RT7251A/B

11

DS7251A/B-01 April 2012

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©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT7251A/B is a synchronous high voltage buck

converter that can support the input voltage range from

4V to 17V and the output current can be up to 1.5A.

Output Voltage Setting

The resistive divider allows the FB pin to sense the output

voltage as shown in Figure 1.

RT7251A/B

GND

FB

R1

R2

Figure 1. Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation :

OUT

FB

R1

V

= V

1

R2

⎛⎞

+

⎜⎟

⎝⎠

External Bootstrap Diode

Connect a 10nF 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 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 1N4148 or BAT54. The external 5V can be a 5V

fixed input from system or a 5V output of the RT7251A/B.

Note that the external boot voltage must be lower than

5.5V.

Figure 2. External Bootstrap Diode

Over Voltage Protection (OVP)

The RT7251A/B provides over voltage protection function

when output voltage is over 125%. The internal MOS will

be turned off. The control will return to normal operation if

over voltage condition is removed.

Under Voltage Protection (UVP)

For the RT7251A/B, it provides Hiccup Mode Under

Voltage Protection (UVP). When the FB voltage drops

below 50% of the feedback reference voltage, the UVP

function will be triggered and the RT7251A/B will shut down

for a period of time and then recover automatically. The

Hiccup Mode UVP can reduce input current in short-circuit

conditions.

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current £GIL increases with higher VIN

and decreases with higher inductance.

OUT

OUT

L

IN

VV

I =

1

fL

V

⎡⎤

⎡⎤

Δ× −

⎢⎥

⎢⎥

×

⎣⎦ ⎣⎦

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can achieve

the highest efficiency operation. However, it requires a

large inductor to achieve this goal. For the ripple current

selection, the value of

starting point. The largest ripple current occurs at the

below the specified maximum, the inductor value should

be chosen according to the following equation :

OUT

OUT

L(MAX)

IN(MAX)

VV

L =

1

fI

V

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎢⎥ ⎢

⎥

⎣⎦ ⎣

⎦

Table 2. Suggested Inductors for Typical

Application Circuit

Component

Supplier

Series

Dimensions

(mm)

TDK

VLF10045

10 x 9.7 x 4.5

TDK

SLF12565

12.5 x 12.5 x 6.5

TAIYO

YUDEN

NR8040

8 x 8 x 4

RT7251A/B

SW

BOOT

5V

10nF