RT8560 datasheet(13/15 Pages) RICHTEK

13 / 15 page

RT8560

DS8560-01 April 2011

www.richtek.com

Where,

VRISET is the voltage of the RISET pin (1.21V typ.) and the

RISET is the resister between RISET pin and GND.

This setting is the reference for the LED current at LED1-

4 and represents the sensed LED current for each string.

The DC/DC converter regulates the LED current according

to the setting.

Brightness Control

The RT8560 features both analog and digital dimming

control. Analog dimming is linearly controlled by an

external voltage (0.3V < VACTL < 1.2V). With an on-chip

output clamping amplifier and a 200k

Ω resistor, PWM

dimming signal is easily low-pass filtered to an analog

dimming signal with one external capacitor for noise-free

PWM dimming. A very high contrast ratio true digital PWM

dimming can be achieved by driving ACTL pin with a PWM

signal and suggest PWM frequency is from 200Hz to

10kHz. Refer to Figure 8, 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 duty is about 5%. If the frequency is increased to

1kHz to 30kHz, the duty will be about 10%.

LED Current vs. ACTL PWM Duty Cycle

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100

Duty Cycle (%)

ACTL = 200Hz

ACTL = 1kHz

ACTL = 10kHz

ACTL = 20kHz

ACTL = 30kHz

ACTL = 0V to 3V, VIN = 12V

100

Figure 8. LED Current vs. PWM Dimming Duty Cycle

Over Voltage Protection

The RT8560 equips over voltage protection (OVP) function.

When the voltage at OVP pin reaches a threshold of

approximately 2.5V, the MOSFET drive output (SW) will

turn

“OFF”. The MOSFET drive output (SW) will turn

“ON” again once the voltage at OVP drops below the

threshold of approximately 2.5V.

So, the output voltage can be clamped at a certain voltage

level and it can be calculated by the following equation :

OUT, OVP

= 2.5

⎛⎞

×⎜⎟

⎝⎠

Where,

R1 and R2 are the voltage divider connected to OVP pin.

If at least one string is in normal operation, the controller

will automatically ignore the open strings and continue to

regulate the current for the string(s) in normal operation.

Current-limit Protection

The RT8560 can limit the peak current to achieve over

current protection. The RT8560 senses the inductor

current of on period that flows through SW pin. The duty

cycle depend on current signal and internal slope

compensation compared with error signal. The internal

N-MOSFET will be turned off when the current signal is

large than internal slope compensation. In the off period,

the inductor current will be descended until the internal

N-MOSFET is on by the oscillator.

Over Temperature Protection

The RT8560 has over temperature protection (OTP)

function to prevent the excessive power dissipation from

overheating. OTP will shut down the IC while junction

temperature exceeds 150

°C. Main converter starts

switching once the junction temperature is cooled

approximately 20

°C.

Inductor Selection

The value of the output inductor (L), where the transition

from discontinuous to continuous mode occurs is

approximated by the following equation :

)

OUT

L =

f V

−×

×× ×

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