9-14

Description Of Operation

LCD Devices

The LCD devices in the family (ICM7211, ICM7211A,

ICM7211M, ICM7211AM) provide 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 connect-

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 charac-

ters in multiples of four and a single backplane. A slave

device represents a load of approximately 200pF (compara-

ble 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 con-

sumption is to keep the backplane rise and fall times less

than about 5

µs. 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 ICM7211 devices be slaved to it. This external signal

should be capable of driving very large capacitive loads with

short (1 - 2

µs) 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 dis-

play temperatures, depending on the display type.

The onboard oscillator is designed to free run at approxi-

mately 19kHz at microampere current levels. The oscillator

frequency is divided by 128 to provide the backplane fre-

quency, which will be approximately 150Hz with the oscillator

free-running; the oscillator frequency may be reduced by

connecting an external capacitor between the OSCillator ter-

The oscillator may also be overdriven if desired, although care

must be taken to ensure that the backplane driver is not dis-

abled 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 over-

driving 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.

LED Devices

The LED device in the family (ICM7212AM) provides outputs

suitable

for

directly

driving

four-digit,

seven-segment

common-anode LED displays. These devices include 28

individual segment drivers, each consisting of a low-leakage,

current-controlled, open-drain, N-Channel transistor.

The drain current of these transistors can be controlled by

varying the voltage at the BRtrighTness input (pin 5). The volt-

age at this pin is transferred to the gates of the output devices

for “on” segments, and thus directly modulates the transistor’s

“on” resistance. A brightness control can be easily imple-

mented with a single potentiometer controlling the voltage at

pin 5, connected as in Figure 9. The potentiometer should be

a high value (100k

Ω to 1MΩ) to minimize power consumption,

which can be significant when the display is off.

The brightness input may also be operated digitally as a dis-

play enable; when high, the display is fully on, and low fully

off. The display brightness may also be controlled by varying

the duty cycle of a signal swinging between the two voltages

at the brightness input.

of these pins should be connected. The double connection is

necessary to minimize effects of bond wire resistance with

the large total display currents possible.

When operating LED devices at higher temperatures and/or

higher supply voltages, the device power dissipation may

need to be reduced to prevent excessive chip temperatures.

It is recommended that if the device is to be operated at

OSCILLATOR

FREQUENCY

BACKPLANE

INPUT/OUTPUT

OFF

ON

64 CYCLES

64 CYCLES

128 CYCLES

SEGMENTS

SEGMENTS

FIGURE 8. DISPLAY WAVEFORMS

100k

Ω TO 1MΩ

BRIGHTNESS

PIN 5

FIGURE 9. BRIGHTNESS CONTROL

ICM7211, ICM7212