RT8292A Datasheet, PDF (9/14 Pages) Richtek Technology Corporation – 2A, 23V, 340kHz Synchronous Step-Down Converter

RT8292A
Application Information
The RT8292A is a synchronous high voltage buck converter
that can support the input voltage range from 4.5V to 23V
and the output current can be up to 2A.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense
the output voltage as shown in Figure 1.
V_OUT
R1
FB
RT8292A
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
Soft-Start
The RT8292A 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
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT8292A 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).
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
RT8292A. Note that the external boot voltage must be
lower than 5.5V.
5V
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
Chip Enable
R_EN
100k
2 _VIN
BOOT ¹
C_IN
RT8292A
7 _EN
SW ³
C_BOOT
L
Q1
8 _SS
C_SS
4,
9 (Exposed Pad)GND
FB ⁵
C_C
COMP ⁶
R_C
C_P
V_OUT
R1 _COUT
R2
Figure 3. Enable Control Circuit for Logic Control with
Low Voltage
BOOT
RT8292A
SW
100nF
Figure 2. External Bootstrap Diode
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.
DS8292A-03 March 2011
www.richtek.com
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