RT8265 datasheet(9/13 Pages) RICHTEK

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RT8265

DS8265-02 March 2011

www.richtek.com

Application Information

The RT8265 is an asynchronous high voltage buck converter

that can support the input voltage range from 4.75V to 24V

and the output current can be up to 3A.

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

RT8265

GND

VOUT

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.).

Figure 2

Soft-Start

The RT8265 contains an external soft-start clamp that

gradually raises the output voltage. The soft-start timming

can be programed by the external capacitor between SS

pin and GND. The chip provides a 15

μA charge current for

the external capacitor. If 0.1

μF capacitor is used to set

the soft-start and it's period will be 10ms(typ.).

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

highest efficiency operation. However, it requires a large

inductor to achieve this goal.

For the ripple current selection, the value of

ΔIL= 0.4(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

⎣⎦ ⎣

⎦

Inductor Core Selection

The inductor type must be selected once the value for L is

known. Generally speaking, high efficiency converters can

not afford the core loss found in low cost powdered iron

cores. So, the more expensive ferrite or mollypermalloy

cores will be a better choice.

The selected inductance rather than the core size for a

fixed inductor value is the key for actual core loss. As the

inductance increases, core losses decrease. Unfortunately,

increase of the inductance requires more turns of wire and

therefore the copper losses will increase.

Ferrite designs are preferred at high switching frequency

due to the characteristics of very low core losses. So,

design goals can focus on the reduction of copper loss

and the saturation prevention.

BOOT

RT8265

10nF

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 BOOT pin for efficiency

improvement. The external 5V can be a 5V fixed input

from system or a 5V output of the RT8265. The bootstrap

diode can be a low cost one such as IN4148 or BAT54.

This diode is also recommended for high duty cycle

operation

applications.

OUT

(when

> 65%)

RT8265 Datasheet(PDF) 9 Page - Richtek Technology Corporation

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