NCL30083(2013_02) Ver la hoja de datos (PDF) - ON Semiconductor

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Fabricante

NCL30083
(Rev.:2013_02)

Dimmable Quasi-Resonant Primary Side Current-Mode Controller for LED Lighting with Thermal Fold-back

ON Semiconductor 

NCL30083

Zero Crossing Detection Block

The ZCD pin allows detecting when the drain−source

voltage of the power MOSFET reaches a valley.

A valley is detected when the voltage on pin 1 crosses

below the VZCD(THD) internal threshold.

At startup or in case of extremely damped free

oscillations, the ZCD comparator may not be able to detect

the valleys. To avoid such a situation, the NCL30083

features a Time−Out circuit that generates pulses if the

voltage on ZCD pin stays below the VZCD(THD) threshold

for 6.5 ms.

The Time−out also acts as a substitute clock for the valley

detection and simulates a missing valley in case of too

damped free oscillations.

The 3rd valley

is validated

high

low

The 2nd valley is detected

By the ZCD comparator

high

low

high

low

high

The 3rd valley is not detected

by the ZCD comp

Time−out circuit adds a pulse to

account for the missing 3rd valley

V ZCD

43 V ZCD(THD)

14

2nd, 3rd

12

15 ZCD comp

16 TimeOut

low

17 Clk

Figure 66. Time−out Chronograms

Because of this time−out function, if the ZCD pin or the

auxiliary winding is shorted, the controller will continue

switching leading to improper regulation of the LED

current. Moreover during an output short circuit, the

controller will strive to maintain the constant current

operation.

In order to avoid these scenarios, a secondary timer starts

counting when the ZCD voltage is below the VZCD(short)

threshold. If this timer reaches 90 ms, the controller detects

a fault and enters the auto−recovery fault mode (controller

shuts−down and waits 4−s before re−starting switching).

Line Feed−forward

Because of the propagation delays, the MOSFET is not

turned−off immediately when the current set−point is

reached. As a result, the primary peak current is higher than

expected and the output current increases. To compensate

the peak current increase brought by the propagation delay,

a positive voltage proportional to the line voltage is added

on the current sense signal. The amount of offset voltage can

be adjusted using the RLFF resistor as shown in Figure 67.

The offset voltage is applied only during the MOSFET

on−time.

This offset voltage is removed at light load during

dimming when the output current drops below 15% of the

programmed output current.

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