RT8253B RICHTEK [Richtek Technology Corporation], RT8253B Datasheet - Page 9

• Manufacturer Part Number: RT8253B
• Description: 3A, 23V, 1.2MHz Synchronous Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Application Information
The RT8253B is a synchronous high voltage buck converter
that can support an input voltage range from 4.5V to
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.
The output voltage is set by an external resistive voltage
divider according to the following equation :
Where V
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
cycle 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
RT8253B. Note that the external boot voltage must be
lower than 5.5V.
DS8253B-02 March 2011
V
OUT
= V
FB
FB
is the feedback reference voltage (0.8V typ.).
Figure 1. Output Voltage Setting
⎛
⎜
⎝
1
+
Chip Enable
R2
R1
RT8253B
⎞
⎟
⎠
GND
V
IN
FB
Figure 3. Enable Control Circuit for Logic Control with Low Voltage
C
100k
Q1
R
SS
EN
V
OUT
9 (Exposed Pad)
R1
R2
C
IN
4,
2
7
8
VIN
EN
SS
GND
RT8253B
23V
COMP
BOOT
SW
FB
Soft-Start
The RT8253B 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, it's period will be 13.5ms(typ.).
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 RT8253B quiescent current will drop below 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
resistor and C
(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 100 kΩ
pull-up resistor, REN, is connected between V
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
6
1
3
5
C
Figure 2. External Bootstrap Diode
C
C
C
BOOT
P
R
C
RT8253B
EN
L
* capacitor from the VIN pin
BOOT
SW
R1
R2
5V
C
OUT
RT8253B
100nF
V
OUT
www.richtek.com
IN
and the
EN
9
*