the nominal design current. Then Figure 3 from
the SuperFlux LED Data Sheet could be used to
calculate the other design currents for higher
luminous flux categories. Then values of
design. Lumileds Lighting recommends that the
designer use realistic assumptions for these
parameters. It is very easy to overly guard-band
these assumptions, which results in an excessive
estimate of the number of LED emitters needed.
external resistor, R, can be calculated with
Equation #3.2 at the reduced design current
using the appropriate nominal forward voltage
models for each forward voltage category.
The luminous efficiency of AlInGaP technology
has significantly improved over the past few
years. Please consult with your Lumileds Lighting
or Agilent Technologies Field Sales Engineer for
the recommended minimum luminous flux
categories of SuperFlux LED emitters for given
future production dates.
After all of these “ideal” values of R are
computed, the designer would need to choose
the closest standard resistor values. In many
cases, the designer can use the same resistor
value for multiple LED emitter categories
provided that the maximum forward current is
not exceeded under worst-case conditions.
Many LED signal lamp requirements also include
operation at higher voltages for a limited duration
(i.e. 24 volts for two minutes). In analyzing the
performance of an LED signal lamp under these
conditions, it is important to analyze the transient
heating effects. Under these conditions the LED
emitters don’t reach thermal equilibrium so the
junction temperatures are lower than indicated
by Equation #3.9. Equation #3.11 can be used
to estimate the maximum junction temperature
Experience has shown that the worst-case
design occurs with the lowest expected
luminous flux category and the highest forward
voltage category. Thus, the lowest value of
external resistor, R, and the highest design
current, IF DES, would be determined for this
particular category combination. For LED
emitters with the lowest expected luminous flux
category and lower forward voltage categories,
Equation #3.2 will generate higher values of R.
For LED emitters with higher luminous flux
categories, the values of R will be even larger
since the design current is reduced.
using the appropriate time constants for RqJ LF
RqLF P, and RqP A. In addition, many test
specifications allow a different operating
temperature for these tests.
,
The assumptions used for maximum ambient
temperature, RqJA, maximum steady-state input
voltage, and the worst-case SuperFlux LED
categories (minimum expected luminous flux
and maximum expected forward voltage) have a
large effect on the nominal design current and
thus the luminous flux output. Thus these
parameters have a large effect on the number of
LED emitters needed for a given signal lamp
23
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
