RT6224D
Feed-Forward Capacitor (CFF)
Enable Operation (EN)
The RT6224D 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.
BW
EN
R
EN
V
IN
EN
RT6224D
C
EN
V
OUT
GND
R1
C
FF
Figure 2. External Timing Control
FB
RT6224D
R2
R
100k
EN
GND
V
EN
RT6224D
GND
IN
Q1
Enable
Figure 1. CFF Capacitor Setting
CFF can be calculated base on below equation :
1
CFF
23.1412R1BW 0.8
Figure 3. Digital Enable Control Circuit
Internal Soft-Start (SS)
The RT6224D soft-start uses an internal soft-start time
R
EN1
V
IN
EN
800μs.
R
EN2
RT6224D
GND
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
COUT VOUT 0.61.2
t
(ILIM Load Current)0.8
Figure 4. ResistorDivider for Lockout Threshold Setting
t 800μs
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS6224D-01 August 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
