RT7253A
Application Information
The RT7253A is a synchronous high voltage buck
converter
that can support the input voltage range from 4.5V to 17V
and the output current can be up to 3A.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense
the output voltage as shown in Figure 1.
V
OUT
Soft-Start
The RT7253A contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
can be programmed by the external capacitor between
SS pin and GND. The chip provides a 6μA charge current
for the external capacitor. If 0.1μF capacitor is used to
set the soft-start, the period will be 13.5ms(typ.).
Chip Enable Operation
R1
FB
RT7253A
GND
R2
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT7253A quiescent current drops to lower than
3μA. Driving the EN pin high (>2.7V, < 5.5V) will turn on
the device again. For external timing control (e.g.RC),
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 5).
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 3. 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
Chip Enable
Q1
8 SS
C
SS
4,
9 (Exposed Pad) GND
FB 5
COMP
6
C
C
C
P
R
C
R2
C
IN
7 EN
2
VIN
BOOT
1
C
BOOT
L
R1
C
OUT
V
OUT
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
V
OUT
= V
FB
⎛
1
+
R1
⎞
⎜
⎟
⎝
R2
⎠
where V
FB
is the feedback reference voltage (0.8V 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
RT7253A. Note that the external boot voltage must be
lower than 5.5V.
5V
RT7253A
SW 3
Figure 3. Enable Control Circuit for Logic Control with
Low Voltage
To prevent enabling circuit when V
IN
is smaller than the
V
OUT
target value, a resistive voltage divider can be placed
between the input voltage and ground and connected to
the EN pin to adjust IC lockout threshold, as shown in
Figure 4. For example, if an 8V output voltage is regulated
from a 12V input voltage, the resistor R
EN2
can be selected
to set input lockout threshold larger than 8V.
BOOT
RT7253A
SW
100nF
Figure 2. External Bootstrap Diode
DS7253A-02 March 2011
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
