SC417EVB Ver la hoja de datos (PDF) - Semtech Corporation
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SC417EVB Datasheet PDF : 29 Pages
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SC417/SC427
Applications Information (continued)
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-fre-
quency switching of the MOSFETs.
Because the on-times are forced to occur at intervals no
greater than 40μs, the frequency will not fall below
~25kHz. Figure 5 shows ultra-sonic power-save
operation.
FB Ripple
Voltage (VFB)
Inductor
Current
FB threshold
(500mV)
DC Load Current
FB Ripple
Voltage (VFB)
minimum fSW ~ 25kHz
Inductor
Current
FB threshold
(500mV)
(0A)
On-time DH on-time is triggered when
(TON) VFB reaches the FB Threshold.
On-time
(TON)
DH On-time is triggered when
VFB reaches the FB Threshold
DH
DH
40μs time-out
DL
DL
DL drives high when on-time is completed.
DL remains high until VFB falls to the FB threshold.
Figure 4 — Forced Continuous Mode Operation
Ultra-sonic Power-save Operation (SC417)
The SC417 provides ultra-sonic power-save operation at
light loads, with the minimum operating frequency fixed
at 25kHz. This is accomplished using an internal timer that
monitors the time between consecutive high-side gate
pulses. If the time exceeds 40μs, DL drives high to turn the
low-side MOSFET on. This draws current from V through
OUT
the inductor, forcing both V and V to fall. When V
OUT
FB
FB
drops to the 500mV threshold, the next DH on-time is trig-
gered. After the on-time is completed the high-side
MOSFET is turned off and the low-side MOSFET turns on.
The low-side MOSFET remains on until the inductor
current ramps down to zero, at which point the low-side
MOSFET is turned off.
After the 40μsec time-out, DL drives high if VFB
has not reached the FB threshold.
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 VFB 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.
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