SC402B
18
The SC402B PSAVE operation reduces the switching fre-
quency according to the load for increased efficiency at
light load conditions.
Forced Continuous Mode Operation
The SC402B operates the switcher in FCM (Forced
Continuous Mode) by floating the EN/PSV pin (see Figure
4). In this mode one of the power MOSFETs is always on,
with no intentional dead time other than to avoid cross-
conduction. This feature results in uniform frequency
across the full load range with the trade-off being poor
efficiency at light loads due to the high-frequency switch-
ing of the MOSFETs. DH is gate signal to drive upper
MOSFET. DL is lower gate signal to drive lower MOSFET.
FB Ripple
Voltage (V
FB
)
FB threshold
DL
DH
Inductor
Current
DC Load Current
DH on-time is triggered when
V
FB
reaches the FB Threshold.
On-time
(T
ON
)
DL drives high when on-time is completed.
DL remains high until V
FB
falls to the FB threshold.
Figure 4 Forced Continuous Mode Operation
Power-save Operation
The SC402B provides power-save operation at light loads
with no minimum operating frequency. With power-save
enabled, the internal zero crossing comparator monitors
the inductor current via the voltage across the low-side
MOSFET during the off-time. If the inductor current falls to
zero for 8 consecutive switching cycles, the controller
enters power-save operation. It will turn off the low-side
MOSFET on each subsequent cycle provided that the
current crosses zero. At this time both MOSFETs remain off
until V
FB
drops to the 600mV threshold. Because the
MOSFETs are off, the load is supplied by the output
capacitor.
If the inductor current does not reach zero on any switch-
ing cycle, the controller immediately exits power-save and
returns to forced continuous mode.
Figure 5 shows power-save operation at light loads.
FB Ripple
Voltage
(V
FB
)
FB threshold
DL
DH
Inductor
Current
Zero (0A)
DH On-time is triggered when
V
FB
reaches the FB Threshold.
On-time (T
ON
)
DL drives high when on-time is completed.
DL remains high until inductor current reaches zero.
Dead time varies
according to load
Figure 5 Power-save Operation
Smart Power-save Protection
Active loads may leak current from a higher voltage into
the switcher output. Under light load conditions with
power-save enabled, this can force V
OUT
to slowly rise and
reach the over-voltage threshold, resulting in a hard shut-
down. Smart power save prevents this condition. When
the FB voltage exceeds 10% above nominal, the device
immediately disables power save, and DL drives high to
turn on the low-side MOSFET. This draws current from
V
OUT
through the inductor and causes V
OUT
to fall. When V
FB
drops back to the 600mV trip point, a normal T
ON
switch-
ing cycle begins. This method prevents a hard OVP shut-
down and also cycles energy from V
OUT
back to V
IN
. It also
minimizes operating power by avoiding forced conduc-
Applications Information (continued)
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