SC174 データシートの表示（PDF） - Semtech Corporation

部品番号

コンポーネント説明

メーカー

SC174

4A EcoSpeed™ Synchronous Step-Down Regulator with Optional Ultrasonic Power Save

Semtech Corporation 

SC174

Applications Information

SC174 Synchronous Buck Converter

The SC174 is a step down synchronous buck dc-dc regu-

lator. The SC174 is capable of 4A operation at very high

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

mable operating frequency range of 200KHz – 1MHz

enables the user to optimize the solution for minimum

board space and optimum efficiency.

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 SC174 can operate with an input voltage ranging

from 3V to 5.5V.

Psuedo-fixed Frequency Adaptive On-time Control

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

VOUT 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 VOUT for the

present VIN 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

The PWM control method used by the SC174 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.

Q1

Q2

VIN

TON

CIN

VFB

L

VLX

ESR

VLX

FB threshold

VOUT

One-Shot Timer and Operating Frequency

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

FB Comparator output goes high when VFB 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 VOUT 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 VIN. When the capacitor

voltage reaches VOUT, the on-time is completed and the

high-side MOSFET turns off.

COUT

+

FB

This method automatically produces an on-time that is

proportional to VOUT and inversely proportional to VIN.

Under steady-state operation conditions, the switching

frequency can be determined from the on-time by the

following equation.

Figure 1 — PWM Control Method, VOUT Ripple

fSW

=

VOUT

TON × VIN

© 2010 Semtech Corporation

11

Share Link: