DM9374 データシートの表示（PDF） - Fairchild Semiconductor
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DM9374 Datasheet PDF : 8 Pages
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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 (VCC = VS).
The power dissipated by the LED and the driver outputs is
(VCC x Iseg x n Segments). The total power dissipated with
a 15 mA LED displaying an eight (8) would be:
PTOT = 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 VF 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 (VS, Figure 1) for the LEDs, which is set to the LED
VF plus the offset voltage of the driver, as much as 280 mW
can be saved per digit. i.e.,
VS = VF (Max) + Voffset
= 2.0V + 0.5V
= 2.5V
PT= 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
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