SC401B datasheet(18/32 Pages) SEMTECH

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SC401B

The SC401B power save operation maintains a minimum

switching frequency of 25kHz, for applications with strin-

gent audio requirements.

Forced Continuous Mode Operation

The SC401B 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 (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

On-time

(TON)

DL drives high when on-time is completed.

DL remains high until VFB falls to the FB threshold.

Figure 4 — Forced Continuous Mode Operation

Power Save Operation

The SC401B 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

(VFB)

FB threshold

Inductor

Current

Zero (0A)

DH On-time is triggered when

VFB reaches the FB Threshold.

On-time (TON)

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

OUT through the inductor and causes VOUT to fall. When VFB

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 VIN. It also

minimizes operating power by avoiding forced conduc-

Applications Information (continued)

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