2000 Fairchild Semiconductor Corporation
DS010210
www.fairchildsemi.com
March 1989
Revised March 2000
DM9374
7-
Segm
ent
Dec
oder/
D
ri
ver/
Latch
wit
h
Cons
tant
Cur
r
ent
Si
nk Out
puts
DM9374
7-Segment Decoder/Driver/Latch
with Constant Current Sink Outputs
General Description
The DM74 is a 7-segment decoder driver incorporating
input latches and output circuits to directly drive common
anode LED displays.
Ordering Code:
Connection Diagram
Logic Symbol
V
CC
=
Pin 16
GND
=
Pin 8
Pin Descriptions
Order Number
Package Number
Package Description
DM9374N
N16E
16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
Pin
Description
Names
A0A3
Address (Data Inputs)
LE
Latch Enable Input (Active LOW)
RBI
Ripple Blanking Input (Active LOW)
RBO
Ripple Blanking as Output (Active LOW)
as Input (Active LOW)
a g
Constant Current Outputs (Active LOW)
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2
D
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9374
Truth Table
H
=
HIGH Voltage Level
L
=
LOW Voltage Level
X
=
Immaterial
Note 1: The RBI will blank the display only if a binary zero is stored in the latches.
Note 2: RBO used as an input overrides all other input conditions.
Numerical Designations
Binary
Inputs
Outputs
Display
State
LE
RBI
A3
A2
A1
A0
a
b
c
d
e
f
g
RBO
--
H
(Note 1)
X
X
X
X
STABLE
H
Stable
0
L
L
L
L
L
L
H
H
H
H
H
H
H
L
Blank
0
L
H
L
L
L
L
L
L
L
L
L
L
H
H
0
1
L
X
L
L
L
H
H
L
L
H
H
H
H
H
1
2
L
X
L
L
H
L
L
L
H
L
L
H
L
H
2
3
L
X
L
L
H
H
L
L
L
L
H
H
L
H
3
4
L
X
L
H
L
L
H
L
L
H
H
L
L
H
4
5
L
X
L
H
L
H
L
H
L
L
H
L
L
H
5
6
L
X
L
H
H
L
L
H
L
L
L
L
L
H
6
7
L
X
L
H
H
H
L
L
L
H
H
H
H
H
7
8
L
X
H
L
L
L
L
L
L
L
L
L
L
H
8
9
L
X
H
L
L
H
L
L
L
L
H
L
L
H
9
10
L
X
H
L
H
L
H
H
H
H
H
H
L
H
--
11
L
X
H
L
H
H
L
H
H
L
L
L
L
H
E
12
L
X
H
H
L
L
H
L
L
H
L
L
L
H
H
13
L
X
H
H
L
H
H
H
H
L
L
L
H
H
L
14
L
X
H
H
H
L
L
L
H
H
L
L
L
H
P
15
L
X
H
H
H
H
H
H
H
H
H
H
H
H
BLANK
X
X
X
X
X
X
X
H
H
H
H
H
H
H
L (Note 2)
BLANK
3
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DM9374
Functional Description
The DM9374 is a 7-segment decoder/driver with latches on
the address inputs and active LOW constant current out-
puts to drive LEDs directly. This device accepts a 4-bit
binary code and produces output drive to the appropriate
segments of the 7-segment display. It has a decode format
which produces numeric codes "0" through "9" and other
codes.
Latches on the four data inputs are controlled by an active
LOW Latch Enable, LE. When LE is LOW, the state of the
outputs is determined by the input data. When LE goes
HIGH, the last data present at the inputs is stored in the
latches and the outputs remain stable. The LE pulse width
necessary to accept and store data is typically 50 ns, which
allows data to be strobed into the DM9374 at normal TTL
speeds. This feature means that data can be routed
directly from high speed counters and frequency dividers
into the display without slowing down the system clock or
providing intermediate data storage.
The latch/decoder combination is a simple system which
drives LED displays with multiplexed data inputs from MOS
time clocks, DVMs, calculator chips, etc. Data inputs are
multiplexed while the displays are in static mode. This low-
ers component and insertion costs, since several circuits--
seven resistors per display, strobe drivers, a separate dis-
play voltage source, and clock failure detect circuits--tradi-
tionally found in multiplexed display systems are
eliminated. It also allows low strobing rates to be used with-
out display flicker.
Another DM9374 feature is the reduced loading on the
data inputs when the Latch Enable is HIGH (only 10
A
typ). This allows many DM9374s to be driven from a MOS
device in multiplex mode without the need for drivers on
the data lines. The DM9374 also provides automatic blank-
ing of the leading and/or trailing-edge zeroes in a multidigit
decimal number, resulting in an easily readable decimal
display conforming to normal writing practice. In an 8-digit
mixed integer fraction decimal representation, using the
automatic blanking capability 0060.0300 would be dis-
played as 60.03. Leading-edge zero suppression is
obtained by connecting the Ripple Blanking Output (RBO)
of a decoder to the Ripple Blanking Input (RBI) of the next
lower stage device. The most significant decoder stage
should have the RIB input grounded; and since suppres-
sion of the least significant integer zero in a number is not
usually desired, the RBI input of this decoder stage should
be left open. A similar procedure for the fractional part of a
display will provide automatic suppression of trailing-edge
zeroes. The RBO terminal of the decoder can be OR-tied
with a modulating signal via an isolating buffer to achieve
duration intensity modulation. A suitable signal can be gen-
erated for this purpose by forming a variable frequency
multivibrator with a cross coupled pair of TTL or DTL gates.
