RT2805A

10

DS2805A-00 November 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT2805A is an asynchronous high voltage buck

converter that can support the input voltage range from

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

REF

R1

V

= V

1

R2

⎛⎞

+

⎜⎟

⎝⎠

Where R1 = 100k

Ω.

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 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

RT2805A.

Figure 2. External Bootstrap Diode

Soft-Start

The RT2805A contains an internal soft-start clamp that

gradually raises the output voltage. The typical soft-start

Chip Enable Operation

The EN pin is the chip enable input. Pull the EN pin low

(<0.4V) will shutdown the device. During shutdown mode,

the RT2805A quiescent current drops to lower than 25

μA.

Drive the EN pin to high (>1.4V, <5.5V) will turn on the

device again. If the EN pin is open, it will be pulled to high

by internal circuit. For external timing control (e.g.RC),

the EN pin can also be externally pulled to High by adding

a 100k

Ω or greater resistor from the VIN pin (see Figure

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current

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

highest efficiency operation. However, it requires a large

inductor to achieve this goal.

ripple current stays 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

⎣⎦ ⎣

⎦

The inductor's current rating (caused a 40

°C temperature

rising from 25

°C ambient) should be greater than the

maximum load current and its saturation current should

be greater than the short circuit peak current limit. Please

see Table 2 for the inductor selection reference.

RT2805A

GND

FB

R1

R2

SW

BOOT

5V

RT2805A

10nF