SC3203 datasheet(14/20 Pages) SEMTECH

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SC3203

Smart Power Save Protection

Loads or circuits which are connected to more than one

DC source may leak current from a higher voltage into a

lower voltage. If the SC3203 provides the lower voltage

and is operating in PSave mode, this leakage can cause

OUT to slowly rise during the dead-time when both

mosfets are off. If the leakage is high it can drive V

OUT up

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

Smart power save prevents this condition. When the FB

pin exceeds 10% above nominal (exceeds 825mV), the

device immediately disables power-save and turns 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 on-time switching cycle

begins. Typically the device will return to normal power-

save operation.

This method prevents a hard OVP shutdown and also

cycles energy from V

OUT back to VIN. Figure 5 shows typical

Smart PSave operation.

FB threshold

VOUT and VFB rise due to leakage

current flowing into COUT

LX drives low (PGND)

when Sm art PSAVE

threshold is reached

Sm art Power Save

Threshold (825m V)

Single LX on-tim e pulse

when FB reaches FB

threshold

VOUT discharges via inductor

through LX to PGND.

Norm al VOUT ripple

LX drives low (PGND)

after LX on-tim e and

returns to Power Save

LX tri-stated

Figure 5 — Smart Power Save

Current Limit Protection

Current limiting is accomplished by sensing the low-side

mosfet current. If this mosfet current exceeds the Current

Limit value (typically 3.75A), the mosfet is kept on. The

controller will not allow another high-side on-time until the

current in the low-side mosfet falls to 3.75A. This method

controls the inductor valley current as shown by ILIM in

Figure 6.

Applications Information (continued)

Time

IPEAK

ILOAD

ILIM

Figure 6 — Valley Current Limit

Enable Input and VIN UVLO

The EN input is used to enable or disable the switch-

ing regulator. The EN pin has two thresholds. The first

threshold at typically 1V activates the internal LDO. The

second threshold at 1.5V turns on the switcher.

The EN input also features a programmable input Under-

Voltage Lockout (VIN UVLO). A resistor divider to the

EN pin allows the user to select a VIN point at which the

switcher will turn off. The EN circuit is shown in Figure 7.

VIN

REN

1.5VREF

O n/O ff

Hysteresis

100m V

Com parator

Figure 7 — Enable Input Circuit

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