DM9374N View Datasheet(PDF) - Fairchild Semiconductor
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DM9374N Datasheet PDF : 8 Pages
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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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