SC417 datasheet(15/29 Pages) SEMTECH

15 / 29 page

SC417/SC427

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.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

(500mV)

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

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

OUT

through

the inductor, forcing both V

OUT

and V

to fall. When V

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.

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)

FB threshold

(500mV)

Inductor

Current

(0A)

40μs time-out

minimum fSW ~ 25kHz

After the 40μsec time-out, DL drives high if VFB

has not reached the FB threshold.

DH On-time is triggered when

VFB reaches the FB Threshold

On-time

(TON)

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

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.

Applications Information (continued)

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