Datasheet

14
Appendix A. Thermal Considerations
The display IC has a maximum junction temperature of
150°C. The IC junction temperature can be calculated
with Equation 1.
A typical value for Rq
J-A
is 100 °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 .062 in. thick PCB
with .020 inch wide, one ounce copper traces.
P
D
can be calculated as in Equa tion 2.
Figure 4 shows how to derate the power of one IC versus
ambient temperature. Opera tion at high ambient tem-
peratures may require the power per IC to be reduced.
The power con sump tion can be reduced by changing
either the N, I
PIXEL
, Osc cyc or V
LED
. Changing V
LOGIC
has
very little impact on the power consumption.
Appendix B. Electrical Considerations
Equation 1:
T
J
MAX = T
A
+ P
D
* Rq
JA
Where:
T
J
MAX = maximum IC junction temperature
T
A
= ambient temperature surrounding the display
Rq
JA
= thermal resistance from the IC junction to ambient
P
D
= power dissipated by the IC
Equation 2:
P
D
= (N * I
PIXEL
* Duty Factor * V
LED
) + I
LOGIC
* V
LOGIC
Where:
P
D
= total power dissipation
N = number of pixels on (maximum 4 char * 5 * 7 = 140)
I
PIXEL
= peak pixel current.
Duty Factor = 1/8 * Osc cyc/64
Osc cyc = number of ON oscillator cycles per row
I
LOGIC
= IC logic current
V
LOGIC
= logic supply voltage
Equation 3:
I
PEAK
= M * 20 * I
PIXEL
Where:
I
PEAK
= maximum instantaneous peak current for the display
M = number of ICs in the system
20 = maximum number of LEDs on per IC
I
PIXEL
= peak current for one LED
Equation 4:
I
LED
(AVG) = N * I
PIXEL
* 1/8 * (oscillator cycles)/64
(see Variable Denitions above)
Figure 4.
P
D
MAX – MAXIMUM POWER
DISSIPATION PER IC – W
0
25
T
A
– AMBIENT TEMPERATURE – °C
0.7
0.6
0.5
0.4
0.3
0.2
0.1
60555045403530
0.8
0.9
1.0
1.1
1.2
8580757065
90
1.3
R = 100 °C/W
J-A
θ