NCL2801LED2GEVB

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4

Dimming

In order to avoid flicker issues caused by skip mode

operation, this deep dimming design uses 2 current control

loops. The main loop controls current through the normal

feedback control to the NCP13992. Below a predetermined

threshold (20% in this case), the second control loop sets the

current using a linear regulator made of Q3 & U9. While the

main loop is in control, Q3 is saturated for maximum

efficiency. Once Q3 enters the active mode and controls the

LED current, Q7 regulates the voltage across Q3 to optimize

power losses while allowing Q3 to be in the linear region and

provide a very stable output current even though the LLC is

in skip mode.

Figure 2. Line Dim Curve

The blue line shows the internal dim voltage (note this is

not the external 0−10 V dim signal) vs the normalized

output current for the main output control. The orange line

represents the internal dim voltage vs normalized output for

the linear control. For any given dim setting, the lower

setting of the 2 control loops will set the output current. As

you can see, the blue line (main loop) sets the output current

above 20% load and the orange line (linear loop) sets the

current below 20%. This dual mode control provides deep

stable dimming while maximizing efficiency for higher

output loads.

PROTECTION

OVP/Thermal Protection

The thermal protection is built into the NCP13992 and

shuts down the NCP13992 when the die temperature

exceeds 150

°C. An NTC can be connected to pin 7 to

program the thermal protection. Pulling up pin 7 through a

Zener diode is a good way to set the OVP on the primary

voltage through the turns ratio of the power transformer.

This is a convenient way to set the OVP threshold.