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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
F(LED), and forward current IF(LED), of
13 – 14.5 mA. This rise averages T
J(LED)=1°C. The Table below
shows the V
F(LED) for the respective displays.
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-
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
R 1
LED Power
LED
T1
IC Power
Rθ2
IC
T2
LED Power
Rθ1
LED
T1
LED Power
LED
θ
R 1
T1
IC Power
θ
R 2
IC
T2
LED Power
R 1
θ
LED
T1
θ
R CA
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
+
=