ICM7211A Datasheet, PDF (6/9 Pages) Intersil Corporation – 4-Digit, LCD Display Driver

ICM7211A
Typical Performance Curves
30
LCD DEVICES, TEST CIRCUIT
DISPLAY BLANK, PIN 36 OPEN
25
T_A = -20^oC
20
T_A = 25^oC
15
180 _{LCD DEVICES, TA = 25oC}
150
C_OSC = 0pF
(PIN 36 OPEN)
120
C_OSC = 22pF
90
10
60
T_A = 70^oC
5
30
C_OSC = 220pF
1
2
3
4
5
6
7
V_SUPP (V)
FIGURE 3. OPERATING SUPPLY CURRENT AS A FUNCTION
OF SUPPLY VOLTAGE
0
1
2
3
4
5
6
V_SUPP (V)
FIGURE 4. BACKPLANE FREQUENCY AS A FUNCTION OF
SUPPLY VOLTAGE
Description Of Operation
LCD Devices
The ICM7211A provides outputs suitable for driving conven-
tional 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-
ing 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 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 5µs. 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 ICM7211A devices be slaved to it. This external signal
should be capable of driving very large capacitive loads with
short (1 - 2µs) 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-
minal and V_DD.
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
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 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.
OSCILLATOR
FREQUENCY
BACKPLANE
INPUT/OUTPUT
OFF
SEGMENTS
128 CYCLES
64 CYCLES
64 CYCLES
ON
SEGMENTS
FIGURE 5. DISPLAY WAVEFORMS
6