Logic Diagram
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9374
Applications
It is possible with common anode 7-segment LED displays
and constant current sink decoder drivers to save substan-
tial amounts of power by carefully choosing operating
points on display supply voltage. First, examine the power
used in the normal display driving method where the dis-
play and decoder driver are both operated from a
+
5.0V
regulated supply (V
CC
=
V
S
).
The power dissipated by the LED and the driver outputs is
(V
CC
x I
seg
x n Segments). The total power dissipated with
a 15 mA LED displaying an eight (8) would be:
P
TOT
=
5.0V x 15 mA x 7
=
525 mW
Of this 525 mW, the power actually required to drive the
LED is dependent on the V
F
drop of each segment. Most
GaAsP LEDs exhibit either a 1.7V or a 3.4V forward volt-
age drop. Therefore, the required total power for seven
segments would be:
P
(1.7)
=
1.7V x 15 mA x 7
=
178.5 mW
P
(3.4)
=
3.4V x 15 mA x 7
=
357 mW
The remaining power is dissipated by the driver outputs
which are maintaining the 15 mA constant current required
by the LEDs. Most of this power is wasted, since the driver
can maintain approximately 15 mA with as little as 0.5V
across the output device. By using a separate power
source (V
S
, Figure 1) for the LEDs, which is set to the LED
V
F
plus the offset voltage of the driver, as much as 280 mW
can be saved per digit. i.e.,
V
S
=
V
F
(Max)
+
V
offset
=
2.0V
+
0.5V
=
2.5V
P
T
=
2.5V x 14 mA (from Figure 6) x 7
=
245 mW
These figures show that using a separate supply to drive
the LEDs can offer significant display power savings. In
battery powered equipment, two rechargeable nickel-cad-
mium cells in series would be sufficient to drive the display,
while four such cells would be needed to operate the logic
units.
Another method to save power is to apply intensity modula-
tion to the displays (Figure 2). It is well known that LED dis-
plays are more efficient when operated in pulse mode.
There are two reasons: one, the quantum efficiency of the
LED material is better; secondly the eye tends to peak
detect. Typically a 20% off duty cycle to displays (GaAsP)
will produce the same brightness as operating under dc
conditions.
FIGURE 1. Separate Supply for LED Displays
All Inverters are DTL 9936 or Open Collector TTL 7405
FIGURE 2. Intensity Control by RBO Pulse Duty Cycle
5
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DM9374
Low Power, Low Cost Display Power Sources--In small
line operated systems using TTL/MSI and LED or incan-
descent displays, a significant portion of the total dc power
is consumed to drive the displays. Since it is irrelevant
whether displays are driven from unfiltered dc or pulsed dc
(at fast rates), a dual power system can be used that
makes better utilization of transformer rms ratings. The
system utilizes a full wave rectified but unsmoothed dc volt-
age to provide the displays with 120 Hz pulsed power while
the reset of the system is driven by a conventional dc
power circuit. The frequency of 120 Hz is high enough to
avoid display flicker problems. The main advantages of this
system are:
Reduced transformer rating
Much smaller smoothing capacitor
Increased LED light output due to pulsed operation
With the standard capacitor filter circuit, the rms current
(full wave) loading of the transformer is approximately
twice the dc output. Most commercial transformer manu-
facturers rate transformers with capacitive input filters as
follows:
Full Wave Bridge Rectifier Circuit
Transformer rms current
=
1.8 x dc current required
Full Wave Center Tapped Rectifier Circuit
Transformer rms current
=
1.2 x dc current required
Therefore, the removal of a large portion of the filtered dc
current requirement (display power) substantially reduces
the transformer loading.
There are two basic approaches. First (Figure 3) is the
direct full wave rectified unregulated supply to power the
displays. The '74 decoder driver constant current feature
maintains the specified segment current after the LED
diode drop and 0.5V saturation voltage has been reached
(
2.2V). Care must be exercised not to exceed the '74
power ratings and the maximum voltage that the decoder
driver sees in both the "on" and "off" modes.
The second approach (Figure 4) uses a 3-terminal voltage
regulator such as the 7805 to provide dc pulsed power to
the display with the peak dc voltage limited to
+
5.0V. This
approach allows easier system thermal management by
heat sinking the regulator rather than the display or display
drivers. When this power source is used with an intensity
modulation scheme or with a multiplexed display system,
the frequencies must be chosen such that they do not beat
with the 120 Hz full wave rectified power frequency.
FIGURE 3. Direct Unregulated Display Supply
FIGURE 4. Pulsed Regulated Display Supply
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