RT6207A/B
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.
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
EN
R1
C
ff
R
EN
V
IN
EN
RT6207A/B
FB
RT6207A/B
R2
C
EN
GND
GND
Figure 1. Cff Capacitor Setting
Figure 2. External Timing Control
Cff can be calculated base on below equation :
R
EN
1
100k
Cff
V
EN
RT6207A/B
GND
IN
23.1412R1BW 0.8
Q1
Enable
Soft-Start (SS)
The RT6207A/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
R
EN1
ISS μA
V
IN
EN
Following below equation to get the minimum capacitance
R
EN2
RT6207A/B
GND
range in order to avoid UV occur.
C
(I
LIM
V
0.61.2
OUT
OUT
T
Load Current)0.8
Figure 4. ResistorDivider for Lockout Threshold Setting
T6μA
V
REF
C
SS
Do not leave SS unconnected.
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
18
DS6207A/B-02 December 2015
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
