Al5801 – Diodes AL5801 User Manual
Page 8
AL5801
Document number: DS35555 Rev. 3 - 2
8 of 11
July 2012
© Diodes Incorporated
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
Ω
So a standard
resistor value of
1.78
Ω
with 1% tolerance is used.
The R
TH
resistance of the NTC thermistor at room temperature is recommended as 10k
Ω
. The value of base resistor R
base
is set to be 470
Ω.
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
Ω - 0.56V
/
10k
Ω = 72µA
When
V
BE(Q2)
is changed to
V
BE
T
as the temperature increases to T
ºC
, the thermistor resistance at T
°C
required to compensate this variation is
given by
R
TH
T
=
V
BE
T
/ (( V
RSET
- V
BE
T
) /
R
base
- I
B(Q2)
)
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
R
TH
85
=
0.44V
/ (( 0.62V – 0.44V ) / 470
Ω - 72µA ) = 1.4kΩ
The NTC thermistor is chosen for compensation whose resistance is 10k
Ω at +
25°C
and 1.38k
Ω 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