Datasheet

15
Current Calculations
The peak and average display current requirements have
a signicant impact on power supply selection. The
maximum peak current is calculated with Equation 3.
The average current required by the display can be
calculated with Equation 4.
The power supply has to be able to supply I
PEAK
tran-
sients and supply I
LED
(AVG) continuously. The range on
V
LED
allows noise on this supply without sig ni 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 vary from 0 to 5.5 V without aecting either
the Dot or the Control Registers. V
LED
can be varied
between 4.0 to 5.5 V with out any noticeable variation in
light output. However, oper at ing V
LED
below 4.0 V may
cause objectionable mismatch between the pixels and is
not recommended. Dimming the display by pulse width
modulat ing V
LED
is also not recommended.
V
LOGIC
can vary from 3.0 V to 5.5 V without aecting
either the displayed message or the display intensity.
However, operation below 4.5 V will change the timing
and logic levels and operation below 3 V may cause the
Dot and Control Registers to be altered.
The logic ground is internally connected to the LED
ground by a substrate diode. This diode becomes for-
ward 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 con-
nections 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. The
LED ground for the LED drivers can be routed separately
from the logic ground until an appropri ate ground
plane is available. On long interconnections between
the display and the host system, voltage drops on the
analog ground can be kept from aecting the display
logic levels by isolating the two grounds.
Electrostatic Discharge
The inputs to the ICs are pro tected against static dis-
charge and input current latchup. How ever, for best
results, standard CMOS handling precautions should
be used. Before use, the HCMS-29xx 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 conduc tive
wrist straps. Lab coats made of synthetic material should
be avoided since they are prone to static buildup. In-
put current latchup 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 latchup and ESD damage, unused
inputs should be con nected to either ground or V
LOGIC
.
Voltages should not be applied to the inputs until V
LOGIC
has been applied to the display.
Appendix C. Oscillator
The oscillator provides the internal refresh circuitry with
a signal that is used to synchron ize the columns and
rows. This ensures that the right data is in the dot driv-
ers for that row. This signal can be supplied from either
an external source or the internal source.
A display refresh rate of 100 Hz or faster ensures
icker-free operation. Thus for an external oscillator
the frequency should be greater than or equal to 512
x 100 Hz = 51.2 kHz. Operation above 1 MHz without
the prescaler or 8 MHz with the prescaler may cause
noticeable pixel to pixel mismatch.
Appendix D. Refresh Circuitry
This display driver consists of 20 one-of-eight column
decoders and 20 constant current sources, 1 one-of-eight
row decoder and eight row sinks, a pulse width modula-
tion control block, a peak current control block, and the
circuit to refresh the LEDs. The refresh counters and oscil-
lator are used to synchronize the columns and rows.
The 160 bits are organized as 20 columns by 8 rows. The
IC illuminates the display by sequentially turning ON each
of the 8 row-drivers. To refresh the display once takes
512 oscillator cycles. Because there are eight row driv-
ers, each row driver is selected for 64 (512/8) oscillator
cycles. Four cycles are used to briey blank the display
before the following row is switched on. Thus, each row
is ON for 60 oscillator cycles out of a possible 64. This
corresponds to the maximum LED on time.