RT8489

12

DS8489-00 November 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

capacitor is determined primarily by the stability of the

regulator rather than the gate charge of the switching

N-MOSFET. A 1

μF capacitor will be adequate for most

applications.

Place the capacitor close to the IC to minimize the trace

length to the GBIAS pin and also to the IC ground. An

internal current limit on the GBIAS protects the RT8489

from excessive on chip power dissipation.

GBIAS pin should be connected to the input supply. Be

aware that a typical 20mA current will load the GBIAS to

shutdown.

Loop Compensation

The RT8489 uses an internal error amplifier, in which

through its compensation pin (VC) the loop response is

optimized for specific applications. The external inductor,

output capacitor, compensation resistor, and

compensation capacitor determine the loop stability. The

inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor at VC are selected to optimize control loop

response and stability.

The compensation resistor and capacitor are connected

in series from the VC pin to GND to provide a pole and a

zero for proper loop compensation. The typical

compensation values for RT8489 is 1.8k

Ω and 3.3nF.

LED Current Setting

The maximum current of channel 1 to 4 is set by placing

an appropriate sense resistor for LED string. When the

voltage of ACTL is higher than 1.4V, the LED current can

be calculated by the following equation :

LED, MAX

Sx

225mV

I

=

(mA)

R

N-MOSFET and GND.

The ACTL pin should be tied to a voltage higher than 1.4V

to get the full scale 225mV (typical) threshold across the

sense resistor. The ACTL pin can also be used to dim the

LED current to zero, although relative accuracy decreases

with the decreasing voltage sense threshold. When the

ACTL pin voltage is less than 1.4V, the LED current is :

ACTL

LED

Sx

(V

0.4) 225mV

I

=

(mA)

R

−×

The ACTL pin can also be used in conjunction with a

thermistor to provide over temperature protection for the

Brightness Control

For LED applications where a wide dimming range is

required, two methods are available: analog dimming and

PWM dimming. The easier method is to simply vary the

DC current through the LED by analog dimming.

However, PWM dimming which switches the LED on and

off via different duty cycle to control the average LED

current is the better dimming method. The PWM dimming

offers several advantages over analog dimming and is more

preferred by LED manufacturers. One advantage is the

chromaticity of the LEDs which remains unchanged since

the LEDcurrent is either zero or at the programmed current.

Another advantage of PWM dimming is that a wider

dimming range is available.

The RT8489 features both analog and digital dimming

control. Analog dimming is linearly controlled by an

external voltage (0.4V to 1.4V) at the ACTL pin. A very

high contrast ratio is true digital PWM dimming which

can be achieved by driving the ACTL pin with a PWM signal

at a recommended PWM frequency of 100Hz to 10kHz.

The PWM dimming frequency can be sufficiently adjusted

from 100Hz to 30kHz. However, LED current cannot be

100% proportional to the duty cycle, especially for high

frequency and low duty ratio, because of physical

limitation caused by internal switching frequency. Referring

to Figure 4, the minimum dimming duty can be as low as

1% for the frequency range from 100Hz to 300Hz. For the

dimming frequency from 300Hz to 1kHz, the minimum

dimming duty is about 5%. If the frequency is increased

from 1kHz to 30kHz, the minimum dimming duty will be

about 10%.