AN874 Datasheet, PDF (2/6 Pages) Maxim Integrated Products – Using the MAX6950 and the MAX6951 LED Display Drivers with Multiplex Dual-Digit Displays

LED
Digit
6*
LED
Digit
7*
SEG dp
SEG dp
SEG g
SEG g
SEG f
SEG f
SEG e
SEG e
SEG d
SEG d
SEG c
SEG c
CC6
SEG b
SEG b
SEG
a
CC7
SEG
a
*These digits are not available for the MAX6950.
The MAX6950 and MAX6951 were intended to drive single-digit displays, and this allows the segments
for each digit to be routed to the correct driver pin. Many dual-digit displays pin out the two digits
separately, treating the digits as two singles in one package. These can be driven directly by the
MAX6950 and MAX6951. Other dual-digit displays internally pair the segment anodes for the two digits
(Figure 1) and these displays present issues.
The problem can be understood by checking the SEG dp entries in Table 1 for Digit 0 and Digit 1. SEG
dp is connected to a different pin for each digit. However, a multiplex dual-digit display commons these
connections internally (Figure 1), effectively shorting Pin 5 and Pin 6 of the driver.
Figure 1. Internal connections for a multiplex dual-digit display
Two simple solutions are presented here.
The first solution simply sacrifices the segment drive that share a cathode drive for each dual-digit. This
is SEG dp for Digit 0 and Digit 1 discussed earlier. By examining Table 1, it can be seen that the other
sacrificed segments are SEG f for Digit 2 and Digit 3, SEG d for Digit 4 and Digit 5, and SEG b for Digit
6 and Digit 7. However, it's usually the DP segments that applications can sacrifice, not segments
within the seven-segment digit.
If the DPs are not required, then the connections shown in Table 2 can be used. This is a rework of the
standard connections shown in Table 1, with the segments moved so that all segments apart from the
DP segments are available. The DP segment connections on the displays are simply left unused. The
no-decode operation should not be selected when this (or any other nonstandard) pin mapping is used.
The MAX6950 or MAX6951's internal hexadecimal fonts display correctly in decode mode only when
the standard display connections are being used.