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SC414

6A Integrated FET Regulator with 5V LDO

Semtech Corporation 

SC414/SC424

Applications Information

Synchronous Buck Converter

The SC414/SC424 is a step down synchronous DC-DC buck

converter with integrated power MOSFETs and a 5V LDO.

The device is capable of 6A operation at very high effi-

ciency. A space saving 4x4 (mm) 28-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 SC414/SC424 requires two input supplies for normal

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

IN

IN

from 3V to 28V. V5V requires a 3.3 or 5V supply input that

can be an external source or the internal LDO configured

to supply 5V. If the LDO is enabled, V5V voltage must be

> 5V.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC414/SC424 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.

VIN

Q1

VLX

Q2

TON

VLX

CIN

VFB

FB Threshold

L

ESR

+

COUT

VOUT

FB

Figure 1 — PWM Control Method, VOUT 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 and

OUT

V . The period is proportional to output voltage and

IN

inversely proportional to input voltage. With this adaptive

on-time configuration, the device automatically antici-

pates the on-time needed to regulate V for the present

OUT

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 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

One-shot timer operation is shown in Figure 2. The FB

Comparator output goes high when V is less than the

FB

internal 750mV reference. This feeds into the gate drive

and turns on the high-side MOSFET, and starts the one-

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

tor and a capacitor. One comparator input is connected to

V , the other input is connected to the capacitor. When

OUT

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 , the on-time is

OUT

completed and the high-side MOSFET turns off.

FB FB Comparator

-

750mV +

Gate

Drives VIN

DH

Q1

VLX L

VOUT

VIN

One-Shot

Timer

ESR

DL

Q2

COUT

+

RTON On-time = K x RTON x (VOUT/VIN)

Figure 2 — On-Time Generation

VOUT

FB

14

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