ISD201X, ISD231X, ISD235X

2006-04-04

11

The small alphanumeric displays are hybrid LED and CMOS

assemblies that are designed for reliable operation in commercial

and industrial 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

ISD displays 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, V

13 – 14.5 mA. This rise averages T

shows the V

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.

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 matrixes and the two driver 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 6) and Figures from page 12 on.

Key to equation symbols

DF

Duty factor

I

CC

Quiescent IC current

I

COL

Column current

n

Number of LEDs on in a 5 x 7 array

P

CASE

Package power dissipation excluding LED

under consideration

P

COL

Power dissipation of a column

P

DISPLAY

Power dissipation of the display

P

LED

Power dissipation of a LED

R

qCA

Thermal resistance case to ambient

R

qJC

Thermal resistance junction to case

T

A

Ambient temperature

T

J(IC)

Junction temperature of an IC

T

J(LED)

Junction temperature of a LED

T

J(MAX)

Maximum junction temperature

V

CC

IC voltage

V

COL

Column voltage

V

F(LED)

Forward voltage of LED

Z

qJC

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 „Normalized Luminous Intensity

vs. Junction Temperature“ (page 12). For optimum light output,

keep the thermal resistance of the socket or PC board as low as

possible.

Equation 1.

Equation 2.

Model Number

VF

Min.

Typ.

Max.

ISD2010

ISD2310

1.6

1.7

2.0

ISD2011/2/3

ISD2311/2/3

ISD2351/2/3

1.9

2.2

3.0

IDDG5321

θ

LED Power

LED

IC Power

IC

LED Power

LED

LED Power

LED

θ

IC Power

θ

IC

LED Power

θ

LED

θ

P

DISPLAY

T

JMAX

() TA

–

RθJC RθCA

+

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

=

P

DISPLAY

5V

COL

I

COL

n35

⁄

() DF V

CC

I

CC

+

=

T

J LED

()

P

LED

Z θJC PCASE RθJC RθCA

+

() T

A

++

=

T

J LED

()

I

COL

28

⁄

()V

F LED

() ZθJC

[]

n35

⁄

()I

COL

DF 5V

COL

() V

CC

I

CC

+

[] RθJC RθCA

+

[] T

A

+

⋅

+

=

T

JIC

()

P

COL

RθJC RθCA

+

() T

A

+

=

T

JIC

()

5V

COL

V

F LED

()

–

() I

COL

2

⁄

() n35

⁄

()DF

⋅⋅

V

CC

I

CC

⋅

+

[] RθJC RθCA

+

[]

⋅

T

A

+

=