SC417MLTRT datasheet(13/29 Pages) SEMTECH

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SC417/SC427

Synchronous Buck Converter

The SC417/SC427 is a step down synchronous DC-DC buck

converter with integrated power MOSFETs and a program-

mable LDO. The device is capable of 10A operation at very

high efficiency. A space saving 5x5 (mm) 32-pin package

is used. The programmable operating frequency range of

200kHz to 1MHz enables optimizing the configuration for

PCB area and efficiency.

The buck controller uses a pseudo-fixed frequency adap-

tive on-time control. This control method allows fast tran-

sient response which permits the use of smaller output

capacitors.

Input Voltage Requirements

The SC417/SC427 requires two input supplies for normal

operation: VIN and V5V. VIN operates over the wide range

from 3V to 28V. V5V requires a 5V supply input that can be

an external source or the internal LDO configured to

supply 5V.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC417/SC427 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 trigger the on-time of the controller.

COUT

ESR

CIN

VOUT

FB Threshold

VFB

VLX

TON

VIN

Figure 1 — PWM Control Method, V

OUT Ripple

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

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

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

side MOSFET. The pulse period is determined by V

OUT

and

; the period is proportional to output voltage and

inversely proportional to input voltage. With this adaptive

on-time arrangement, the device automatically antici-

pates the on-time needed to regulate V

OUT

for the present

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 components.

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 typi-

cally 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

is less than the

internal 500mV 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 com-

parator and a capacitor. One comparator input is con-

nected to V

OUT

, the other input is connected to the

capacitor. When the on-time begins, the internal capaci-

tor charges from zero volts through a current which is

proportional to V

. When the capacitor voltage reaches

OUT

, the on-time is completed and the high-side MOSFET

turns off.

•

Applications Information

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