RT7285C
11
DS7285C-03 July 2014
www.richtek.com
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Feed-forward Capacitor (C
ff
)
The RT7285C 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 (V
OUT
> 3.3V)
transient response is improved by adding a small
“
feed-
forward
”
capacitor (C
ff
) 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
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.
Figure 1. C
ff
Capacitor Setting
C
ff
can be calculated base on below equation :
ff
1
C
2 3.1412 R1 BW 0.8
Enable Operation (EN)
For automatic start-up the EN pin can be connected to
V
IN
, through a 100k
Ω
resistor. Its large hysteresis band
makes EN useful for simple delay and timing circuits. EN
can be externally pulled to V
IN
by adding a resistor-
capacitor delay (R
EN
and C
EN
in Figure 2). Calculate the
delay time using EN's internal threshold where switching
operation begins (1.4V, typical).
An external MOSFET can be added to implement digital
control of EN when no system voltage above 2V is available
(Figure 3). In this case, a 100k
Ω
pull-up resistor, R
EN
, is
connected between VIN and the EN pin. MOSFET Q1 will
be under logic control to pull down the EN pin. To prevent
enabling circuit when VIN is smaller than the VOUT target
value or some other desired voltage level, a resistive voltage
divider can be placed between the input voltage and ground
and connected to EN to create an additional input under
voltage lockout threshold (Figure 4).
Figure 4. Resistor Divider for Lockout Threshold Setting
Figure 2. External Timing Control
Figure 3. Digital Enable Control Circuit
RT7285C
EN
GND
V
IN
R
EN
C
EN
EN
RT7285C
EN
GND
100k
V
IN
R
EN
Q1
Enable
RT7285C
EN
GND
V
IN
R
EN1
R
EN2
RT7285C
GND
FB
R1
R2
V
OUT
C
ff
Internal Soft-Start (SS)
The RT7285C soft-start uses an internal soft-start time
800
μ
s.
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
OUT
OUT
LIM
C
V
0.6 1.2
T
(I
Load Current) 0.8
T 800
μ
s
BW
