SC508 datasheet(16/32 Pages) SEMTECH

16 / 32 page

SC508/SC508A

Because the period between on-times is limited to a

maximum value, a minimum operating frequency is main-

tained. Figure 6 shows ultrasonic power-save operation.

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

(0A)

minimum frequency

After the programmable 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)

programmable time-out

Figure 6 — Ultrasonic Power-Save Operation

The equation for determining the R

PSV resistor value is

shown next. The desired minimum frequency is f

SWMIN.

SWMIN

PSV

350

Power-Save Mode Operation

The device provides power-save operation at light loads

with no minimum operating frequency, selected by float-

ing the PSV pin (no connection). In this mode of opera-

tion, the zero cross comparator monitors inductor current

via the voltage across the low-side MOSFET during the

off-time. If the inductor current falls to zero for 8 consecu-

tive switching cycles, the controller enters power-save

operation. It will then turn off the low-side MOSFET on

each subsequent cycle, provided that the current falls to

zero. After the low-side MOSFET is off, both high-side and

low-sides MOSFETs remain off until V

FB drops to the 600mV

threshold. While the MOSFETs are off the load is supplied

by the output capacitor. If the inductor current does not

reach zero on any switching cycle, the controller immedi-

ately exits power-save and returns to forced continuous

mode. Figure 7 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 7 — 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 an over-

voltage shutdown. Smart power-save prevents this condi-

tion. When the FB voltage exceeds 10% above nominal

(exceeds 660mV), 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 VFB drops back to the 600mV trip

point, a normal T

ON switching cycle begins. This method

prevents over-voltage shutdown by cycling energy from

OUT back to VIN. It also minimizes operating power under

light load conditions by avoiding forced continuous mode

operation.

Figure 8 shows typical waveforms for the Smart Power-

save feature.

Applications Information (continued)

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