SC3102 datasheet(12/17 Pages) SEMTECH

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SC3102

Applications Information (continued)

Detailed Description

The SC3102 is a synchronous step-down Pulse Width

Modulated (PWM), DC-DC converter utilizing a 1.5MHz

fixed-frequency voltage mode architecture. The device is

designed to operate in fixed-frequency PWM mode and

has the option to enter forced power save mode (PSAVE) at

light loads to improve efficiency. The switching frequency

is chosen to minimize the size of the external inductor

and capacitors while maintaining high efficiency.

Operation

During normal operation, the PMOS MOSFET is activated

on each rising edge of the internal oscillator. The period

is set by the onboard oscillator when in PWM mode. The

device has an internal synchronous NMOS rectifier and

does not require a Schottky diode on the LX pin. The

device operates as a buck converter in PWM mode with

a fixed frequency of 1.5MHz at medium to high loads.

The MODE input is used to select between forced PWM

and forced PSAVE modes. To improve the efficiency at

light loads, the MODE pin can be set low to force PSAVE

operation. When the MODE pin is held high, the device

operates in forced continuous PWM mode regardless of

the output load condition.

Forced Power Save Mode Operation

Connect the MODE pin to ground to force PSAVE mode.

The maximum load current supported in forced PSAVE

mode is 350mA.

Operation in PSAVE:

When the MODE pin toggles low, SC3102 operates in

PSAVE mode after waiting for 64 switching cycles. Figure

1 shows the operating conditions in this mode. When the

output current is less than 350mA, the switching fre-

quency depends upon the load, and the output voltage

does not fall below its regulation threshold. When the

output current is higher than 350mA, the switching fre-

quency depends upon the zero current crossing timing

and the output voltage droops below its regulation

threshold. The output voltage recovers when the load

current is less than 350mA.

Figure 1 — Operation Conditions in PSAVE

Exiting from PSAVE:

Figure 2 shows the case of no change in the output current

and the MODE pin toggling from low to high. SC3102

enters PWM mode at the end of the PSAVE cycle, where

the output voltage crosses the lower PSAVE threshold.

Figure 2 — Exiting PSAVE at Light Load

Figure 3 shows the behavior when there is a step increase

in output current right after the MODE pin toggles high.

The output voltage decreases initially due to the output

capacitor supplying the load current. SC3102 changes the

operation to PWM mode at t

1 and recovers the output

voltage.

Load Current

Vout

Inductor Current

Zero-crossing

Lower Threshold

Upper Threshold

MODE = Low

Vout

Inductor Current

Load Current

MODE = High

Mode change request

Forced PWM mode

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