RT7272A
Application Information
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
V
OUT
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 RT7272A quiescent current drops to lower than
3μA. Driving the EN pin high (>2.5V, < 18V) will turn on
R1
FB
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
RT7272A
GND
RT7272A
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 and SW pins. 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
RT7272A. Note that the external boot voltage must be
lower than 5.5V
5V
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.
R
EN
100k
Q1
V
IN
EN
EN
RT7272A
GND
Figure 4. Digital Enable Control Circuit
Under Voltage Protection
Hiccup Mode
The RT7272A provides Hiccup Mode Under Voltage
Protection (UVP). When the V
FB
voltage drops below 0.4V,
BOOT
RT7272A
SW
100nF
Figure 2. External Bootstrap Diode
the UVP function will be triggered to shut down switching
operation. If the UVP condition remains for a period, the
RT7272A will retry automatically. When the UVP condition
is removed, the converter will resume operation. The UVP
is disabled during soft-start period.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7272A-01 January 2013
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
