SC417/SC427
19
FB Ripple
Voltage (V
FB
)
FB threshold
(500mV)
Inductor
Current
DH
DL
(0A)
40祍 time-out
minimum f
SW
~ 25kHz
After the 40祍ec time-out, DL drives high if V
FB
has not reached the FB threshold.
DH On-time is triggered when
V
FB
reaches the FB Threshold
On-time
(T
ON
)
Figure 5 Ultrasonic Power-save Operation
Power-save Mode Operation (SC427)
The SC427 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 500mV threshold. Because the
MOSFETs are off, the load is supplied by the output capaci-
tor. If the inductor current does not reach zero on any
switching cycle, the controller immediately exits power-
save and returns to forced continuous mode. Figure 6
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.
(500mV)
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 6 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 (exceeds
550mV), 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 500mV trip point, a normal
T
ON
switching cycle begins. This method prevents a hard
OVP shutdown and also cycles energy from V
OUT
back to
V
IN
. It also minimizes operating power by avoiding forced
conduction mode operation. Figure 7 shows typical wave-
forms for the Smart Power-save feature.
SmartDrive
TM
For each DH pulse the DH driver initially turns on the high-
side MOSFET at a lower speed, allowing a softer, smooth
turn-off of the low-side diode. Once the diode is off and
the LX voltage has risen 0.5V above PGND, the SmartDrive
circuit automatically drives the high-side MOSFET on at a
rapid rate. This technique reduces switching while main-
taining high efficiency and also avoids the need for snub-
bers or series resistors in the gate drive.
Applications Information (continued)
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