 2000 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA

HDSP200LP/1LP/2LP/3LP

www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)

OSRAM Opto Semiconductors GmbH & Co. OHG • Regensburg, Germany

www.osram-os.com • +49-941-202-7178

6

April 4, 2000-11

Thermal Considerations

The small alphanumeric displays are hybrid LED and CMOS

assemblies that are designed for reliable operation in commer-

cial, industrial, and military environments. Optimum reliability

and optical performance will result when the junction tempera-

ture of the LEDs and CMOS ICs are kept as low as possible.

Thermal Modeling

HDSP200XLP displays consist of two driver ICs and four 5 x 7

LED matrixes. A thermal model of the display is shown in Fig-

ure 5. It illustrates that the junction temperature of the semi-

conductor = junction self heating + the case temperature rise +

the ambient temperature. Equation 1 shows this relationship.

Figure 5. 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),

The junction rise within the LED driver IC is the combination of

the power dissipated by the IC quiescent current and the 28

row 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.

Model Number

VF

Min.

Typ.

Max.

HDSP2000LP

1.6

1.7

2.0

HDSP2001/2/3LP

1.9

2.2

3.0

1

2

1

1

2

1

LED Power IC Power

LED Power

LED Power IC Power

LED Power

For ease of calculations the maximum allowable electrical oper-

ating condition is dependent upon the aggregate thermal resis-

tance of the LED matrixes and the two driver ICs. All of the

thermal management calculations are based upon the parallel

combination of these two networks which is 15

°C/W. Maxi-

mum allowable power dissipation is given in Equation 3.

Equation 3.

For further reference see Figures 2, 7, 8, 9, 10 and 11.

Key to equation symbols

DF

Duty factor

Quiescent IC current

Column current

n

Number of LEDs on in a 5 x 7 array

Package power dissipation excluding LED

under consideration

Power dissipation of a column

Power dissipation of the display

Power dissipation of a LED

Thermal resistance case to ambient

Thermal resistance junction to case

Ambient temperature

Junction temperature of an IC

Junction temperature of a LED

Maximum junction temperature

IC voltage

Column voltage

Forward voltage of LED

Thermal impedance junction to case

Optical Considerations

The light output of the LEDs is inversely related to the LED

diode’s junction temperature as shown in Figure 6. For opti-

mum light output, keep the thermal resistance of the socket or

PC board as low as possible.

P

DISPLAY

T

J MAX

–

+

---------------------------------

=

P

DISPLAY

5V

⁄

() DF V

+

=

See Equation 2 below.

Equation 1.

Equation 2.

T

J LED

()

P

θJC

P

CASE

+

() T

A

++

=

T

J LED

()

I

⁄

()V

F LED

[]

n35

⁄

()I

() V

+

+

[] T

A

+

⋅

+

=

T

JIC

()

P

θJC

+

() T

A

+

=

T

JIC

()

5V

COL

V

F LED

()

–

() I

⁄

() n35

⁄

()DF

⋅⋅

V

CC

I

CC

⋅

+

+

[]

⋅

T

A

+

=