NCL30081/3: Step Dimming Guidelines for LED Drivers

Design Note – DN05065/D
Step Dimming Guidelines for NCL30081/83 LED Drivers
Input Voltage
Output Power
I/O Isolation
LED Lighting
100 - 265 V ac
reducing the output current.
The ability to dim household lighting is a
desirable feature that has been common in
residential lighting for over 50 years. A dimmer
accomplishes this is but requires installation
replacing a switch in the appropriate electrical
Dimming Table 1
box. The installation is often beyond the typical
The internal circuit architecture is shown below
home repair requiring a qualified electrician. The
for the BO detection.
NCL30081/83 incorporates a step dimming
feature that changes the LED bulb output
whenever the AC is interrupted briefly using a
existing wall switch. Toggling the wall switch
on/off for a 1-5 s steps down the bulb output in 6
The Vcc voltage will drop when the BO detector
discrete steps. After the last step, the bulb goes
shuts off the output. When Vcc drops to the
back to 100% brightness. This approach allows
Vcc_off threshold (~9 V), the current draw on the
the customer to add dimming capability to a
Vcc pin drops to about 50 µA. The state of the
fixture without the need to make changes to the
dimming register is retained until Vcc reaches
electrical wiring.
Vcc_reset which is about 5 V. When AC power
returns, the BO detector senses good power and
Theory of Operation
turns on the output again at the new current
The NCL30081/83 detects the presence of the AC
setting. The size of the Vcc capacitor determines
line through the Vin pin. If Vin is lost for more
the hold up time between Vcc_off and Vcc_reset.
than 50ms, the Brown Out detector turns off the
This is the window of time to toggle the dimming
output and decrements the step dimming register
register. If Vcc drops below Vcc_reset, the
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dimming register is cleared. When power is
The decay of the Vcc voltage is dependent on Icc
restored, the output will be at 100%.
and the available stored energy in the Vcc filter
circuit. If the Vcc voltage falls below Vcc_off
before the BO timer has timed to 50ms, the
Design Guidelines
dimming register will not decrement the current.
Use of a split Vcc supply is necessary to ensure
It will hold the previous state. The result will be a
adequate hold up time to provide 1s - 5s of step
driver that fails to decrement the current when the
dimming time while not inhibiting fast start up.
input power is toggled. The NCL30081/83
The split supply is shown below.
accepts a wide range of Vcc up to 26V while
operating down to about 10V. Therefore it is
advisable to operate Vcc as close to 26V as
practical without causing an OVP condition.
Setting Vcc can be a problem for designs that
have wide output voltage requirements. One must
also consider that the LED voltage will decrease
some with reduced current. The driver may be
able to drive a wide output voltage range but the
step dimming range may be narrower because the
Vcc decays before the BO timer decrements the
The 47uF capacitor does not load the Vcc
dimming current state.
terminal during start up thus assuring a fast start.
After the converter starts switching, the 47uF
Output Voltage Rise time
capacitor provides a large energy storage element
to keep the Vcc in the active region for a
The output voltage rise time is dependent on the
driver output current and the output capacitance.
minimum of 50ms (the brown out detect period).
The output capacitor is sized for ripple current at
maximum load. The NCL30081/83 uses an open
Timing Guidelines
loop control for output current. Consequently,
Poor dimming performance can be traced back to
there is no slew rate improvement due to the gain
timing race issue.
of the feedback loop. The energy in the output
1. Vcc Decay
capacitor must be satisfied before the current in
2. Output Voltage rise time
the LEDs. Since the output current is fixed as an
Vcc Decay
When the input power is interrupted, the output
decays as the input energy storage is depleted.
August 2014, Rev. 0
open loop event, the output voltage rise varies
directly as the output current setting. Vcc voltage
rise follows the output voltage rise because they
are coupled through the aux winding of the
Design Recommendations for Step
transformer. If the output current drive is at the
lowest setting and the output voltage is low, the
1. Operate Vcc as high as practical without
Vcc winding may not deliver enough voltage to
causing an OVP at the maximum current and
bootstrap the Vcc supply before the Vcc capacitor
maximum output voltage.
gets to Vcc_off. In this case, the circuit gets stuck
2. Use the split Vcc supply to provide energy
in an endless loop until the controller is reset to
storage for Vcc
100% output. While it may seem as though this is
3. Size the output capacitor to be either small
a fundamental limitation, there are some special
enough or large enough.
characteristics of LEDs that make this scenario
4. Testing with a resistive load will not work
1. Very small Output Capacitance
2. Very large Output Capacitance
In the first scenario, the output capacitance is
chosen to be sufficiently small that the output
voltage rises in time to bootstrap the Vcc/aux
winding. The LED ripple current will be high at
maximum current as a consequence. The second
scenario is counter-intuitive because a very large
output capacitor would only serve to further slow
© 2014 ON Semiconductor.
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Design note created by Frazier Pruett e-mail:
[email protected]
the output voltage rise. This would indeed be the
case for a resistor load; however, LEDs act more
like zener diodes and leave a large residual
voltage on the output capacitor during brief power
interruptions. It is the residual voltage on the
output capacitor that allow the converter to
recover at low output currents. Testing with a
resistor load is not recommended with a step
dimming driver.
August 2014, Rev. 0