RT7232 datasheet(12/18 Pages) RICHTEK

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RT7231/32/33/34

DS7231/32/33/34-00

March 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

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 2A. It adopts

ACOTTM mode control to provide a very fast transient

response with few external compensation components.

PWM Operation

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.

Advanced Constant On-Time Control

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

700kHz over input voltage range and loading range.

Soft-Start

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 2

μA

charge current for the external capacitor. If a 3.9nF

capacitor is used, the soft-start will be 2.6ms (typ.). The

available capacitance range is from 2.7nF to 220nF.

C5 (nF) 1.365

(ms) =

( A)

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 RT7231/32/33/34's quiescent current drops to

lower than 10

μA. Driving the EN pin high (>1.6V, <18V)

will turn on the device again. For external timing control,

Figure 3. Resistor Divider for Lockout Threshold Setting

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.

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 100k

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.

Figure 2. Digital Enable Control Circuit

the EN pin can also be externally pulled high by adding a

REN resistor and CEN capacitor from the VIN pin (see Figure

1).

RT7231/32/33/34

GND

VIN

REN

CEN

GND

100k

VIN

REN

RT7231/32/33/34

GND

VIN

REN1

REN2

RT7231/32/33/34

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