RT7255 Datasheet, PDF(13/17 Page) Richtek Technology Corporation

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RT7255 Datasheet, PDF (13/17 Pages) Richtek Technology Corporation – The RT7255 is a synchronous step-down

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The amplitude of the capacitive soar is a function of the

load step, the output capacitor value, the inductor value

and the output voltage :

VSOAR

L ï´(ïIOUT )2

2ï´ COUT ï´ VOUT

Output Capacitors Stability Criteria

The RT7255's ACOTTM control architecture uses an

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 :

COUT

ï³

2ï´ fSW

ï´ VIN

VOUT

ï´(RESR ï«13647ï´L ï´ VOUT )

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.

Feed-forward Capacitor (Cff)

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

voltage circuits (VOUT > 3.3V) transient response is

improved by adding a small âfeed-forwardâ capacitor

(Cff) across the upper FB divider resistor (Figure 1), to

increase the circuit's Q and reduce damping to speed

up the transient response without affecting the

DS7255-00 June 2015

RT7255

steady-state stability of the circuit. Choose a suitable

capacitor value that following below step.

ï 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.

VOUT

RT7255

GND

R1 Cff

Figure 1. Cff Capacitor Setting

ï Cff can be calculated base on below equation :

Cff

ï½

2ï´3.1412ï´R1ï´BW ï´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

can be programmed by the external capacitor CSS

between SS and GND. An internal current source ISS

(6ïA) charges an external capacitor to build a soft-start

ramp voltage. The VFB voltage will track the internal

ramp voltage during soft-start interval. The typical soft

start time is calculated as follows :

Soft-Start time tSS = CSS x 0.6 / 6ïA

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

connected to VIN, through a 100kï resistor. Its large

hysteresis band makes EN useful for simple delay and

timing circuits. EN can be externally pulled to VIN by

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