RT7247C datasheet(10/15 Pages) RICHTEK

10 / 15 page

RT7247C

DS7247C-02

September 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

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

The output voltage is set by an external resistive voltage

divider according to the following equation :

OUT

REF

= V

⎛⎞

⎜⎟

⎝⎠

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

External Bootstrap Diode

Connect a 0.1

μF 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

RT7247C. Note that the external boot voltage must be

lower than 5.5V

Figure 2. External Bootstrap Diode

Soft-Start

The RT7247C provides 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 programmed 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 :

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 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.

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 RT7247C quiescent current drops to lower than

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

0.8 C

Soft-Start time t

, if C

capacitor

0.8 0.1

is 0.1 F, then soft-start time =

13.5ms

≒

Figure 3. Enable Timing Control

Figure 4. Digital Enable Control Circuit

RT7247C

GND

VOUT

BOOT

RT7247C

0.1µF

RT7247C

GND

VIN

REN

CEN

RT7247C

GND

100k

VIN

REN

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