Series AMLDL-Z
Up to 1000mA | LED Driver
Thermal Feedback Circuit
+Vin
R1
4.7K
BZX284C6V2
1
R2
10K
Q1
1
VADJ
10K
+Vin
AMLDL-Z
-Vin
+Vout
1
st
LED
1
R3
10K
Thermistor
B 3900
-Vout
Last LED
1
GND
Thermally Connected
The selection of components for the thermal feedback circuit is depends on the choice of R2 and R3 and the effectiveness of
the LED heatsink. To optimize the LED brightness control at high temperatures, the LEDs must have a sufficient thermal
extraction path, if not the reduction in drive current will not be optimal.
The thermal control threshold points are set by adjusting R2. Three values (33K, 22K, and 10K) were tested. These values
were chosen to provide thermal break points of approximately 25ºC, 40ºC, and 60ºC.
Note, that the LED drive current will not continually dim to zero – the thermal controls applying DC control to that VADJ pin
has a dimming ratio from maximum current of approximately 5:1.
Once the reduced DC level drops below the shutdown threshold of around 200mV, the LED drive current will fall to zero and
the LEDs will be off.
The slope of the current reduction is determined by the beta value of the thermistor. The larger the Beta value the sharper
will be the resultant current control response. The slope of the current reduction is also affected by Q1’s base emitter voltage
variation with temperature.
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