SC403 datasheet(18/32 Pages) SEMTECH

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SC403

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 PSAVE operation.

FB Ripple

Voltage

(VFB)

FB threshold

(750mV)

Inductor

Current

(0A)

Minimum fSW ~ 20kHz

DH On-Time is triggered when

VFB reaches the FB Threshold

On-time

(TON)

50µsec time-out

After the 50µsec time-out, DL drives high if VFB

has not reached the FB threshold.

Figure 5 — Ultrasonic PSAVE Operation

Smart PSAVE Protection

Active loads may leak current from a higher voltage into

the switcher output. Under light load conditions with

PSAVE enabled, this can force V

OUT to slowly rise and reach

the over-voltage threshold, resulting in a hard shutdown.

Smart PSAVE prevents this condition. When the FB voltage

exceeds 10% above nominal, the device immediately dis-

ables PSAVE, and DL drives high to turn on the low-side

MOSFET. This draws current from V

OUT through the induc-

tor 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. It also minimizes operating

power by avoiding forced conduction mode operation.

Figure 6 shows typical waveforms for the Smart PSAVE

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 (825mV)

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 6 — Smart PSAVE

SmartDriveTM

For each DH pulse the DH driver initially turns on the high-

side MOSFET at a lower speed, allowing a softer, smooth

turn-off of the low-side diode. Once the diode is off and

the LX voltage has risen 1V above PGND, the SmartDrive

circuit automatically drives the high-side MOSFET on at a

rapid rate. This technique reduces switching power loss

while maintaining high efficiency and also avoids the

need for snubbers or series resistors in the gate drive.

Current Limit Protection

Programmable current limiting is accomplished by using

the RDS

ON of the lower MOSFET for current sensing. The

current limit is set by the R

ILIM resistor. The RILIM resistor con-

nects from the ILIM pin to the LXS 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 nega-

tive and current limit will activate. 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 7.

Applications Information (continued)

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