sc424evb Semtech Corporation, sc424evb Datasheet - Page 17

• Manufacturer Part Number: sc424evb
• Description: 6a Integrated Fet Regulator With 5v Ldo
• Manufacturer: Semtech Corporation

Applications Information (continued)
Because the on-times are forced to occur at intervals no
greater than 40μs, the frequency will not fall far below
25kHz. Figure 5 shows ultrasonic power-save operation.
Power-save Mode Operation (SC424)
The SC424 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
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.
Voltage (V
Inductor
DH
DL
Current
FB Ripple
After the 40μsec time-out, DL drives high if V
Figure 5 — Ultrasonic Power-save Operation
FB
FB
has not reached the FB threshold.
)
drops to the 750mV threshold. Because the
40μs time-out
minimum f
On-time
(T
ON
)
SW
~ 25kHz
DH On-time is triggered when
V
FB
reaches the FB Threshold
FB
FB threshold
(750mV)
(0A)
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
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
825mV), the device immediately disables power-save, and
DL drives high to turn on the low-side MOSFET. This draws
current from V
fall. When V
T
OVP shutdown and also cycles energy from V
V
conduction mode operation. Figure 7 shows typical wave-
forms for the Smart Power-save feature.
SmartDrive
For each DH pulse, the DH driver initially turns on the
high-side MOSFET at a slower speed, allowing a softer,
smooth turn-off of the low-side diode. Once the DH node
is high and the LX voltage has risen 1V above PGND, the
SmartDrive circuit automatically drives the high-side
MOSFET on at a rapid rate. This technique reduces ringing
while maintaining high efficiency and also avoids the
need for snubbers or series resistors in the gate drive.
ON
IN
. It also minimizes operating power by avoiding forced
switching cycle begins. This method prevents a hard
DH
DL
Inductor
Current
FB Ripple
Voltage
(V
FB
)
Figure 6 — Power-save Operation
DL remains high until inductor current reaches zero.
FB
drops back to the 750mV trip point, a normal
TM
OUT
DL drives high when on-time is completed.
through the inductor and causes V
On-time (T
V
DH On-time is triggered when
FB
reaches the FB Threshold.
ON
according to load
Dead time varies
)
SC414/SC424
Zero (0A)
OUT
FB threshold
(750mV)
to slowly rise and
OUT
back to
OUT
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