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BQ24721 Datasheet, PDF (28/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR
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bq24721
SLUS683 – NOVEMBER 2005
www.ti.com
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 output capacitors. The compensation input stage is
connected between the feedback output (FBO) pin and the error amplifier input (EAI) pin. The feedback
compensation stage is connected between the error amplifier input (EAI) pin and error amplifier output (EAO) pin.
An internal saw-tooth ramp is compared to the EAO pin error control signal to vary the duty-cycle of the
converter. The ramp height is one-tenth 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 99.98% 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
4.5 V for more than 3 cycles, then the high-set 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 voltage is detected to fall low again due to leakage
current discharging the BTST capacitor below the 4.5 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. The switching frequency can be changed from 300 kHz to 500 kHz by the Charger Mode (0x12)
register PWM FS bit (b5) – a HI is 500 kHz, a LO is 300 kHz. The switching frequency is 300 kHs by default after
power-on-reset. Typical chargers use 300 kHz, but 500 kHz allows a smaller inductance value
The charge current sense resistor 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 sense
accuracy. The type III compensation is already providing phase boost near the cross-over frequency, giving
sufficient phase margin.
ISYNSET
The ISYNSET pin is used to program the charge current threshold at which the charger changes from
non-synchronous operation into synchronous operation. This is important to prevent negative inductor current.
Negative inductor current may cause a boost effect in which the input voltage increases as power is transferred
from the battery to the input capacitors—this can lead to an overvoltage on the PVCC node and potentially cause
some damage to the system.
This programmable value allows setting the current threshold for any inductor current ripple, and avoiding
negative inductor current. The SYNP and SYNN pins are used to sense across the charge current sense resistor.
To program the threshold, a resistor is connected from the ISYNSET pin to AGND. The minimum synchronous
threshold should be set from the inductor current ripple to the full ripple current, where the inductor current ripple
is given by.
I(RIPPLE_MAX)
2
£ I(SYN) £ I(RIPPLE_MAX)
(1)
where
??V(IN_MAX) - V(BAT_MIN)
V(BAT_MIN)
?x
? V(IN_MAX)
??x
1
¦S
R(RIPPLE_MAX) =
L(MIN)
(2)
V(IN_MAX) is the maximum adapter voltage, V(BAT_MIN) is the minimum battery voltage, fS is the switching
frequency, and LMIN is the minimum output inductor value.
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