SC417 datasheet(16/29 Pages) SEMTECH

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SC417/SC427

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

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

550mV), 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

fall. When V

drops back to the 500mV trip point, a normal

switching cycle begins. This method prevents a hard

OVP shutdown and also cycles energy from V

OUT

back to

. It also minimizes operating power by avoiding forced

conduction mode operation. Figure 7 shows typical wave-

forms for the Smart Power-save feature.

threshold

High-side

Drive (DH)

Low-side

Drive (DL)

VOUT drifts up to due to leakage

current flowing into COUT

DH and DL off

DL turns on when Smart

PSAVE threshold is reached

Smart Power Save

Threshold (550mV)

DL turns off when FB

threshold is reached

Single DH on-time pulse

after DL turn-off

VOUT discharges via inductor

and low-side MOSFET

Normal DL pulse after DH

on-time pulse

Normal VOUT ripple

Figure 7 — Smart Power-save

Current Limit Protection

The device features programmable current limiting, which

is accomplished by using the RDSON of the lower MOSFET

for current sensing. The current limit is set by R

ILIM

resistor.

The R

ILIM

resistor connects from the ILIM pin to the LX pin

which is also the drain of the low-side MOSFET. When the

low-side MOSFET is on, an internal ~10μA current flows

from the ILIM pin and through the R

ILIM

resistor, creating a

voltage drop across the resistor. While the low-side

MOSFET is on, the inductor current flows through it and

creates a voltage across the RDS

(ON)

. The voltage across the

MOSFET is negative with respect to ground. If this MOSFET

voltage drop exceeds the voltage across R

ILIM

, the voltage

at the ILIM pin will be negative and current limit will acti-

vate. The current 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 bring

the ILIM voltage back up to zero. This method regulates

the inductor valley current at the level shown by ILIM in

Figure 8.

Applications Information (continued)

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