Datasheet
13
Figure 9. Improper method to manually insert a display.
Thermal Considerations
The HDSP-253X can operate from -40°C to +85°C. The
display’s low thermal resistance allows heat to ow from
the CMOS IC to the 24 package pins. Typically, this heat
is conducted through the printed circuit board traces to
free air. For most applications, no additional heatsinking
is needed. Illuminating all 280 LEDs simultaneously at full
brightness is not recommended for continuous operation.
However, all 280 LEDs can be illuminated simultaneously
at full brightness for 10 seconds at 25°C as a lamp test.
The IC has a maximum allowable junction temperature of
150°C. The IC junction temperature can be calculated with
the following equation:
T
J
MAX = T
A
+ (P
D
x Rq
J-A
)
T
J
MAX is the maximum allowable IC junction temperature.
T
A
is the ambient temperature surrounding the display.
P
D
is the power dissipated by the IC.
Rq
J-A
is the thermal resistance from the IC through the
display package and printed circuit board to the ambient.
A typical value for Rq
J-A
is 39°C/W. This value is typical for
a display mounted in a socket and covered with a plastic
lter. The socket is soldered to a 0.062 in. thick printed
circuit board with 0.020 in. wide one-ounce copper
traces.
P
D
can be calculated as follows:
P
D
= V
DD
x I
DD
V
DD
is the supply voltage and I
DD
is the supply current.
V
DD
can vary from 4.5 V to 5.5 V.
I
DD
changes with V
DD
, temperature, brightness level, and
number of on-pixels.
For AlGaAs
I
DD
(#) = (83.8 x V
DD
-0.35 x T
J
) x B x N/8
I
DD
(V) = (63 x V
DD
-0.79 x T
J
) x B x N/8
For the other colors
I
DD
(#) = (75.4 x V
DD
-0.28 x T
J
) x B x N/8
I
DD
(V) = (54 x V
DD
-0.6 x T
J
) x B x N/8
I
DD
(#) is the supply current using “#” as the displayed
character.
I
DD
(V) is the supply current using “V” as the displayed
character.
T
J
is the IC junction temperature.
B is the percent brightness level.
N is the number of characters illuminated.
Operation in high temperature ambients may require
power derating or heatsinking. Figure 10 shows how to
derate the power for an HDSP-253X. You can reduce the
power by tighter supply voltage regulation or lowering
the brightness level.
Figure 8. Proper method to manually insert a display.