RT7257A
Application Information
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
V
OUT
Soft-Start
The RT7257A provides soft-start function. The soft-start
function is used to prevent large inrush current while
converter is being powered-up. The soft-start timing can
be programmed by the external capacitor between SS and
R1
FB
GND. An internal current source I
SS
(6μA) charges an
external capacitor to build a soft-start ramp voltage. The
V
FB
voltage will track the internal ramp voltage during soft-
start interval. The typical soft start time is calculated as
follows :
RT7257A
GND
R2
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
V
OUT
= V
REF
⎛
1
+
R1
⎞
⎜
⎟
⎝
R2
⎠
Where V
REF
is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65% .The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT7257A. Note that the external boot voltage must be
lower than 5.5V
5V
Soft-Start time t
SS
=
0.8
×
C
SS
, if C
SS
capacitor
I
SS
0.8
×
0.1
μ
≒
13.5ms
6
μ
is 0.1
μ
F, then soft-start time =
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7257A quiescent current drops to lower than
3μA. Driving the EN pin high (>2.5V, < 18V) will turn on
the device again. For external timing control, the EN pin
can also be externally pulled high by adding a R
EN
resistor
and C
EN
capacitor from the VIN pin (see Figure 3).
EN
V
IN
R
EN
EN
RT7257A
GND
C
EN
Figure 3. Enable Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2.5V
is available, as shown in Figure 4. In this case, a 100kΩ
pull-up resistor, R
EN
, is connected between V
IN
and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
V
IN
R
EN
100k
Q1
EN
RT7257A
GND
BOOT
RT7257A
SW
100nF
Figure 2. External Bootstrap Diode
EN
Figure 4. Digital Enable Control Circuit
Copyright
©
2012 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257A-02
September 2012
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
