SC173AEVB datasheet(11/26 Pages) SEMTECH

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Applications Information

SC173A Synchronous Buck Converter

The SC173A is a step down synchronous buck dc-dc regu-

lator. The SC173A is capable of 3A operation at very high

efficiency in a tiny 3x3-10 pin package. The programma-

ble operating frequency range of 200kHz – 1MHz (con-

tinuous conduction mode) enables the user to optimize

the solution for minimum board space and optimum ef-

ficiency.

The buck regulator employs pseudo-fixed frequency

adaptive on-time control. This control scheme allows

fast transient response thereby lowering the size of the

power components used in the system.

Input Voltage Range

The SC173A can operate with an input voltage ranging

from 3V to 5.5V.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC173A is pseudo-

fixed frequency, adaptive on-time, as shown in Figure 1.

The ripple voltage generated at the output capacitor ESR

is used as a PWM ramp signal. This ripple is used to trig-

ger the on-time of the controller.

The adaptive on-time is determined by an internal one-

shot timer. When the one-shot is triggered by the out-

put ripple, the device sends a single on-time pulse to the

high-side MOSFET. The pulse period is determined by

OUT and VIN; the period is proportional to output voltage

and inversely proportional to input voltage. With this

adaptive on-time arrangement, the device automatically

anticipates the on-time needed to regulate V

OUT for the

present V

IN condition and at the selected frequency.

The advantages of adaptive on-time control are:

Predictable operating frequency compared to

other variable frequency methods.

Reduced component count by eliminating

the error amplifier and compensation compo-

nents.

Reduced component count by removing the

need to sense and control inductor current.

Fast transient response — the response time

is controlled by a fast comparator instead of a

typically slow error amplifier.

Reduced output capacitance due to fast tran-

sient response

One-Shot Timer and Operating Frequency

The one-shot timer operates as shown in Figure 2. The

FB Comparator output goes high when V

FB is less than

the internal 750mV reference. This feeds into the gate

drive and turns on the high-side MOSFET, and also starts

the one-shot timer. The one-shot timer uses an internal

comparator, timing capacitor, and a low pass filter (LPF)

which regenerates V

OUT from LX. One comparator input

is connected to the filtered LX voltage, the other input is

connected to the capacitor. When the on-time begins,

the internal capacitor charges from zero volts through a

current which is proportional to V

IN. When the capacitor

voltage reaches V

OUT, the on-time is completed and the

high-side MOSFET turns off.

This method automatically produces an on-time that is

proportional to V

OUT and inversely proportional to VIN.

Under steady-state operation conditions, the switching

frequency can be determined from the on-time by the

following equation.

•

COUT

VIN

ESR

VLX

CIN

VOUT

TON

VLX

FB threshold

VFB

Figure 1 — PWM Control Method, V

OUT Ripple

OUT

SC173A

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