SC419EVB datasheet(13/24 Pages) SEMTECH

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SC419

maintain the desired on-time. On-time is calculated

according to the voltage at the VOUT pin.

Programmable Ultra-sonic Power-Save Operation

The device provides programmable ultra-sonic power-

save operation at light loads. The minimum operating fre-

quency is programmed by connecting a resistor from PSV

to AGND. The SC419 uses the PSV resistor to set an inter-

nal timer that monitors the time between consecutive

high-side gate pulses. If the time exceeds the programmed

timer, DL drives high to turn the low-side MOSFET on. This

draws current from V

OUT through the inductor, forcing both

OUT and VFB to fall. When VFB drops to the 500mV thresh-

old, the next DH on-time is triggered. After the on-time is

completed the high-side MOSFET is turned off and the

low-side MOSFET turns on, and the internal timer is

restarted. The low-side MOSFET remains on until the

inductor current ramps down to zero, at which point the

low-side MOSFET is turned off. This ends the cycle until

FB falls below the 500mV threshold, or the internal timer

forces another DL turn-on.

Because the period between on-times is limited to a

maximum value, a minimum operating frequency is main-

tained. Figure 5 shows ultrasonic power-save operation.

FB Ripple

Voltage (VFB)

FB threshold

(500mV)

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 5 — 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 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 then turn off the low-

side MOSFET on each subsequent cycle, provided that the

current crosses zero. After the low-side MOSFET is off,

both high-side and low-sides MOSFETs remain off until V

drops to the 500mV threshold. While the MOSFETs are off

the load is supplied by the output capacitor. If the induc-

tor 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.

FB Ripple

Voltage

(VFB)

FB threshold

Inductor

Current

Zero (0A)

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 inductor current reaches zero.

Dead time varies

according to load

Figure 6 — Power-Save Operation

Applications Information (continued)

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