RT7263B datasheet(10/15 Pages) RICHTEK

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RT7263B

DS7263B-01

September 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The IC is a synchronous high voltage step-down converter

that can support the input voltage range from 4.5V to 21V

and the output current can be up to 3A.

Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation :

⎛⎞

⎜⎟

⎝⎠

OUT

= V

Figure 1. Output Voltage Setting

Figure 2. External Bootstrap Diode

where VFB is the feedback reference voltage 0.603V (typ.).

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

External Bootstrap Diode

Connect a 100nF low ESR ceramic capacitor between

the BOOT pin and SW pin as shown in Figure 2. 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

IC. Note that the external boot voltage must be lower than

5.5V.

Soft-Start

The IC contains an external soft-start clamp that gradually

raises the output voltage. The soft-start timing is

programmed by the external capacitor between SS pin

and GND. The chip provides an internal 10

μAcharge current

for the external capacitor. If 47nF capacitor is used to set

the soft-start, the period will be 4.7ms (typ.).

Under Voltage Lockout Threshold

The IC includes an input Under Voltage Lockout Protection

(UVLO). If the input voltage exceeds the UVLO rising

threshold voltage (4.2V), the converter resets and prepares

the PWM for operation. If the input voltage falls below the

UVLO falling threshold voltage (3.8V) during normal

operation, the device stops switching. The UVLO rising

and falling threshold voltage includes a hysteresis to

prevent noise caused reset.

Chip Enable Operation

The EN pin is the chip enable input. Pulling the EN pin

low (<0.4V) will shutdown the device. During shutdown

mode, the RT7263B quiescent current drops to lower than

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

BOOT

100nF

RT7263B

Figure 3. Enable Timing Control

An external MOSFET can be added to implement digital

control on the EN pin, as shown in Figure 4. In this case,

a 100k

Ω pull-up resistor, REN, is connected between VIN

pin and the EN pin. MOSFET Q1 will be under logic control

to pull down the EN pin.

RT7263B

GND

VOUT

RT7263B

GND

VIN

REN

CEN

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