HCMS-29XX Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – 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 temperatures
may require the power per IC to
be reduced. The power con-
sumption 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.
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)
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.
Appendix B. Electrical
Considerations
Current Calculations
The peak and average display
current requirements have a
significant impact on power
supply selection. The maximum
peak current is calculated with
Equation 3 below.
The average current required by
the display can be calculated
with Equation 4 below.
The power supply has to be able
to supply I_PEAK transients and
supply I_LED(AVG) continuously.
The range on V_LED allows noise
on this supply without signifi-
cantly changing the display
brightness.
V_LOGIC and V_LED Considerations
The display uses two indepen-
dent electrical systems. One
system is used to power the
display’s logic and the other to
power the display’s LEDs. These
two systems keep the logic
supply clean.
Separate electrical systems
allow the voltage applied to V_LED
and V_LOGIC to be varied
independently. Thus, V_LED can
14