RT7255
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor value
and the output voltage :
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.
2
L(IOUT
)
VSOAR
=
2COUT 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.
BW
V
OUT
R1
C
ff
FB
RT7255
GND
R2
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 :
Figure 1. Cff Capacitor Setting
Cff can be calculated base on below equation :
V
ESR
OUT
13647LV
C
OUT
1
2f V (R
)
SW
IN
OUT
Cff
23.1412R1BW0.8
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.
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
(6A) 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 :
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 “feed-forward” 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
Soft-Start time tSS = CSS x 0.6 / 6A
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 100k resistor. Its large
hysteresis band makes EN useful for simple delay and
timing circuits. EN can be externally pulled to VIN by
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7255-00 June 2015
www.richtek.com
13
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