6-12

TELCOM SEMICONDUCTOR, INC.

BUS COMPATIBLE, 4-DIGIT

CMOS DECODER/DRIVER

TC7211AM

If more than four devices are slaved together, the

backplane signal should be derived externally and all

TC7211AM devices slaved to it. The external drive signal

must drive a high capacitive load with 1

µsec to 2 µsec rise

and fall times. The backplane frequency is normally 125

Hz. At lower display ambient temperatures, the frequency

may be reduced to compensate for display response time.

The on-chip RC oscillator free-runs at approximately

16 kHz. A divide-by 128 circuit provides the125 Hz backplane

frequency. The oscillator frequency may be reduced by

connecting an external capacitor between the oscillator

curves.)

The free-running oscillator may be overridden (if de-

sired) by an external clock. The backplane driver, however,

must not be disabled during the external clock’s negative or

lOW portion, as this will result in a DC drive component

being applied to the LCD, limiting the LCD’s life. To

prevent backplane driver disabling, the oscillator input

should be driven from the positive supply to no less than

one-fifth the supply voltage above ground. A backplane

disable signal will not be sensed if the driving signal remains

above ground by one-fifth the supply voltage. An alternate

method for externally driving the oscillator permits the

oscillator input to swing the full supply voltage range. The

oscillator input signal duty cycle is skewed so the LOW

BASIC OPERATION

The TC7211AM drives 4-digit, 7-segment LCDs. This

device contains 28 individual segment drivers, a backplane

driver, a self-contained oscillator, and a divider chain to

generate the backplane signal.

The 28 CMOS segment drivers and backplane driver

contain ratioed N- and P-channel transistors for identical ON

resistance. The equal resistances eliminate the DC output

driver component resulting from unequal rise and fall times.

This ensures maximum LCD life.

The backplane output driver can be disabled by ground-

ing the OSCILLATOR input (pin 36). The 28 output segment

drivers can therefore be synchronized directly to an input

signal at the backplane (BP) terminal (pin 5). Several slave

devices may be cascaded to the backplane output of a

master device. The backplane signal may also be derived

from an external source. These features permit interfacing to

single backplane LCDs with characters in multiples of four.

Each slave’s backplane input represents a 200pF

capacitive load to the master backplane driver (comparable

to one additional segment). The number of slave devices

drivable by a master device is therefore set by the larger

display backplane capacitive load. The master backplane

output will drive the display backplane of 16 one-half-inch

characters with rise and fall times under 5

µsec. This

represents a system with three slave devices and a fourth

master device driving the backplane.

TYPICAL CHARACTERISTICS

30

25

20

15

10

5

1

234

5

6

7

Operating Supply Current

vs Supply Voltage

TA= –20°C

TA= +70°C

TA= +25°C

V+ VOLTS

0

LCD DEVICES

DISPLAY BLANK

PIN 36 OPEN

180

150

120

90

60

30

0

Backplane Frequency

vs Supply Voltage

COSC = 0 pF

V+ VOLTS

12

3

4

5

6

7

COSC = 22 pF

COSC = 220 pF

(PIN 36 OPEN)

TA = +25°C