SIC403 datasheet(10/25 Pages) VISHAY

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Document Number: 66550

S11-1638-Rev. B, 15-Aug-11

Vishay Siliconix

SiC403

This document is subject to change without notice.

THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000

APPLICATIONS INFORMATION

SiC403 Synchronous Buck Converter

The SiC403 is a step down synchronous buck DC/DC

converter with integrated power FETs and programmable

LDO. The SiC403 is capable of 6 A operation at very high

efficiency in a tiny 5 mm x 5 mm - 32 pin package. The

programmable operating frequency range of 200 kHz to

1 MHz, enables the user to optimize the solution for minimum

board space and optimum efficiency.

The

buck

controller

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 SiC403 requires two input supplies for normal operation:

VIN and VDD. VIN operates over the wide range from 3 V to

28 V. VDD requires a 5 V supply input that can be an external

source or the internal LDO configured to supply 5 V.

Pseudo-Fixed Frequency Adaptive On-Time Control

The PWM control method used for the SiC403 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.

The adaptive on-time is determined by an internal oneshot

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

device sends a single on-time pulse to the highside

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 adaptive on-time control has significant advantages over

traditional control methods used in the controllers today.

• Reduced component count by eliminating DCR sense or

current sense resistor as no need of a sensing inductor

current.

• Reduced

saves

external

components

used

for

compensation by eliminating the no error amplifier and

other components.

• Ultra fast transient response because of fast loop,

absence of error amplifier speeds up the transient

response.

• Predictable frequency spread because of constant on-time

architecture.

• Fast transient response enables operation with minimum

output capacitance

Overall, superior performance compared to fixed frequency

architectures.

On-Time One-Shot Generator (tON) and Operating

Frequency

The SiC403 have an internal on-time one-shot generator

which is a comparator that has two inputs. The FB

Comparator output goes high when VFB is less than the

internal 750 mV 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 and a

capacitor. One comparator input is connected to VOUT, 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. The figure 2 shows the

on-chip implementation of on-time generation.

This method automatically produces an on-time that is

proportional to VOUT and inversely proportional to VIN.

The SiC403 uses an external resistor to set the ontime which

indirectly

sets

the

frequency.

The

on-time

can

programmed to provide operating frequency from 200 kHz to

1 MHz using a resistor between the tON pin and ground. The

resistor value is selected by the following equation.

Figure 1 - Output Ripple and PWM Control Method

VIN

CIN

VLX

ESR

VLX

tON

VFB

COUT

VOUT

FB threshold

Figure 2 - On-Time Generation

750 mV

VOUT

VIN

Rton

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

FB comparator

One-shot

timer

Gate

drives

ESR

VOUT

COUT

VLX

Rton =

(tON - 10 ns) x VIN

25 pF x VOUT

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