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

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 5

s. 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 - 2

s) 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

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

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

ICM7211AM

B3

B2

B1

BO

000

0

000

1

001

0

001

1

010

0

010

1

011

0

011

1

100

0

100

1

101

0

101

1

OSCILLATOR

FREQUENCY

BACKPLANE

INPUT/OUTPUT

OFF

ON

64 CYCLES

64 CYCLES

128 CYCLES

SEGMENTS

SEGMENTS

FIGURE 4. DISPLAY WAVEFORMS

ICM7211AM

ICM7211AM