RT8509

6

DS8509-01 March 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT8509 is a high performance step-up DC/DC

converter that provides a regulated supply voltage for panel

source driver ICs. The RT8509 incorporates current mode,

fixed frequency, Pulse Width Modulation (PWM) circuitry

with a built in N-MOSFET to achieve high efficiency and

fast transient response. The following content contains

detailed description and information for component

selection.

Boost Regulator

The RT8509 is a current mode boost converter integrated

from 2.8V to 14V. It performs fast transient responses to

generate source driver supplies for TFT LCD display. The

high operation frequency allows use of smaller

components to minimize the thickness of the LCD panel.

The output voltage can be adjusted by setting the resistive

voltage-divider sensing at the FB pin. The error amplifier

varies the COMP voltage by sensing the FB pin to regulate

the output voltage. For better stability, the slope

compensation signal summed with the current sense

signal will be compared with the COMP voltage to

determine the current trip point and duty cycle.

Soft-Start

The RT8509 provides soft-start function to minimize the

inrush current. When powered on, an internal constant

current charges an external capacitor. The rising voltage

and the inductor peak current will also be limited at the

same time. When powered off, the external capacitor will

be discharged until the next soft-start time.

The soft-start function is implemented by the external

capacitor with a 5

μA constant current charging to the soft-

start capacitor. Therefore, the capacitor should be large

enough for output voltage regulation. A typical value for

soft-start capacitor is 33nF. The available soft-start capacitor

range is from 10nF to 100nF.

on and period time is approximately 1ms.

OUT

REF

REF

R1

V

= V

x 1

, where V

= 1.25V (typ.)

R2

⎛⎞

+

⎜⎟

⎝⎠

Ω

without some sacrificing. Place the resistive voltage divider

as close as possible to the chip to reduce noise sensitivity.

Loop Compensation

The voltage feedback loop can be compensated with an

external compensation network consisting of R3. Choose

R3 to set high frequency integrator gain for fast transient

response and C1 to set the integrator zero to maintain

μF x 3, L1 = 4.7μH, while the

recommended value for compensation is as follows :

R3 = 56k

Ω, C1 = 1nF.

Over Current Protection

The RT8509 boost converter has over current protection

to limit the peak inductor current. It prevents large current

from damaging the inductor and diode. During the On-time,

once the inductor current exceeds the current limit, the

internal LX switch turns off immediately and shortens the

duty cycle. Therefore, the output voltage drops if the over

current condition occurs. The current limit is also affected

by the input voltage, duty cycle, and inductor value.

Over Temperature Protection

The RT8509 boost converter has thermal protection function

to prevent the chip from overheating. When the junction

temperature exceeds 155

°C, the function shuts down the

device. Once the device cools down by approximately

10

°C, it will automatically restart to normal operation. To

guarantee continuous operation, do not operate over the

maximum junction temperature rating of 125

°C.

Inductor Selection

The inductance depends on the maximum input current.

As a general rule, the inductor ripple current range is 20%

to 40% of the maximum input current. If 40% is selected

Output Voltage Setting

The regulated output voltage is shown as the following

equation :