RT7272B
Application Information
Output Voltage Setting
Chip Enable Operation
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7272B 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).
V
OUT
R1
FB
RT7272B
GND
R2
EN
R
EN
V
IN
EN
RT7272B
Figure 1. Output Voltage Setting
C
EN
GND
The output voltage is set by an external resistive voltage
divider according to the following equation :
Figure 3. Enable Timing Control
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = VREF 1+
⎜
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.
Where VREF is the reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. This capacitor provides the gate driver
voltage for the high side MOSFET.
R
EN
100k
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
RT7272B. Note that the external boot voltage must be
lower than 5.5V
V
EN
RT7272B
GND
IN
Q1
EN
Figure 4. Digital Enable Control Circuit
Under Voltage Protection
5V
Hiccup Mode
The RT7272B provides Hiccup Mode Under Voltage
Protection (UVP). When the VFB voltage drops below 0.4V,
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT7272B will retry automatically. When the UVP condition
is removed, the converter will resume operation. The UVP
is disabled during soft-start period.
BOOT
100nF
RT7272B
SW
Figure 2. External Bootstrap Diode
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS7272B-01 January 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
