RT7249 datasheet(13/17 Pages) RICHTEK

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RT7249

DS7249-02 March 2017

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

(Buck Converter)

Output Voltage Setting

The resistive divider allows the FB pin to sense the output

voltage as shown in Figure 1.

Figure 1. Output Voltage Setting

OUT

REF

= V









where VREF is the reference voltage (0.8V typ.).

External Bootstrap Diode

Connect a 1

μF low ESR ceramic capacitor between the

BOOT and LX pins. 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 the 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 RT7249. Note that the external

boot voltage must be lower than 5.5V.

Figure 2. External Bootstrap Diode

0.8 C

Soft-Start time t

, if C

capacitor

0.8 0.1μ

is 0.1μF, then soft-start time =

13.5ms

6μ



≒

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 4. 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 3. Enable Timing Control

Figure 4. Digital Enable Control Circuit

The output voltage is set by an external resistive voltage

divider according to the following equation :

Soft-Start

The RT7249 provides a soft-start function. The soft-start

function is used to prevent large inrush current while

converter is being powered-up. The soft-start timing can

be adjusted by the external capacitor between SS and

GND. An internal current source ISS (6

μA) charges an

external capacitor to build a soft-start ramp voltage. The

VFB voltage will track the internal ramp voltage during soft-

start interval. The typical soft-start time is calculated as

follows :

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 RT7249's quiescent current drops to lower than

μA. Driving the EN pin high (>2V, <5.5V) will turn on the

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 3).

RT7249

GND

VOUT

RT7249

GND

VIN

REN

CEN

RT7249

GND

100k

VIN

REN

BOOT

RT7249

0.1µF

RT7249 Datasheet(PDF) 13 Page - Richtek Technology Corporation

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