RT7278 datasheet(10/14 Pages) RICHTEK

10 / 14 page

RT7278

DS7278-00 January 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

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.

Application Information

The RT7278 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 3A. 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 timer 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 RT7278 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 RT7278 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 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 RT7278's quiescent current drops to lower than

μA. Driving the EN pin high (>2V, <18V) will turn on the

Figure 1. External Timing Control

An external MOSFET can be added to implement digital

control on the EN pin when the EN pin input voltage is

lower than 2V, as shown in Figure 2. In this case, a 100k

pull-up resistor, REN, is connected between VIN and the

EN pin. MOSFET Q1 will be under logic control to pull

down the EN pin.

Figure 2. Digital Enable Control Circuit

Figure 3. Resistor Divider for Lockout Threshold Setting

device again. For external timing control, the EN pin can

also be externally pulled high by adding a REN resistor

and CEN capacitor from the VIN pin (see Figure 1).

RT7278

GND

VIN

REN

CEN

RT7278

GND

100k

VIN

REN

RT7278

GND

VIN

REN1

REN2

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