BQ24747 Datasheet, PDF (19/36 Pages) Texas Instruments – SMBus-Controlled Level 2 Multi-Chemistry Battery Charger With Input Current Detect Comparator and Charge Enable Pin

bq24747
www.ti.com ............................................................................................................................................................................................. SLUS988 – OCTOBER 2009
SYNCHRONOUS AND NON-SYNCHRONOUS OPERATION
The charger operates in non-synchronous mode when the sensed charge current is below the internal ISYNSET
value of 13 mV (1.3 A) falling, and 0.8 mV (800 mA) rising (with built-in hysteresis). Otherwise, the charger
operates in synchronous mode.
In synchronous mode, the low-side n-channel power MOSFET is on, and the high-side n-channel power
MOSFET is off. The internal gate-drive logic enforces break-before-make switching to prevent shoot-through
currents. During the 30-ns dead time when both FETs are off, the back diode of the low-side power MOSFET
conducts the inductor current. Having the low-side FET turned on keeps the power dissipation low, and safely
allows high-current charging. In synchronous mode, the inductor current is always flowing and operates in
Continuous Conduction Mode (CCM), creating a fixed two-pole system.
In 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 turns on for approximately 80 ns, then the
low-side power MOSFET turns off and stays off until the beginning of the next cycle, when the high-side power
MOSFET is turned on again. The 80-ns low-side MOSFET on-time is required to ensure that the bootstrap
capacitor is always charged and able to keep the high-side power MOSFET turned 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 PHASE node (connection
between high and low-side MOSFET) down, allowing the bootstrap capacitor to recharge up to the VDDP LDO
value. After the 80 ns, the low-side MOSFET is kept off to prevent negative inductor current from flowing. The
inductor current is blocked by the off-state low-side MOSFET, and the inductor current becomes discontinuous.
This mode is called Discontinuous Conduction Mode (DCM).
In DCM operation, the loop response automatically changes, and acts as 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. 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. This should be low enough to be absorbed
by the input capacitance.
When the converter goes into 0% duty cycle, neither MOSFET turns on (no 80-ns recharge pulse), and there is
no discharge from the battery.
ISYNSET CONTROL (CHARGE UNDERCURRENT)
In bq24747, ISYN is the internally-set ISYNSET value as the charge-current threshold at which the charger
switches from non-synchronous operation 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 an overvoltage condition on the DCIN node, and potentially can damage the system. This
programmable value allows setting the current threshold for any inductor ripple current to avoid negative inductor
current. The minimum synchronous threshold should be set from 50%–100% of the inductor ripple current, where
the inductor ripple current is calculated using Equation 1.
I _{ripple _ max}
2
£ I _SYN
£ I _{ripple _ max}
and
I _ripple
=
(V_in
-
V_bat
)
´
V_bat
V_in
L
´
1
f_s
V_in ´ (1- D) ´ D ´
=
L
1
f_s
Where:
V_{IN_MAX}: maximum adapter voltage
V_{BAT_MIN}: minimum BAT voltage
f_S: switching frequency
L_MIN: minimum output inductor
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