SC410A Datasheet, PDF (14/26 Pages) Semtech Corporation – 2A EcoSpeed Step-Down Regulator with LDO and Power Save

SC410A
Applications Information (continued)
FB Ripple
Voltage (V_FB)
FB threshold
(750mV)
Inductor
Current
DC Load Current
FB Ripple
Voltage
(V_FB)
Inductor
Current
Dead time varies
according to load
FB threshold
(750mV)
Zero (0A)
On-time DH on-time is triggered when
(T_ON) VFB reaches the FB Threshold.
On-time (T_ON)
DH On-time is triggered when
DH
DH
V_FB reaches the FB Threshold.
DL
DL drives high when on-time is completed.
DL remains high until V_FB falls to the FB threshold.
Figure 4 — Continuous Mode Operation
DL
DL drives high when on-time is completed.
DL remains high until inductor current reaches zero.
Figure 5 — Power Save Operation
Power-save Mode Operation
The SC410A 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 750mV 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.
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
825mV), 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 750mV trip point,
a normal T_ON switching cycle begins. This method pre-
vents a hard OVP shutdown and also cycles energy from
V_OUT back to V_IN. Figure 6 shows typical waveforms for the
smart power save feature.
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