HCMS-29X5 Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – High Performance CMOS 5 x 7 Alphanumeric Displays

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
below.
A typical value for Rθ_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 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.
Equation 1:
T_JMAX = T_A + P_D * Rθ_JA
Where:
T_JMAX = maximum IC junction temperature
T_A = ambient temperature surrounding the display
Rθ_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 significantly
changing the display brightness.
V_LOGIC and V_LED Considerations
The display uses two independent
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 vary from 0 to 5.5
V without affecting either the Dot
or the Control Registers. V_LED can