BQ24750_07 Datasheet, PDF (21/38 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger with Integrated System Power Selector and AC Over-Power Protection

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bq24750
SLUS735 – DECEMBER 2006
should be divided and placed on 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.
Synchronous and Non-Synchronous Operation
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 uses break-before-make switching to prevent shoot-through
currents. During the 30-ns 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 safe
charging at high currents. During synchronous mode, the inductor current always flows, and the device 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 low-side MOSFET 80-ns on-time is required to ensure that 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 turned-off low-side MOSFET, and the inductor current becomes
discontinuous. This mode is called Discontinuous Conduction Mode (DCM).
During the DCM mode, the loop response automatically changes and has a single-pole system at which the pole
is proportional to the load current, because the converter does not sink current, and only the load provides a
current sink. This means that at very low currents, the loop response is slower, because there is less sinking
current available to discharge the output voltage. At very low currents during non-synchronous operation, there
may be a small amount of negative inductor current during the 80-ns recharge pulse. The charge should be low
enough to be absorbed by the input capacitance.
Whenever the converter goes into 0% duty-cycle mode, and BTST – PH < 4 V, the 80-ns recharge pulse occurs
on LODRV, the high-side MOSFET does not turn on, and the low-side MOSFET does not turn on (no 80-ns
recharge pulse), and there is no discharge from the battery.
In the bq24750, ISYN is internally set as the charge-current threshold at which the charger changes from
non-synchronous to synchronous operation. The low-side driver turns on for only 80 ns to charge the boost
capacitor. This is important to prevent negative inductor current, which 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
excessive voltage on the PVCC node and potential system damage. This programmable value allows setting the
current threshold for any inductor ripple current, and avoiding negative inductor current. The minimum
synchronous threshold should be set within a range from ½ the inductor ripple current to the full ripple current,
where the inductor ripple current is given by
I _{RIPPLE_MAX}
2
v
I_SYN
v
I _{RIPPLE_MAX}
ǒ Ǔ ǒ Ǔ ǒ Ǔ _{VIN_MAX * VBAT_MIN}
V_{BAT_MIN}
V_{IN_MAX}
1
fs
and I_{RIPPLE_MAX} +
L_MIN
(4)
where
V_{IN_MAX} = maximum adapter voltage
V_{BAT_MIN} = minimum BAT voltage
f_S = switching frequency
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