HCMS-2975_13 Datasheet, PDF (14/16 Pages) AVAGO TECHNOLOGIES LIMITED – High Performance CMOS 5 x 7 Alphanumeric Displays

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 below.
A typical value for Rq_JA is 100°C/W. This value is typical
for a display mounted in a socket and covered with a
plastic filter. The socket is soldered to a .062 in. thick PCB
with .020 inch wide, one ounce copper traces.
P_D can be calculated as Equa­tion 2 below.
Figure 4 shows how to derate the power of one IC versus
ambient temperature. Opera­tion at high ambient tem-
peratures may require the power per IC to be reduced.
The power con­sump­tion 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
1.3
1.2
R_θJ-A = 100°C/W
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
25 30 35 40 45 50 55 60 65 70 75 80 85 90
T_A – AMBIENT TEMPERATURE – °C
Figure 4.
Equation 1:
T_JMAX = T_A + P_D * Rq_JA
Where:
T_JMAX = 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 * Osccyc/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 Definitions above)
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