TC7116/A/TC7117/A
DS21457D-page 14 2002-2012 Microchip Technology Inc.
6.0 TYPICAL APPLICATIONS
The TC7117/TC7117A sink the LED display current,
causing heat to build up in the IC package. If the inter-
nal voltage reference is used, the changing chip tem-
perature can cause the display to change reading. By
reducing the LED common anode voltage, the TC7117/
TC7117A package power dissipation is reduced.
Figure 6-1 is a curve tracer display showing the rela-
tionship between output current and output voltage for
typical TC7117CPL/TC7117ACPL devices. Since a
typical LED has 1.8V across it at 8mA and its common
anode is connected to +5V, the TC7117/TC7117A out-
put is at 3.2V (Point A, Figure 6-1). Maximum power
dissipation is 8.1mA x 3.2V x 24 segments = 622mW.
However, notice that once the TC7117/TC7117A’s out-
put voltage is above 2V, the LED current is essentially
constant as output voltage increases. Reducing the
output voltage by 0.7V (Point B Figure 6-1) results in
7.7mA of LED current, only a 5% reduction. Maximum
power dissipation is now only 7.7mA x 2.5V x 24 =
462mW, a reduction of 26%. An output voltage reduc-
tion of 1V (Point C) reduces LED current by 10%
(7.3mA), but power dissipation by 38% (7.3mA x 2.2V
x 24 = 385mW).
FIGURE 6-1: TC7117/TC7117A Output
vs. Output Voltage
Reduced power dissipation is very easy to obtain.
Figure 6-2 shows two ways: either a 5.1, 1/4W resis-
tor, or a 1A diode placed in series with the display (but
not in series with the TC7117/TC7117A). The resistor
reduces the TC7117/TC7117A’s output voltage (when
all 24 segments are ON) to Point C of Figure 6-1. When
segments turn off, the output voltage will increase. The
diode, however, will result in a relatively steady output
voltage, around Point B.
In addition to limiting maximum power dissipation, the
resistor reduces change in power dissipation as the
display changes. The effect is caused by the fact that,
as fewer segments are ON, each ON output drops
more voltage and current. For the best-case of six
segments (a “111” display) to worst-case (a “1888”
display), the resistor circuit will change about 230mW,
while a circuit without the resistor will change about
470mW. Therefore, the resistor will reduce the effect of
display dissipation on reference voltage drift by about
50%.
The change in LED brightness caused by the resistor is
almost unnoticeable as more segments turn off. If
display brightness remaining steady is very important
to the designer, a diode may be used instead of the
resistor.
FIGURE 6-2: Diode or Resistor Limits
Package Power Dissipation
C
B
A
6.000
7.000
8.000
9.000
10.000
2.00 2.50 3.00 3.50 4.00
Output Voltage (V)
TP2
TP5
100
k
Ω
TP1
24k
Ω
1k
Ω
0.1
μF
TP3
0.01
μF
+
In
–
0.22
μ
F
Display
Display
100
pF
+5V
1M
Ω
-5V
150k
Ω
0.47
μF
TC7117
TC7117A
40 TP
4
3035 21
20101
47
k
Ω
1N4001
1.5W, 1/4Ω
