HDSP2000LP, HDSP2001LP, HDSP2002LP, HDSP2003LP
2009-03-31 7
Thermal Considerations
The small alphanumeric displays are hybrid LED and CMOS
assemblies that are designed for reliable operation in commercial,
industrial, and military environments. Optimum reliability and
optical performance will result when the junction temperature of the
LEDs and CMOS ICs are kept as low as possible.
Thermal Modeling
HSDP200XLP consist of two driver ICs and four 5 x 7 LED
matrixes. A thermal model of the display is shown in Figure
„Thermal Model“. It illustrates that the junction temperature of the
semiconductor = junction self heating + the case temperature rise
+ the ambient temperature.
Equation 1 shows this relationship.
Thermal Model
See Equation 1 below.
The junction rise within the LED is the product of the thermal
impedance of an individual LED (37°C/W, DF=20%, F=200 Hz),
times the forward voltage, VF(LED), and forward current IF(LED), of
13 – 14.5 mA. This rise averages TJ(LED)=1°C. The Table below
shows the VF(LED) for the respective displays.
The junction rise within the LED driver IC is the combination of the
power dissipated by the IC quiescen t cur rent and the 28 ro w driver
current sinks. The IC junction rise is given in Equation 2.
A thermal resistance of 28°C/W results in a typical junction rise of
6°C.
See Equation 2 below.
For ease of calculations the maximum allowable electrical operat-
ing condition is dependent upon the aggregate thermal resistance
of the LED matrixe s and the tw o driv e r ICs. All of the thermal man-
agement calculations are based upon the parallel combination of
these two networks which is 15°C/W. Maximum allowable power
dissipation is given in Equation 3.
Equation 3.
For further reference see Figures „Maximum Allowable Power Dis-
sipation vs. Temperature“ (page 3) and Figures from page 8 on.
Key to equation symbols
DF Duty factor
ICC Quiescent IC current
ICOL Column current
n Number of LEDs on in a 5 x 7 array
PCASE Package power dissipation excluding LED
under consideration
PCOL Power dissipation of a column
PDISPLAY Power dissipation of the display
PLED Power dissipation of a LED
RqCA Thermal resistance case to ambient
RqJC Thermal resistance junction to case
TAAmbient temperature
TJ(IC) Junction temperature of an IC
TJ(LED) Junction temperature of a LED
TJ(MAX) Maximum junction temperature
VCC IC voltage
VCOL Column voltage
VF(LED) Forward voltage of LED
ZqJC Thermal impedance junction to case
Equation 1.
Equation 2.
Model Number VF
Min. Typ. Max.
HDSP2000LP 1.6 1.7 2.0
HSDP2001/2/3LP 1.9 2.2 3.0
IDDG5321
θ
R
1
LED Power
LED
T
1
IC Power
R
θ2
IC
T
2
LED Power
R
θ1
LED
T
1
LED Power
LED
θ
R
1
T
1
IC Power
θ
R
2
IC
T
2
LED Power
R
1θ
LED
T
1
θ
R
CA
PDISPLAY
TJMAX()
TA
–
RθJC RθCA
+
----------------------------------------=
PDISPLAY 5VCOL ICOL n35⁄()DF VCC ICC
+=
TJ LED()
PLED ZθJC PCASE RθJC RθCA
+()TA
++=
TJ LED()ICOL 28⁄()VF LED()
ZθJC
[]n35⁄()ICOL DF 5VCOL
()VCC ICC
+[]RθJC RθCA
+[]TA
+⋅+=
TJIC() PCOL RθJC RθCA
+()TA
+=
TJIC() 5V
COL VFLED()
–()ICOL 2⁄()n35⁄()DF⋅⋅ VCC ICC
⋅+[]RθJC RθCA
+[]⋅TA
+=