ICM7228 Datasheet, PDF (14/17 Pages) Intersil Corporation – 8-Digit, Microprocessor- Compatible, LED Display Decoder Driver

ICM7228
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DP
FIGURE 11. DIGITS SEGMENT ASSIGNMENTS
The No Decode mode of the ICM7228A and ICM7228B
allows the direct segment-by-segment control of all 64 seg-
ments driven by the device. In the No Decode mode, the
input data directly control the outputs as shown in Table 7.
TABLE 7. NO DECODE SEGMENT LOCATIONS
DATA
INPUT
ID7
ID6 ID5 ID4 ID3 ID2 ID1 ID0
Controlled Decimal a b c e g f d
Segment
Point
An input high level turns on the respective segment, except
for the decimal point, which is turned on by an input low level
on ID7.
The No Decode mode can be used in different applications
such as bar graph or status panel driving where each
segment controls an individual LED.
The ICM7228C and ICM7228D have only the Hexadecimal
and Code B character sets. The HEXA/CODE
B/SHUTDOWN input, pin 9, requires a three level input. Pin
9 selects the Hexadecimal format when pulled high, the
Code B format when floating or driven to mid-supply, and the
shutdown mode when pulled low (See Tables 3 and 4).
Table 6 also applies to the ICM7228C/D devices.
Shutdown and Display Banking
When shutdown, the ICM7228 enters a low power standby
mode typically consuming only 1µA of supply current for the
ICM7228A/B and 2.5µA for the ICM7228C/D. In this mode
the ICM7228 turns off the multiplex scan oscillator as well as
the digit and segment drivers. However, input data can still
be entered when in the shutdown mode. Data is retained in
memory even with the supply voltage as low as 2V.
The ICM7228A/B is shutdown by writing a control word with
Shutdown (lD4) low. The ICM7228C/D is put into shutdown
mode by driving pin 9, HEXA/CODE B/SHUTDOWN, low.
The ICM7228 operating current with the display blanked is
within 100µA - 200µA for all versions. All versions of the
ICM7228 can be blanked by writing Hex FF to all digits and
selecting Code B format. The ICM7228A and ICM7228B can
also be blanked by selecting No Decode mode and writing
Hex 80 to all digits (See Tables 6 and 7).
Common Anode Display Drivers, ICM7228A and
ICM7228C
The common anode digit and segment driver output schemat-
ics are shown in Figure 12. The common anode digit driver
output impedance is approximately 4Ω. This provides a nearly
constant voltage to the display digits. Each digit has a
minimum of 200mA drive capability. The N-Channel segment
driver’s output impedance of 50Ω limits the segment current to
approximately 25mA peak current per segment. Both the seg-
ment and digit outputs can directly drive the display, current
limiting resistors are not required.
Individual segment current is not significantly affected by
whether other segments are on or off. This is because the seg-
ment driver output impedance is much higher than that of the
digit driver. This feature is important in bar graph applications
where each bar graph element should have the same bright-
ness, independent of the number of elements being turned on.
Common Cathode Display Drivers, ICM7228B and
ICM7228D
The common cathode digit and segment driver output
schematics are shown in Figure 13. The N-channel digit driv-
ers have an output impedance of approximately 15Ω. Each
digit has a minimum of 50mA drive capability. The segment
drivers have an output impedance of approximately 100Ω
with typically 10mA peak current drive for each segment.
The common cathode display driver output currents are only
1_{/4 of the common anode display driver currents. Therefore,}
the ICM7228A and ICM7228C common anode display
drivers are recommended for those applications where high
display brightness is desired. The ICM7228B and ICM7228D
common cathode display drivers are suitable for driving
bubble-lensed monolithic 7 segment displays. They can also
drive individual LED displays up to 0.3 inches in height when
high brightness is not required.
Display Multiplexing
Each digit of the ICM7228 is on for approximately 320µs,
with a multiplexing frequency of approximately 390Hz. The
ICM7228 display drivers provide interdigit blanking. This
ensures that the segment information of the previous digit is
gone and the information of the next digit is stable before the
next digit is driven on. This is necessary to eliminate display
ghosting (a faint display of data from previous digit superim-
posed on the next digit). The interdigit blanking time is 10µs
typical with a guaranteed 2µs minimum. The ICM7228 turns
off both the digit drivers and the segment drivers during the
interdigit blanking period. The digit multiplexing sequence is:
D2, D5, D1, D7, D8, D6, D4 and D3. A typical digit’s drive
pulses are shown on Figure 4.
Due to the display multiplexing, the driving duty cycle for
each digit is 12%
for each segment
(i1s0₁0/₈xo¹_f/i8ts)
This means the average
peak current. This must
current
be con-
sidered while designing and selecting the displays.
Driving Larger Displays
If very high display brightness is desired, the ICM7228
display driver outputs can be externally buffered. Figures 14
thru 16 show how to drive either common anode or common
cathode displays using the ICM7228 and external driver
circuit for higher current displays.
Another method of increasing display currents is to connect
two digit outputs together and load the same data into both
digits. This drives the display with the same peak current, but
the average current doubles because each digit of the
display is on for twice as long, i.e., ¹/₄ duty cycle versus ¹/₈.
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