SC3303 datasheet(14/26 Pages) SEMTECH

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SC3303

FB Ripple

Voltage (VFB)

FB threshold

Inductor

Current

DC Load Current

DH on-time is triggered when

VFB reaches the FB Threshold.

(750mV)

On-time

(TON)

DL drives high when on-time is completed.

DL remains high until VFB falls to the FB threshold.

Figure 4 — Continuous Mode Operation

Ultrasonic Power Save Operation

The SC3303 provides ultrasonic power save operation at

light loads, with the minimum operating frequency fixed

at 22kHz. 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 750mV 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

~22kHz. Figure 5 shows ultrasonic power save operation.

Applications Information (continued)

FB Ripple

Voltage

(VFB)

FB threshold

(750mV)

Inductor

Current

(0A)

Minimum fSW ~ 25kHz

DH On-Time is triggered when

VFB reaches the FB Threshold

On-time

(TON)

40μsec time-out

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

has not reached the FB threshold.

Figure 5 — Ultrasonic 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 (exceeds

825mV), 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

OUT

to fall. When V

drops back to the 750mV trip point,

a normal T

switching cycle begins. This method pre-

vents a hard OVP shutdown and also cycles energy from

OUT

back to V

. Figure 6 shows typical waveforms for the

smart power save feature.

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