SC173 Datasheet, PDF(12/27 Page) Semtech Corporation – 3A EcoSpeedTM Synchronous Step-Down Regulator with Optional Ultrasonic Power Save

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SC173 Datasheet, PDF (12/27 Pages) Semtech Corporation – 3A EcoSpeedTM Synchronous Step-Down Regulator with Optional Ultrasonic Power Save

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SC173

Applications Information (continued)

PWM

REF +

VOUT

VIN On-Shot Timing

Generator

RTON

Time = K x VOUT/VIN

Hi-Side

and

Lo-Side

Gate

Drivers

VIN

LPF VOUT

VLX

Q2 ESR

COUT +

Figure 2 â On-Time Generation

The SC173 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.

RTON

25pF â fSW

VOUT Voltage Selection

The switcher output voltage is regulated by comparing

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

the internal 750mV reference voltage, see Figure 3.

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 VDD. 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 VDD),

the switching regulator turns on with ultrasonic power-

save enabled. The SC173 ultrasonic power-save opera-

tion maintains a minimum switching frequency of 25kHz,

for applications with stringent audio requirements.

Forced Continuous Mode Operation

The SC173 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.

VOUT

To FB pin

Figure 3 â Output Voltage Selection

Note that this control method regulates the valley of the

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

voltage VOUT is offset by the output ripple according to

the following equation.

VOUT

0.75V

â ï£¬ï£¬ï£ï£«1+

ï£·ï£·ï£¸ï£¶

VRIPPLE

Figure 4 â Forced Continuous Mode Operation

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