HCMS-3901 Datasheet, PDF (17/18 Pages) AVAGO TECHNOLOGIES LIMITED – 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.
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
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 5.5V 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 2.6V. 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 5.5V 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
connections are used, the LED ground can vary from
-0.3 V to +0.3 V with respect to the logic ground. Voltages
below -0.3 V can cause all the dots to be ON. Voltage
above +0.3 V can cause dimming and dot mismatch.
Using a decoupling capacitor between the power supply
and ground will help prevent any supply noise in the
frequency range greater than that of the functioning
display from interfering with the display’s internal
circuitry. The value of the capacitor depends on the series
resistance from the ground back to the power supply
and the range of frequencies that need to be suppressed.
It is also advantageous to use the largest ground plane
possible.
Electrostatic Discharge
The inputs to the ICs are protected against static
discharge and input current latch up. However, for best
results, standard CMOS handling precautions should
be used. Before use, the HCMS-39XX should be stored
in antistatic tubes or in conductive material. During
assembly, a grounded conductive work area should be
used and assembly personnel should wear conductive
wrist straps. Lab coats made of synthetic material
should be avoided since they are prone to static
buildup. Input current latch up is caused when the
CMOS inputs are subjected to either a voltage below
ground (V_IN < ground) or to a voltage higher than
V_LOGIC (V_IN >V_LOGIC) and when a high current is forced
into the input. To prevent input current latch up and ESD
damage, unused inputs should be connected to either
ground or V_LOGIC. Voltages should not be applied to the
inputs until V_LOGIC has been applied to the display.
Table 3. Equations.
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)
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