Datasheet

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bq24610
bq24617
SLUS892B –DECEMBER 2009–REVISED SEPTEMBER 2013 www.ti.com
Asymmetrical gate drive (fast turnoff and slow turnon) for the ACDRV and BATDRV drivers provides fast turnoff
and slow turnon of the ACFET and BATFET to help the break-before-make logic and to allow a soft start at
turnon of either FET. The soft-start time can be further increased by putting a capacitor from gate to source of
the p-channel power MOSFETs.
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 into 8 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 12 kHz to 17 kHz for bq24610/7, where
the resonant frequency, f
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, is given by:
(7)
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 sawtooth ramp signal in order to get a 100% duty-
cycle PWM request. Internal gate-drive logic allows achieving 99.5% 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 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 PH node down and recharge the BTST capacitor. Then the high-side driver
returns to 100% duty-cycle operation until the (BTST-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 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 complimentary 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 lower-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 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
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