8/13/2019 Max16820 24v Hb-led Pwm 1-10v Dimming

1/2

High-brightness LEDs (HBLEDs) are making inroads into more traditional lighting applications that

include a dc distribution system (for example, !-" #$-%& track lights) . HBLEDs are more efficient, and

they have a potentially longer lifespan than do halogen or xenon lamps.

Because hysteretic controllers are inexpensive, simplify lighting designs, and require no compensation networks,

they re well!suited for driving HBLEDs. Hysteretic controllers usually have a pulsewidth! modulator "#$%& input that

ena'les a pulse train of varying duty cycle to provide the dimming function. (ne pro'lem, however, in converting a

traditional lighting system is that many dimmers provide a )! to )*!+ dc signal rather than a #$% signal. lso, to

increase the HBLEDs operating lifetimes, a controller should provide temperature'ased current fold'ack.

-onverting a dc voltage to a #$% signal is easy. he #$% signal appears at the output of a comparator when you

apply the dc voltage at one input and a triangle wave at the other. Headaches can arise, however, when trying to align

the triangle wave with the control voltage. /ou need a linear relationship 'etween duty cycle and control voltage, with a

*0 duty cycle at the minimum control voltage and a )**0 duty cycle at the maximum.

he circuit in 1igure ) includes the hysteretic controller, 2) "% 3)456*&7 related power components7 and a control

circuit 'ased on a quad op amp, 26 "L%3869&. 2) drives five HBLEDs from a 69!+ source, using only inductor L),

http://electronicdesign.com/files/29/19151/fig_01.gifhttp://electronicdesign.com/files/29/19151/fig_01.gifhttp://electronicdesign.com/files/29/19151/fig_01.gifhttp://electronicdesign.com/files/29/19151/fig_01.gif

8/13/2019 Max16820 24v Hb-led Pwm 1-10v Dimming

2/2

%(:1E ;), and catch diode D). sense resistor "