AL5892

Document number: DS42212 Rev. 1 - 2

8 of 11

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October 2019

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AL5892

Functional Description and Application Information (continued)

By using resistors divider, a proportion of rectified mains voltage can be got as input to a phase-detection comparator. The comparator has a

nominal threshold of 0.3V. The comparator generates a 0V to 5V PWM signal with a duty cycle proportional to TRIAC dimmer conduction angle.

PWM signal is presented to a R-C low-pass filter with a bandwidth on the order of 1Hz. This RC filter finally produces a DC level signal (DIMD)

which corresponds to the

TRIAC’s conduction angle.

Figure 5. AL5892 Dimmer Detection PWM Waveform

As input signal of dimmer-detection comparator, DIMD signal is compared with a threshold of 2.4V. When no dimmer is present, DIMD will be

higher than 2.4V; the AL5892 makes the bleeder inactive to improve efficiency. When a dimmer exists, DIMD will be lower than 2.4V; the bleeder

circuit will be active.

According to the detection method, those TRIAC dimmers with lower than 161°conduction angle can be identified by AL5892.

Based on this measured conduction angle information the dimming curve can be produced. It is clear that the RC filter will produce the maximum

DC level, when no dimmer is present, and in response the AL5892 makes the bleeder inactive to improve efficiency.

LED Open and Short Detection

The AL5892 can survive LED open/short fault with rated line voltage range input. Within an AC cycle, in short condition, the time of current through

main MOSFET will be much longer than in normal condition. The AL5892 detects the width of CS, when the CS current width is longer than a

threshold, it means LED short happening.

In LED short condition, the AL5892 pulls down the COMP pin voltage to minimize the input power, and the main MOSFET will turn on one time in

five half AC sine cycle to prevent IC overheat. That also makes it auto-recovery when LED short condition is removed.

When LED is open, the voltage of E-CAP between LED+ and LED- will rise up to the peak value of line voltage, the current through the main

MOSFET will decrease to 0 naturally.

Thermal Fold Back and Shutdown (OTP)

Thermal Fold Back happens when the IC junction temperature reaches about +150°C

, which limits AL5892’s power dissipation by reducing more

than 50% LED current.

Thermal Shutdown turns off the output MOSFET when the IC junction temperature exceeds +165°C typical.

Figure 6. Thermal Fold Back

TJ (°C)