RT8282B RICHTEK [Richtek Technology Corporation], RT8282B Datasheet - Page 9

• Manufacturer Part Number: RT8282B
• Description: 2A, 23V, 1.2MHz Synchronous Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Application Information
The RT8282B is a synchronous high voltage buck converter
that can support the input voltage range from 4.5V to
and the output current can be up to 2A.
Output Voltage Setting
The resistive 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
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
RT8282B. Note that the external boot voltage must be
lower than 5.5V.
DS8282B-01 March 2011
V
OUT
= V
FB
FB
Figure 2. External Bootstrap Diode
is the feedback reference voltage (0.8V typ.).
Figure 1. Output Voltage Setting
⎛
⎜
⎝
1
+
R2
R1
RT8282B
RT8282B
⎞
⎟
⎠
BOOT
GND
SW
FB
V
5V
OUT
100nF
R1
R2
23V
Chip Enable
Soft-Start
The RT8282B 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 RT8282B quiescent current will drops 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
(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, REN, is connected between V
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
To prevent enabling circuit when V
V
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
to set input lockout threshold larger than 8V.
OUT
Figure 3. Enable Control Circuit for Logic Control with
EN
* resistor and C
V
target value, a resistive voltage divider can be placed
IN
C
100k
SS
9 (Exposed Pad)
R
Q1
EN
C
IN
4,
Low Voltage
EN
2
7
8
VIN
EN
SS
GND
* capacitor from the VIN pin
RT8282B
COMP
BOOT
SW
FB
IN
RT8282B
1
3
5
6
is smaller than the
EN2
C
C
www.richtek.com
C
can be selected
C
BOOT
P
R
L
C
IN
and the
R1
R2
V
OUT
C
9
OUT