RT6236A/B
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.
Enable Operation (EN)
For automatic start-up the low-voltage 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.
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.
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 VINand the ENpin. MOSFET Q1 will
be under logic control to pull down the ENpin. 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).
BW
V
OUT
R1
C
ff
FB
RT6236A/B
R2
EN
R
EN
GND
V
IN
EN
RT6236A/B
C
EN
Figure 1. Cff Capacitor Setting
GND
Cff can be calculated base on below equation :
Figure 2. External Timing Control
1
Cff
23.1412R1BW 0.8
R
EN
100k
V
EN
RT6236A/B
GND
IN
Soft-Start (SS)
Q1
Enable
The RT6236A/B soft-start uses an external capacitor at
SS to adjust the soft-start timing according to the following
equation :
Figure 3. Digital Enable Control Circuit
CSS nF 0.7V
t ms
ISS μA
R
EN1
Following below equation to get the minimum capacitance
range in order to avoid UV occur.
V
IN
EN
R
EN2
RT6236A/B
GND
C
V
0.61.2
OUT
OUT
T
(I
Load Current)0.8
LIM
T6μA
V
REF
C
SS
Figure 4. ResistorDivider for Lockout Threshold Setting
Do not leave SS unconnected.
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
18
DS6236A/B-03 January 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
