SiC461, SiC462, SiC463, SiC464

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Vishay Siliconix

S18-0393-Rev. K, 16-Apr-18

7

Document Number: 65124

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

pulse is generated for a fixed time. During the on-time pulse,

the high side MOSFET will be turned on. Once the on-time

pulse expires, the low side MOSFET will be turned on after

a dead time period. The low side MOSFET will stay on for a

repeated.

Fig. 6 illustrates the basic block diagram for voltage mode,

constant on time architecture with external ripple injection,

Fig. 5 - SiC46x Operational Principle

The need for ripple injection in this architecture is explained

below. First, let us understand the basic principles of this

control architecture:

• The reference of a basic voltage mode COT regulator

is replaced with a high gain error amplifier loop. The loop

ensures the DC component of the output voltage follows

the

internal

accurate

reference

voltage,

providing

excellent regulation

scheme

ensures

rapid

correction

of

the

transient

perturbation

• This establishes two voltage loops, one is the steady state

voltage feedback path (via the FB pin) and the other is the

user the fast transient response of a COT regulator and

the stable, jitter free, line and load regulation performance

of a PWM controller

Choosing the Ripple Injection Component Values

For stability purposes the SiC46x requires adequate ripple

injection amplitude. Adequate ripple amplitude is required

for two main reasons:

1. To reduce jitter due to noise coupled into the system

2. To provide stable operation. Sub harmonic oscillation

can occur with constant on time ripple control if below

condition is not met

Therefore, when the converter design uses an all ceramic

output capacitor or other low ESR output capacitors,

enough to avoid instability or noise sensitivity but not too

large that it degrades transient performance. To ensure

stable operation under CCM, DCM and ultrasonic mode,

900 mV is recommended so as not to degrade transient

response.

Fig. 6 - SiC46x Control Block Diagram

Equation 1

switching frequency is changed.

frequency is typically fixed, while the converter may be

value, should take into account the maximum power

dissipation at the expected operating conditions.

where

of a 0603 sized resistor is typically 25 mW. Power

dissipation derating must be taken into account for high

ambient temperatures

calculated above

and

Please

use

the

follow

formula:

This procedure allows for a maximum range of operation. In

compensation

components,

a

calculator

is

provided

Fixed on-time

V

RAMP

V

COMP

PWM

ESR

C

OUT



t

ON

2

---------



C

x

R

x

L

C

y

EA

Ripple

based

controller

R1

R2

REF

R

COMP

C

COMP

Load

Q1

Q2

V

IN

C

OUT

C

IN

V

RAMP

V

IN

V

OUT

–

 V

OUT



V

IN

f

sw



C

x



R

x





------------------------------------------------------

=

R

x

V

IN_MAX.

V

OUT

1D

–







P

RX_MAX.

--------------------------------------------------------------------

=

C

X_MIN.

P

RX_MAX.

V

IN_MAX.

f

sw

V

RAMP_MAX.





---------------------------------------------------------------------------

=