TC7117 TelCom, TC7117 Datasheet - Page 13

• Manufacturer Part Number: TC7117
• Description: 3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS WITH HOLD
• Manufacturer: TelCom

3-1/2 DIGIT ANALOG-TO-DIGITAL
CONVERTERS WITH HOLD
APPLICATIONS INFORMATION
causing heat to build up in the IC package. If the internal
voltage reference is used, the changing chip temperature
can cause the display to change reading. By reducing the
LED common anode voltage, the TC7117/TC7117A pack-
age power dissipation is reduced.
ship between output current and output voltage for typical
TC7117CPL/TC7117ACPL devices. Since a typical LED
has 1.8V across it at 8 mA and its common anode is
connected to +5V, the TC7117/TC7117A output is at 3.2V
(Point A, Figure 17). Maximum power dissipation is 8.1 mA
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 17) results in 7.7 mA of LED
current, only a 5% reduction. Maximum power dissipation is
now only 7.7 mA 2.5V 24 = 462 mW, a reduction of 26%.
An output voltage reduction of 1V (Point C) reduces LED
current by 10% (7.3 mA), but power dissipation by 38% (7.3
mA
Figure 15. Recommended Component Values for 2V Full Scale
3.2V
The TC7117/TC7117A sink the LED display current,
Figure 17 is a curve-tracer display showing the relation-
However, notice that once the TC7117/TC7117A's out-
TELCOM SEMICONDUCTOR, INC.
TC7116A
TC7117A
2.2V
TC7116
TC7117
24 segments = 622 mW.
(TC7116/TC7116A and TC7117/TC7117A)
24 = 385 mW).
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
0.047 µF
0.22 µF
TO DISPLAY
0.1 µF
100 k
100 pF
25 k
SET V REF = 1V
470 k
0.01 µF
1M
24 k
+
–
IN
V +
V –
Figure 18 shows two ways: Either a 5.1 , 1/4W resistor, 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 17. When segments turn off, the
output voltage will increase. The diode, however, will result
in a relatively steady output voltage, around Point B.
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 230 mW, while a circuit without the resistor will
change about 470 mW. Therefore, the resistor will reduce
the effect of display dissipation on reference voltage drift by
about 50%.
almost unnoticeable as more segments turn off. If display
brightness remaining steady is very important to the de-
signer, a diode may be used instead of the resistor.
Figure 16. TC7117/TC7117A Operated from Single +5V Supply
Reduced power dissipation is very easy to obtain.
In addition to limiting maximum power dissipation, the
The change in LED brightness caused by the resistor is
TC7117A
TC7117
(An External Reference Must Be Used in This
Application.)
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
0.22 µF
0.47 µF
TO DISPLAY
0.1 pF
100 k
100 pF
1 k
47 k
0.01 µF
SET V REF = 100 mV
1.2V
10 k
TC7116A
TC7117A
1 M
10 k
TC7116
TC7117
TC9491CZM
+
–
3-215
IN
V
+
1
2
3
4
5
6
7
8