SC173EVB datasheet(13/27 Pages) SEMTECH

13 / 27 page

Applications Information (continued)

Ultrasonic Power Save Operation

The SC173 provides ultrasonic 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

OUT through the inductor, forcing both VOUT and VFB to

fall. When V

FB drops to the 750mV threshold, the next DH

on-time is triggered. After the on-time is completed the

high-side MOSFET is turned off and the low-side MOS-

FET 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 ultrasonic power save opera-

tion.

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

shutdown. Smart power save prevents this condition.

When the FB voltage exceeds 10% above nominal (ex-

ceeds 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 V

OUT to fall. When VFB drops back to the 750mV trip

point, a normal T

ON switching cycle begins. This method

prevents a hard OVP shutdown and also cycles energy

from V

OUT back to VIN. Figure 6 shows typical waveforms

for the Smart Power Save feature.

Figure 6 — Smart Power Save

Current Limit Protection

The device features fixed current limiting, which is ac-

complished by using the R

DS(ON) of the lower MOSFET for

current sensing. While the low-side MOSFET is on, the

inductor current flows through it and creates a voltage

across the R

DS(ON). During this time, the voltage across the

MOSFET is negative with respect to ground. During this

time, If this MOSFET voltage drop exceeds the internal

reference voltage, the current limit will activate. The cur-

rent limit then keeps the low-side MOSFET on and will

not allow another high side on-time, until the current in

the low-side MOSFET reduces enough to drop below the

internal reference voltage once more. This method regu-

lates the inductor valley current at the level shown by I

LIM

in Figure 7.

SC173

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