SC414EVB datasheet(14/29 Pages) SEMTECH

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SC414/SC424

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

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.

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 configuration, 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

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

Comparator output goes high when V

is less than the

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

OUT

, 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

When the capacitor voltage reaches V

OUT

, the on-time is

completed and the high-side MOSFET turns off.

Gate

Drives

FB Comparator

One-Shot

Timer

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

VOUT

VIN

750mV

COUT

VIN

ESR

VOUT

VLX

RTON

Figure 2 — On-Time Generation

•

Applications Information

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