AL8863

Document number: DS42198 Rev. 1 - 2

8 of 13

www.diodes.com

August 2019

© Diodes Incorporated

AL8863

LED Open-Circuit Protection

The AL8863 has by default open LED protection. If the LEDs should become open circuit, the resulted voltage drop on sensing resistor is nearly

zero and the DRV pin output will be constantly high, leading to output voltage VOUT equals VIN. No excessive voltages will be seen by the

AL8863 and external components. If open circuit condition lasts for continuous 416µs, FAULT goes low. Once open circuit condition is removed,

the AL8863 resumes normal operation and FAULT resumes high.

For PWM dimming operation, ensure at least one switching cycle during each PWM cycle to make FAULT detection correct. FAULT goes low after

open circuit condition lasts for accumulated 416µs during on time of PWM cycles.

LED Short-Circuit Protection

If the LED string should become shorted together (the anode of the top LED becomes shorted to the cathode of the bottom LED), the AL8863 will

continue to switch and the output current will still be at the regulated current - so no damages will be caused within the AL8863 and external

components. However, the duty cycle at which it operates will change dramatically and the switching frequency will most likely decrease.

The AL8863 includes a dedicated OSP pin, used in combination with CSN pin for LED short circuit fault detection. A resistor divider is set up

between CSN node (the anode of LED string) to the cathode of LED string, and the CSN pin and OSP pin senses output voltage. When the

differential voltage between CSN and OSP is continuously below 1V for 23ms, FAULT goes low. Once short circuit condition is removed, the

AL8863 resumes normal operation and FAULT resumes high.

For PWM dimming operation, ensure at least one switching cycle during each PWM cycle to make FAULT detection correct. FAULT goes low

when the differential voltage between CSN and OSP is continuously below 1V for 23ms.

Over-Temperature Protection

The AL8863 includes Over-Temperature Protection (OTP) circuitry to protect the device from excessive heat damage. When the junction

temperature exceeds approximately +165°C, the AL8863 is forced to shutdown and FAULT goes low. Once the junction temperature cools down

by approximately +40°C, the AL8863 automatically restarts and FAULT resumes high.

Thermal Fold-Back Operation

The AL8863 includes a programmable thermal fold-back feature to reduce power dissipation at high ambient temperature. An external NTC

thermistor can be connected from NTC pin to GND as shown below. The NTC bias current is 100µA, flowing across the NTC thermistor and

generating a voltage on the NTC pin. The AL8863 monitors the voltage on the NTC pin and reduces output current if the voltage on this pin falls

below 1.3V. The value of NTC thermistor can be adjusted to set the voltage on the NTC pin to 1.3V at the required temperature threshold Tbreak.

The output current is gradually reduced to 35% of its nominal value until the voltage on the NTC pin falls below 1V if the temperature continues to

rise.

Vref

-

+

LOGIC

CONTROL

Iref

NTC

Iout(%)

T(deg C)

Tbreak

Tclamp

OTP

35%

100%

EA

CLAMP

NTC

Fault Indicator (FAULT)

The AL8863 includes an active low, open-drain Fault indicator (FAULT). The FAULT pin goes low when one of the following conditions occurs:

(1) Open circuit across the LED string

(2) Short circuit across the LED string

(3) Over-temperature condition