RT7257B

2

DS7257B-03

September 2012

www.richtek.com

©

Copyright

2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Functional Pin Description

Pin No.

Pin Name

Pin Function

1

BOOT

Bootstrap for High Side Gate Driver. Connect a 0.1

μF or greater ceramic

capacitor from BOOT to SW pins.

2

VIN

Input Supply Voltage, 4.5V to 18V. Must bypass with a suitable large ceramic

capacitor.

3

SW

Switch Node. Connect this pin to an external L-C filter.

4,

9 (Exposed Pad)

GND

Ground. The exposed pad must be soldered to a large PCB and connected to

GND for maximum power dissipation.

5

FB

Feedback Input. It is used to regulate the output of the converter to a set value

via an external resistive voltage divider.

6

COMP

Compensation Node. COMP is used to compensate the regulation control

loop. Connect a series RC network from COMP to GND. In some cases, an

additional capacitor from COMP to GND is required.

7

EN

Enable Input Pin. A logic high enables the converter; a logic low forces the

RT7257B into shutdown mode reducing the supply current to less than 3

μA.

Attach this pin to VIN with a 100k

Ω pull up resistor for automatic startup.

8

SS

Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor

from SS to GND to set the soft-start period. A 0.1

μF capacitor sets the

soft-start period to 13.5ms.

Typical Application Circuit

R2 (k

Ω)

L (

μH)

8

27

3

51

2.2

10

22 x 2

5

62

11.8

33

2.2

6.8

22 x 2

3.3

75

24

22

2.2

3.6

22 x 2

2.5

25.5

12

16

2.2

3.6

22 x 2

1.5

10.5

12

10

2.2

2

22 x 2

1.2

12

24

8.2

2.2

2

22 x 2

1

3

12

6.8

2.2

2

22 x 2

Table 1. Suggested Components Selection

VIN

EN

GND

BOOT

FB

SW

7

5

2

3

1

L

3.6µH

0.1µF

22µF x 2

R1

75k

R2

24k

3.3V

10µF x 2

4.5V to 18V

RT7257B

SS

8

COMP

2.2nF

22k

Open

6

4, 9 (Exposed Pad)

0.1µF