HCMS-39X6 Datasheet, PDF (19/22 Pages) Agilent(Hewlett-Packard) – 3.3 V High Performance CMOS 5x7 AlphaNumeric Displays

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 in Table 3.
Table 3. Equations.
The average current required by
the display can be calculated with
Equation 4 in Table 3.
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.
Equation 1:
T_J MAX = T_A + P_D * Rθ_JA
Where: T_J MAX = maximum IC junction temperature
T_A = ambient temperature surrounding the display
Rθ_JA = thermal resistance from the IC junction to ambient
P_D = total power dissipation
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)
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 inde-
pendently. Thus, V_LED can vary
from 0 to 5.5 V without affecting
either the Dot or the Control
Registers. V_LED can be varied
between 3.1 to 5.5 V without
much noticeable variation in light
output to the human eyes. There
is also no pixel mismatch
observed.
The intensity of the light output
takes a plunge if operated less
than 3.1 V. There is also no pixel
mismatch observed at voltage as
low as 2.6 V. However, operating
below 3.1 V is not recommended.
Dimming the display by pulse
width modulating V_LED is also not
recommended.
V_LOGIC can vary from 3.0 to 5.5 V
without affecting either the
displayed message or the display
intensity. However, operating
below 3 V may change the timing
and logic levels and may cause
Dot and Control Registers to be
altered. Thus, operation of the
display below 3.0 V is not
recommended.
The logic ground is internally
connected to the LED ground by
a substrate diode. This diode
becomes forward biased and
conducts when the logic ground is
0.4 V greater than the LED
ground. The LED ground and the
logic ground should be connected
to a common ground, which can
withstand the current introduced
by the switching LED drivers.
When separate ground
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