RT6224A/B
Feed-Forward Capacitor (CFF)
Enable Operation (EN)
The RT6224A/B 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 steady-state stability of the circuit. Choose
a suitable capacitor value that following below step.
For automatic start-up the EN pin can be connected to
VIN, through a 100kΩ resistor. Its large hysteresis band
makes EN useful for simple delay and timing circuits. EN
can be externally pulled to VIN by adding a resistor-
capacitor delay (REN and CEN in Figure 2). Calculate the
delay time using EN's internal threshold where switching
operation begins (1.5V, 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 100kΩ pull-up resistor, REN, is
connected between VIN and the EN pin. The MOSFET
Q1 will be under logic control to pull down the EN pin. To
prevent enabling circuit when VINis 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).
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.
EN
BW
R
EN
V
EN
RT6224A/B
IN
C
EN
GND
V
OUT
Figure 2. External Timing Control
R1
C
FF
FB
RT6224A/B
R2
R
EN
100k
GND
V
EN
RT6224A/B
GND
IN
Q1
Enable
Figure 1. CFF Capacitor Setting
CFF can be calculated base on below equation :
1
CFF
Figure 3. Digital Enable Control Circuit
23.1412R1BW 0.8
Internal Soft-Start (SS)
R
EN1
The RT6224A/B soft-start uses an internal soft-start time
V
IN
EN
800μs.
R
EN2
RT6224A/B
GND
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
COUT VOUT 0.61.2
t
Figure 4. ResistorDivider for Lockout Threshold Setting
(ILIM Load Current)0.8
t 800μs
Copyright 2018 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS6224A/B-02 August 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
