15

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.

The  average  current  required  by  the  display  can  be 

calculated with Equation 4.

changing the display brightness.

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 

between 4.0 to 5.5 V without any noticeable variation in 

cause objectionable mismatch between the pixels and is 

not recommended. Dimming the display by pulse width 

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  appropriate  ground 

plane  is  available.  On  long  interconnections  between 

the display and the host system, voltage drops on the 

analog  ground  can  be  kept  from  affecting  the  display 

logic levels by isolating the two grounds.

Electrostatic Discharge

The  inputs  to  the  ICs  are  protected  against  static  dis-

charge  and  input  current  latchup.  However,  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  conductive 

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 

and  when  a  high  current  is  forced  into  the  input.  To 

prevent input current latchup and ESD damage, unused 

has been applied to the display.

Appendix C. Oscillator

The oscillator provides the internal refresh circuitry with 

a  signal  that  is  used  to  synchronize  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 

flicker-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 briefly 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.