-side MOSFET on-time is required to ensure the bootstrap capacitor
is always recharged and able to keep the high-side power MOSFET on during the next cycle. This is important
for battery chargers, where unlike regular dc-dc converters, there is a battery load that maintains a voltage and
can both source and sink current. The 80-ns low-side pulse pulls the PH node (connection between high- and
low-side MOSFETs) down, allowing the bootstrap capacitor to recharge up to the REGN LDO value. After 80 ns,
the low-side MOSFET is kept off to prevent negative inductor current from occurring.
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