CHMSL Design Example
This worst-case design procedure will be
illustrated with an example. Let suppose that an
LED array is being designed using 4-LED
strings of HPWT-MH00 from luminous flux
categories F through L and forward voltage
categories 2 through 6. For this example RqJA =
350° C/W, TA MAX = 70° C, VIN MAX = 15.0 V, and
VIN DES = 12.8 V. In addition, a silicon diode with
a forward voltage of 0.8 V is connected in series
with the circuit in order to provide protection
against negative EMC transients. Then, the
design steps are shown below:
For HPWT-MH00, forward voltage category 6:
VO LL = 1.85 V, RS LL = 14.4 ohm
Then:
3. Determine nominal design current, IF DES
:
The nominal forward current through the LED
emitters at the design voltage is determined with
Equation #3.12. The equation should use the
value of R from the Step 2 at the design voltage
and the nominal forward voltage of the SuperFlux
LED emitters using the same forward voltage
category used in Step 2.
1. Determine IF MAX
:
The maximum allowable DC forward current
through the SuperFlux LED emitters is
determined from the maximum ambient
temperature, TA MAX, estimated overall thermal
resistance, RqJA, of the LED signal lamp, and
Figure 4 of the Data Sheet.
For HPWT-MH00, forward voltage category 6:
VO NOM = 2.03 V, RS NOM = 12.4 ohm
For TA MAX = 70° C, and RqJA = 350° C/W:
Then: IF MAX = 55 mA, from HPWT-MH00
DataSheet, Figure 4
2. Determine minimum value of current limiting
resistor, R:
The minimum value of R is determined with
Equation #3.2 at the maximum input voltage
and maximum forward current from Step 1 for
the maximum forward voltage category Super
Flux LED to be used in the assembly.
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