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TELCOM SEMICONDUCTOR, INC.

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3-1/2 DIGIT ANALOG-TO-DIGITAL

CONVERTERS WITH HOLD

TC7116

TC7116A

TC7117

TC7117A

APPLICATIONS INFORMATION

The TC7117/TC7117A sink the LED display current,

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.

Figure 17 is a curve-tracer display showing the relation-

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

× 3.2V × 24 segments = 622 mW.

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 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

× 2.2V × 24 = 385 mW).

Reduced power dissipation is very easy to obtain.

Figure 18 shows two ways: Either a 5.1

Ω, 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.

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 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%.

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 de-

signer, a diode may be used instead of the resistor.

Figure 15. Recommended Component Values for 2V Full Scale

(TC7116/TC7116A and TC7117/TC7117A)

Figure 16. TC7117/TC7117A Operated from Single +5V Supply

(An External Reference Must Be Used in This

Application.)

100 k

Ω

100 pF

0.047 µF

470 k

Ω

0.22 µF

TO DISPLAY

0.1 µF

25 k

Ω

24 k

V

+

SET VREF = 1V

0.01 µF

+

IN

1M

Ω

–

V

–

21

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24

25

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31

32

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39

40

TC7116

TC7116A

TC7117

TC7117A

Ω

100 pF

0.47 µF

47 k

Ω

TO DISPLAY

0.1 pF

1 k

Ω

Ω

V

SET VREF= 100 mV

+

10 k

10 k

Ω

1.2V

0.01 µF

–

IN

1 M

Ω

+

100 k

Ω

0.22 µF

21

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24

25

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TC9491CZM

TC7117

TC7117A