SC174 datasheet(12/27 Pages) SEMTECH

12 / 27 page

Hi-Side

and

Lo-Side

Gate

Drivers

PWM

On-Shot Timing

Generator

Time = K x VOUT/VIN

VOUT

VIN

REF

COUT

VIN

ESR

VOUT

VLX

LPF

RTON

The SC174 uses an external resistor to set the on-time

which indirectly sets the frequency. The on-time can

be programmed to provide operating frequency from

200kHz to 1MHz using a resistor between the TON pin

and ground. The resistor value is

selected by the fol-

lowing equation.

OUT Voltage Selection

The switcher output voltage is regulated by comparing

OUT as seen through a resistor divider at the FB pin to

the internal 750mV reference voltage, see Figure 3.

Figure 3 — Output Voltage Selection

Note that this control method regulates the valley of the

output ripple voltage, not the DC value. The DC output

voltage V

OUT is offset by the output ripple according to

the following equation.

Figure 2 — On-Time Generation

TON

25pF

⋅

VOUT

To FB pin

0.75V

RIPPLE

OUT













⋅

Enable and Power-save Inputs

The EN/PSV inputs are used to enable or disable the

switching regulator. When EN/PSV is low (grounded),

the switching regulator is off and in its lowest power

state. When off, the output power switches are tri-stat-

ed. When EN/PSV is allowed to float, the pin voltage will

float to 41% of the voltage at V

DD. The switching regula-

tor turns on with power-save disabled and all switching

is in forced continuous mode.

When EN/PSV is high (above 60% of the voltage at V

DD),

the switching regulator turns on with ultrasonic power-

save enabled. The SC174 ultrasonic power-save opera-

tion maintains a minimum switching frequency of 25kHz,

for applications with stringent audio requirements.

Forced Continuous Mode Operation

The SC174 operates in Forced Continuous Mode (FCM)

by floating the EN/PSV pin (see Figure 4). In this mode

one of the power MOSFETs is always on, with no inten-

tional dead time other than to avoid cross-conduction.

This feature results in uniform frequency across the full

load range with the trade-off being poor efficiency at

light loads due to the high-frequency switching of the

MOSFETs.

Figure 4 — Forced Continuous Mode Operation

Applications Information (continued)

SC174

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