RT7231/32/33/34
Application Information
The RT7231/32/33/34 is a synchronous high voltage Buck
converter that can support the input voltage range from
4.5V to 18V and the output current up to 4A. It adopts
ACOTTM mode control to provide a very fast transient
response with few external compensation components.
the ENpin can also be externally pulled high by adding a
REN resistor and CEN capacitor from the VINpin (see Figure
1).
EN
R
EN
V
IN
EN
RT7231/32/33/34
C
EN
PWM Operation
GND
It is suitable for low external component count
configuration with appropriate amount of Equivalent Series
Resistance (ESR) capacitors at the output. The output
ripple valley voltage is monitored at a feedback point
voltage. The synchronous high side MOSFET is turned
on at the beginning of each cycle. After the internal
on-time expires, the MOSFET is turned off. The pulse
width of this on-time is determined by the converter's input
and output voltages to keep the frequency fairly constant
over the entire input voltage range.
Figure 1. External Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2V
is available, as shown in Figure 2. In this case, a 100kΩ
pull-up resistor, REN, is connected between the VIN and
EN pins. MOSFET Q1 will be under logic control to pull
down the EN pin.
R
EN
100k
Advanced Constant On-Time Control
V
IN
EN
The RT7231/32/33/34 has a unique circuit which sets the
on-time by monitoring the input voltage and SW signal.
The circuit ensures the switching frequency operating at
650kHz over input voltage range and loading range.
RT7231/32/33/34
Q1
EN
GND
Figure 2. Digital Enable Control Circuit
Soft-Start
To prevent enabling circuit when VIN is smaller than the
VOUT 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 3. For example, if an 8V output voltage is regulated
from a 12V input voltage, the resistor REN2 can be selected
to set input lockout threshold larger than 8V.
The RT7231/32/33/34 contains an external soft-start clamp
that gradually raises the output voltage. The soft-start
timing can be programmed by the external capacitor
between the SS and GND pins. The chip provides a 6μA
charge current for the external capacitor. If a 3.9nF
capacitor is used, the soft-start will be 0.87ms (typ.).
The available capacitance range is from 2.7nF to 220nF.
R
C5 (nF)1.365
tSS (ms) =
EN1
V
IN
EN
ISS (A)
RT7231/32/33/34
R
EN2
GND
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.85V) will shut down the device. During shutdown
mode, the RT7231/32/33/34's quiescent current drops to
lower than 10μA. Driving the EN pin high (>1.25V, <18V)
will turn on the device again. For external timing control,
Figure 3. ResistorDivider for Lockout Threshold Setting
Copyright 2016 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
www.richtek.com
12
DS7231/32/33/34-09 October 2016
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
