RT7252B datasheet(11/14 Pages) RICHTEK

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RT7252A/B

DS7252A/B-01 April 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT7252A/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 2A.

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 divider

according to the following equation :

OUT

= V

⎛⎞

⎜⎟

⎝⎠

where VFB is the feedback reference voltage (0.8V typ.).

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 RT7252A/B.

Note that the external boot voltage must be lower than

5.5V

Figure 2. External Bootstrap Diode

Over Voltage Protection (OVP)

The RT7252A/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)

Latch-Off Mode

For the RT7252A/B, it provides Hiccup Mode Under

Voltage Protection (UVP). When the FB voltage drops

below 50% of the feedback reference voltage, VFB, the

UVP function will be triggered and the RT7252A/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

ΔIL increases with higher VIN

and decreases with higher inductance.

OUT

I =

⎡⎤

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

ΔIL = 0.2(IMAX) will be a reasonable

starting point. The largest ripple current occurs at the

highest VIN. To guarantee that the ripple current stays

below the specified maximum, the inductor value should

be chosen according to the following equation :

OUT

L(MAX)

IN(MAX)

L =

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×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

RT7252A/B

GND

VOUT

RT7252A/B

BOOT

10nF

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