#3.12, for the same value of external current
limiting resistor, R.
The maximum forward current through the LED
string is determined by the effects of thermal
resistance of the LED signal lamp, RqJA, and
maximum ambient temperature, TA MAX, as shown
in Figure 4 of the SuperFlux LED Data Sheet.
Where:
R = external current limiting resistor
VIN = input voltage applied to the circuit
VD = voltage drop across optional reverse
transient EMC protection diode
IF = design forward current through the circuit
VO = turn-on voltage of the linear forward
voltage model
So, the worst-case design procedure is to
determine the maximum forward current, IF MAX
,
based on parameters RqJA and TA MAX from Figure
4 of the SuperFlux LED Data Sheet. Then the
value of the external resistor, R, is determined
RS = series resistance of the linear forward
voltage model
y = number of series connected LED emitters
x = number of paralleled strings used with
external current limiting resistor
with Equation #3.2, using values of IF MAX, VIN MAX
and LED linear model parameters (VO LL, RS LL).
,
The nominal design current, IF DES, which occurs
at the design voltage, VIN DES, can be calculated
with Equation #3.12 using values of VIN DES, R,
and LED emitters with a typical forward voltage
[i.e. with linear model parameters (VO NOM, RS NOM).
In practice, the LED signal lamp needs to be
designed to accommodate a range of forward
voltage categories and luminous flux categories.
In most cases, the overall goal is to design an
LED signal lamp with a fixed light output. For
LED emitters from the same luminous flux
category, this can be accomplished by
Figure 3.19 Worst-Case Variations in Forward
Current Through Several LED Strings as a
Function of the Applied Voltage.
calculating different values of external resistor, R,
using Equation #3.2, with the same design
current and voltage, IF DES and VIN DES, and using
the nominal forward voltage models (VO NOM, RS
NOM) for each forward voltage category.
Note, for the example shown in this section:
x = 1 and y = 4.
Figure 3.19 shows how the variation in forward
voltage of the LED emitters affects the forward
current through the string. These curves are
generated by substituting the worst-case linear
forward voltage model parameters [(VO NOM, RS
NOM), (VO LL, RS LL), or (VO HH, RS HH)] into Equation
For LED emitters with higher luminous flux
categories, the design current would be reduced
to keep the light output constant. First, the
minimum luminous flux would be calculated for
the lowest expected luminous flux category at
22
LUMILEDS [ LUMILEDS LIGHTING COMPANY ]
