AL5801 datasheet(8/11 Pages) DIODES

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AL5801

Document number: DS35555 Rev. 3 - 2

8 of 11

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July 2012

AL5801

Constant LED Current Temperture Compensation

Variation in the junction temperature of Q2 will cause variations in the value of controlled LED current ILED. The base-emitter VBE 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 Rbase, for stabilizing the LED current.

Figure 14 Constant LED Current Temperature Compensation for AL5801

The voltage drop VRSET in the sense resistor REXT should be set to be 40 to 100mV higher than the VBE(Q2) at 25ºC. Figure 11 shows the typical

VBE(Q2) is 0.56V at room temperature with 0.1mA IBIAS, so VRSET is selected to be 0.62V.

With the VRSET chosen, the sense resistor value for 350mA ILED is determined by

REXT = VRSET / ILED = 0.62V / 350mA = 1.77Ω

So a standard resistor value of 1.78

Ω with 1% tolerance is used.

The RTH resistance of the NTC thermistor at room temperature is recommended as 10kΩ. The value of base resistor Rbase is set to be 470Ω.

Q2’s base current is obtained as

IB(Q2) = ( VRSET - VBE(Q2) ) / Rbase - VBE(Q2) / RTH = ( 0.62V - 0.56 ) / 470Ω - 0.56V / 10kΩ = 72µA

When VBE(Q2) is changed to VBE

T as the temperature increases to TºC, the thermistor resistance at T°C required to compensate this variation is

given by

RTH

T = VBET / (( VRSET - VBET ) / Rbase - IB(Q2) )

At -2mV/°C, VBE(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

RTH

85 = 0.44V / (( 0.62V – 0.44V ) / 470Ω - 72µA ) = 1.4kΩ

The NTC thermistor is chosen for compensation whose resistance is 10k

Ω at +25°C and 1.38kΩ at +85°C with a β value of 3530.

Figure 15 shows the ILED variation with temperature with and without temperature compensation.

Figure 15 LED Current Variation with and

without Temperature Compensation

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