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BQ24705 Datasheet(PDF) 17 Page - Texas Instruments

Part # BQ24705
Description  Host-Controlled Multi-Cell Battery Charger
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

BQ24705 Datasheet(HTML) 17 Page - Texas Instruments

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CONVERTER OPERATION
Where resonant frequency, fo, is given by:
fo +
1
2p LoCo where (from Figure 2 schematic)
SYNCHRONOUS AND NON-SYNCHRONOUS OPERATION
bq24705
www.ti.com............................................................................................................................................... SLUS779B – DECEMBER 2007 – REVISED MARCH 2009
The synchronous buck PWM converter uses a fixed frequency (600 kHz) 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 8–12.5 kHz nominal.
• C
O = C11 + C12
• L
O = L1
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 200 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 99.98% 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 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 the 4 V, and the reset pulse is reissued.
The 600 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 RSR should be placed with at least half or more of the
total output capacitance placed before the sense resistor contacting both sense resistor and the output inductor;
and the other half or remaining capacitance placed after the sense resistor. The output capacitance should be
divided and placed onto both sides of the charge current sense resistor. A ratio of 50:50 percent gives the best
performance; but the node in which the output inductor and sense resistor connect should have a minimum of
50% of the total capacitance. This capacitance provides sufficient filtering to remove the switching noise and give
better current sense accuracy. The type III compensation provides phase boost near the cross-over frequency,
giving sufficient phase margin.
The charger operates in non-synchronous mode when the sensed charge current is below the ISYNSET value.
Otherwise, the charger operates in synchronous mode.
During synchronous mode, the low-side n-channel power MOSFET is on, when the high-side n-channel power
MOSFET is off. The internal gate drive logic ensures there is break-before-make switching to prevent
shoot-through currents. During the 30ns dead time where both FETs are off, the back-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
operates in Continuous Conduction Mode (CCM), creating a fixed two-pole system.
During non-synchronous operation, after the high-side n-channel power MOSFET turns off, and after the
break-before-make dead-time, the low-side n-channel power MOSFET will turn-on for around 80ns, then the
low-side power MOSFET will turn-off and stay off until the beginning of the next cycle, where the high-side power
MOSFET is turned on again. The 80ns 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
MOSFET) down, allowing the bootstrap capacitor to recharge up to the REGN LDO value. After the 80 ns, the
low-side MOSFET is kept off to prevent negative inductor current from occurring. The inductor current is blocked
by the off low-side MOSFET, and the inductor current will become discontinuous. This mode is called
Discontinuous Conduction Mode (DCM).
Copyright © 2007–2009, Texas Instruments Incorporated
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