RT7255

Copyright © 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS7255-00

June

2015

www.richtek.com

13

The amplitude of the capacitive soar is a function of the

load step, the output capacitor value, the inductor value

and the output voltage :

2

OUT

SOAR

OUT

OUT

L ( I

)

V

=

2 C

V





Output Capacitors Stability Criteria

internal virtual inductor current ramp and other

compensation

that

ensures

stability

with

any

reasonable output capacitor. The internal ramp allows

the IC to operate with very low ESR capacitors and the

IC is stable with very small capacitances. Therefore,

output capacitor selection is nearly always a matter of

meeting output voltage ripple and transient response

requirements, as discussed in the previous sections.

For the sake of the unusual application where ripple

voltage is unimportant.

Any ESR in the output capacitor lowers the required

minimum output capacitance, sometimes considerably.

For the rare application where that is needed and

useful, the equation including ESR is given here :

OUT

OUT

SW

IN

ESR

OUT

V

C

2 f

V

(R

13647 L V

)











 

As can be seen, setting RESR to zero and simplifying

the equation yields the previous simpler equation. To

allow for the capacitor's temperature and bias voltage

coefficients,

use

at

least

double the

calculated

capacitance and use a good quality dielectric such as

X5R or X7R with an adequate voltage rating since

ceramic capacitors exhibit considerable capacitance

reduction as their bias voltage increases.

The RT7255 is optimized for ceramic output capacitors

and for low duty cycle applications. However for

high-output voltages, with high feedback attenuation,

the circuit's response becomes over-damped and

transient response can be slowed. In high-output

improved by adding a small “feed-forward” capacitor

increase the circuit's Q and reduce damping to speed

up the transient

response

without

affecting the

steady-state stability of the circuit. Choose a suitable

capacitor value that following below step.



Get the BW the quickest method to do transient

response form no load to full load. Confirm the

damping frequency. The damping frequency is BW.

RT7255

GND

FB

R1

R2

BW



ff

1

C

2 3.1412 R1 BW 0.8











External Soft-Start (SS)

The RT7255 provides soft-start function. The soft-start

function is used to prevent large inrush current while

converter is being powered-up. The soft-start timing

(6

A) charges an external capacitor to build a soft-start

ramp voltage during soft-start interval. The typical soft

start time is calculated as follows :

The available capacitance range is from 2.7nF to

120nF. Do not leave SS unconnected

Enable Operation (EN)

For automatic start-up the high-voltage EN pin can be

hysteresis band makes EN useful for simple delay and