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When V BE(Q2) is changed to V BE as the temperature increases to T oC , the thermistor resistance at T °C required to compensate this variation is
R TH = V BE / (( V RSET - V BE ) / R base - I B(Q2) )
R TH = 0.44V / (( 0.62V – 0.44V ) / 470 ? - 72μA ) = 1.4k ?
AL5801
Constant LED Current Temperture Compensation
Variation in the junction temperature of Q2 will cause variations in the value of controlled LED current I LED . The base-emitter V BE voltage of Q2
decreases with increasing temperature at a rate of approximately 2mV/ °C . Figure 14 shows a simple temperature compensation network, which
comprises of an NTC thermistor and resistor R base , for stabilizing the LED current.
Figure 14 Constant LED Current Temperature Compensation for AL5801
The voltage drop V RSET in the sense resistor R EXT should be set to be 40 to 100mV higher than the V BE(Q2) at 25 oC . Figure 11 shows the typical
V BE(Q2) is 0.56V at room temperature with 0.1mA I BIAS , so V RSET is selected to be 0.62V.
With the V RSET chosen, the sense resistor value for 350mA I LED is determined by
R EXT = V RSET / I LED = 0.62V / 350mA = 1.77 ?
So a standard resistor value of 1.78 ? with 1% tolerance is used.
The R TH resistance of the NTC thermistor at room temperature is recommended as 10k ? . The value of base resistor R base is set to be 470 ? .
Q2’s base current is obtained as
I B(Q2) = ( V RSET - V BE(Q2) ) / R base - V BE(Q2) / R TH = ( 0.62V - 0.56 ) / 470 ? - 0.56V / 10k ? = 72μA
T
given by
T T T
At - 2mV/ °C, V BE(Q2) reduces to 0.44V from 0.56V as the temperature increases from +25°C to +85°C. From the above equation, the thermistor’s
resistance at +85°C to keep the same output current is given by
85
The NTC thermistor is chosen for compensation whose resistance is 10k ? at + 25°C and 1.38k ? at + 85°C with a β value of 3530 .
Figure 15 shows the I LED variation with temperature with and without temperature compensation.
Figure 15 LED Current Variation with and
without Temperature Compensation
AL5801
Document number: DS35555 Rev. 3 - 2
8 of 11
www.diodes.com
July 2012
? Diodes Incorporated
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