RT7248
Application Information
Output Voltage Setting
Soft-Start
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The RT7248 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
GND. An internal current source ISS (6μA) charges an
external capacitor to build a soft-start ramp voltage. The
VFB voltage will track the internal ramp voltage during soft-
start interval. The typical soft start time is calculated as
V
OUT
R1
FB
RT7248
GND
R2
follows :
0.923×C
SS , if CSS capacitor
Figure 1. Output Voltage Setting
Soft-Start time tSS
=
ISS
0.923×0.1μ
is 0.1μF, then soft-start time =
≒ 15.5ms
The output voltage is set by an external resistive voltage
divider according to the following equation :
6μ
R1
R2
⎛
⎝
⎞
⎟
⎠
Chip Enable Operation
VOUT = VREF 1+
⎜
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT7248 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 REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
EN
Where VREF is the reference voltage (0.923V typ.).
External Bootstrap Diode
Connect a 100nF 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
RT7248.Note that the external boot voltage must be lower
than 5.5V
R
EN
V
IN
EN
RT7248
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, REN, is connected between VIN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
5V
BOOT
100nF
RT7248
R
100k
EN
SW
V
IN
EN
RT7248
GND
Q1
EN
Figure 2. External Bootstrap Diode
Figure 4. Digital Enable Control Circuit
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS7248-01 September 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
