6
FN3158.8
October 22, 2015
Description of Operation
Device
The ICM7211AM provides outputs suitable for driving
conventional four-digit, seven-segment LCD displays. These
devices include 28 individual segment drivers, backplane
driver, and a self-contained oscillator and divider chain to
generate the backplane frequency.
The segment and backplane drivers each consist of a
CMOS inverter, with the N-Channel and P-Channel devices
ratioed to provide identical on resistances, and thus equal
rise and fall times. This eliminates any DC component, which
could arise from differing rise and fall times, and ensures
maximum display life.
The backplane output devices can be disabled by
connecting the OSCillator input (pin 36) to V
SS
. This allows
the 28 segment outputs to be synchronized directly to a
signal input at the BP terminal (pin 5). In this manner,
several slave devices may be cascaded to the backplane
output of one master device, or the backplane may be
derived from an external source. This allows the use of
displays with characters in multiples of four and a single
backplane. A slave device represents a load of
approximately 200pF (comparable to one additional
segment). Thus the limitation of the number of devices that
can be slaved to one master device backplane driver is the
additional load represented by the larger backplane of
displays of more than four digits. A good rule of thumb to
observe in order to minimize power consumption is to keep
the backplane rise and fall times less than about 5s. The
backplane output driver should handle the backplane to a
display of 16 one-half inch characters. It is recommended, if
more than four devices are to be slaved together, the
backplane signal be derived externally and all the
ICM7211AM devices be slaved to it. This external signal
should be capable of driving very large capacitive loads with
short (1 - 2s) rise and fall times. The maximum frequency
for a backplane signal should be about 150Hz although this
may be too fast for optimum display response at lower
display temperatures, depending on the display type.
The onboard oscillator is designed to free run at
approximately 19kHz at microampere current levels. The
oscillator frequency is divided by 128 to provide the
backplane frequency, which will be approximately 150Hz
with the oscillator free-running; the oscillator frequency may
be reduced by connecting an external capacitor between the
OSCillator terminal and V
DD
.
The oscillator may also be overdriven if desired, although care
must be taken to ensure that the backplane driver is not
disabled during the negative portion of the overdriving signal
(which could cause a DC component to the display). This can
be done by driving the OSCillator input between the positive
supply and a level out of the range where the backplane disable
is sensed (about one fifth of the supply voltage above V
SS
).
Another technique for overdriving the oscillator (with a signal
swinging the full supply) is to skew the duty cycle of the
overdriving signal such that the negative portion has a duration
shorter than about one microsecond. The backplane disable
sensing circuit will not respond to signals of this duration.
Input Configurations and Output Codes
The ICM7211AM accepts a four-bit true binary (i.e., positive
level = logical one) input at pins 27 thru 30, least significant
bit at pin 27 ascending to the most significant bit at pin 30. It
decodes the binary input into seven-segment alphanumeric
“Code B” output, i.e., 0-9, dash, E, H, L, P, blank. These
codes are shown explicitly in Table 1. It will correctly decode
true BCD to a seven-segment decimal output.
TABLE 1. OUTPUT CODES
BlNARY
CODE B
ICM7211AMB3 B2 B1 BO
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
OSCILLATOR
FREQUENCY
BACKPLANE
INPUT/OUTPUT
OFF
ON
64 CYCLES
64 CYCLES
128 CYCLES
SEGMENTS
SEGMENTS
FIGURE 4. DISPLAY WAVEFORMS
ICM7211AMICM7211AM
