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LDS8160
Appendix 3
LED TEMPERATURE MEASUREMENT
To implement the temperature correction of the I LED
vs. Temperature in respect to compensation curve,
the LED temperature should be known.
the PTAT (proportional to absolute temperature)
technique, as the ΔV F has a linear and positive
(proportional) tracking coefficient with temperature.
? V F ?
ln ? F 2 ? ? R S F 2 ? I F 1 ? (3)
? I
? I
? F 1 ?
A very common and reliable method of measuring
temperature in integrated circuits is to take
advantage of the forward voltage (V F ) behavior of a
? kT
q
I
? ?
?
P-N junction semiconductor diode with respect to
temperature.
At any given current, the forward voltage (V F ) of a P-
N junction diode is
A second method for diode temperature sensing
measures the diode V F at two different temperatures
(T 1 and T 2 ) at a constant I F , and it is also commonly
employed.
V F ?
x ln ? F ? (1)
? ( kT )
q
? I ?
? I ?
? S ?
This is referred to as the CTAT (complementary to
absolute temperature) technique as the V F has a
linear but negative (complementary) tracking rate
k = Boltzmann constant = 1.38 x 10 , (Joules)/deg K
q = Charge of electron = 1.602 x 10
coulombs
ln ? F ? ? R S F (2)
V F _ LED ? ?
? I ?
? S ?
Where:
? = ideality factor; ~ 1 for silicon
-23
-19
T = Absolute Temperature, deg K
I F = the diode forward current, A
I S = the diode reverse saturation current, A.
LEDs, based on compound semiconductors other
than silicon structures, have complex dependency
between forward voltage and current
I
E g ? kT ? ?
I
q q
Where
R S – is LED series resistance, Ω ,
E g – is the bandgap energy of the material that
with temperature. This method requires that the V F
temperature coefficient be pre-characterized.
The LDS8160 utilizes both techniques for improved
temperature estimation accuracy. A proprietary digital
arbitration algorithm resolves the final temperature
estimation every 2.5 seconds from both techniques
and a combination of on-chip silicon diode and LED
device measurements.
The ideality factor term, η, is based on the physic al
properties of the diode construction and directly
relates to the recombination leakage current caused
by defects. For an ideal diode ? = 1, and the V F
increases at the rate of 60 mV per decade change in
I F . Non-ideal P-N junctions (i.e. LEDs) have ? > 1;
therefore the change in V F increases more per
decade change in I F .
?
?
h = 6.626 x 10
(Joules x s) - is Planck's constant;
c = 3.0 x 10 m/s - is the speed of light;
determines the wavelength of emitted light E g
-34
8
? = wavelength in m
hc
,
This factor varies across manufactures and devices,
and it requires a calibration before direct temperature
sensing of the LEDs. The ideality factor ? may be
determined as the slope of the logarithmic I vs. V
diode characteristic in the low operating current
region (where effects of R S are negligible).
The problem with measuring V F directly is that the I S
term is highly temperature dependent and very
difficult to measure or predict. Additionally,
generating a precise current that does not vary with
power supply, processing variations and temperature
is also very difficult.
Measuring V F at two separate forward currents, I F2
and I F1 allows avoiding these issues. Due to the
nature of logarithms, the difference, Δ V F , between the
two measurements will be linearly dependent on
temperature, and the I S terms will cancel. In addition,
the linear term is a function of a ratio of currents that
are relatively straightforward to implement and
independent of the operating conditions. This
temperature measurement method is also known as
Series resistance, R S , is another non-ideal
characteristic. LEDs typically operate at forward
currents in the range of several of milliamps,
therefore, LEDs series resistance in the range of 10’s
of ohms results in a significant deviation from ideal
behavior. The actual R S value can be extracted from
the logarithmic I vs. V F curve of the diode in the high
current operating region.
Figure A3.1 shows a curve that represents a Nichia
WLED (NSSW020BT-P1) used for mobile display
backlighting. The R S value extracted from this curve
is ~17 Ω and ? = ~ 1.55.
? 2009 IXYS Corp.
Characteristics subject to change without notice
31
Doc. No. 8160_DS, Rev. N1.0
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