SC418 datasheet(15/30 Pages) SEMTECH

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SC418

maintain the desired on-time; on-time is calculated

according to the voltage at the VOUT pin.

OUT Voltage Selection

The switcher output voltage is regulated by comparing

OUT as seen through a resistor divider at the FB pin to the

internal 500mV reference voltage (see Figure 3).

To FB pin

VOUT

Figure 3 — Output Voltage Selection

Note that this control method regulates the valley of the

output ripple voltage, not the DC value. The DC value of

OUT is offset by the output ripple according to the follow-

ing equation.

RIPPLE

OUT

Forced Continuous Mode Operation

The SC418 operates the switcher in Forced Continuous

Mode (FCM) by connecting the PSV pin to VDDA. (The PSV

pin should never exceed the VDDA supply.) See Figure 4

for FCM waveforms. In this mode one of the power

MOSFETs is always on, with no intentional dead time other

than to avoid cross-conduction. This results in more

uniform frequency across the full load range with the

trade-off being reduced efficiency at light loads due to

the high-frequency 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

Programmable Ultrasonic Power-Save Operation

The device provides programmable ultrasonic power-save

operation at light loads; the minimum operating fre-

quency is programmed by connecting a resistor from PSV

to AGND. The SC418 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 V

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.

Applications Information (continued)

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