RT8253B Datasheet, PDF (9/15 Pages) Richtek Technology Corporation – 3A, 23V, 1.2MHz Synchronous Step-Down Converter

RT8253B
Application Information
The RT8253B is a synchronous high voltage buck converter
that can support an input voltage range from 4.5V to 23V
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
5V
BOOT
RT8253B
SW
100nF
Figure 2. External Bootstrap Diode
R1
Soft-Start
FB
RT8253B
R2
GND
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
Figure 1. Output Voltage Setting
SS pin and GND. The chip provides a 6μA charge current
for the external capacitor. If 0.1μF capacitor is used to
The output voltage is set by an external resistive voltage
set the soft-start, it's period will be 13.5ms(typ.).
divider according to the following equation :
Chip Enable Operation
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
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
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_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 100 kΩ
pull-up resistor, REN, is connected between V_IN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
lower than 5.5V.
V_IN
Chip Enable
2 _VIN
BOOT ¹
R_EN
C_IN
RT8253B
100k
SW ³
7 _EN
Q1
8 _SS
C_SS
4,
9 (Exposed Pad)
GND
FB ⁵
COMP ⁶
C_{BOOT L}
V_OUT
R1
C_OUT
C_{C RC}
R2
C_P
Figure 3. Enable Control Circuit for Logic Control with Low Voltage
DS8253B-02 March 2011
www.richtek.com
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