TC7107 데이터 시트보기 (PDF) - TelCom Semiconductor Inc => Microchip

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TC7107

3-1/2 DIGIT A/D CONVERTERS

TelCom Semiconductor Inc => Microchip 

TC7106

TC7106A

TC7107

TC7107A

3-1/2 DIGIT A/D CONVERTERS

POWER SUPPLIES

The TC7107A is designed to work from ±5V supplies.

However, if a negative supply is not available, it can be

generated from the clock output with two diodes, two capaci-

tors, and an inexpensive IC. (Figure 12)

In selected applications a negative supply is not re-

quired. The conditions to use a single +5V supply are:

• The input signal can be referenced to the center of the

common-mode range of the converter.

• The signal is less than ±1.5V.

• An external reference is used.

The TSC7660 DC to DC converter may be used to

generate – 5 V from +5 V (Figure 13).

V+

CD4009

V+

OSC1

OSC2

OSC3

TC7107A

GND

V–

0.047

µF

1N914

10 +

µF –

1N914

TC7107 Power Dissipation Reduction

The TC7107A sinks the LED display current and this

causes 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 TC7107A package power

dissipation is reduced.

Figure 14 is a photograph of a curve-tracer display

showing the relationship between output current and output

voltage for a typical TC7107CPL. Since a typical LED has

1.8 volts across it at 7mA, and its common anode is con-

nected to +5V, the TC7107A output is at 3.2V (point A on

Figure 13). Maximum power dissipation is 8.1mA x 3.2V x

24 segments = 622mW.

Notice, however, that once the TC7107A output voltage

is above two volts, the LED current is essentially constant as

output voltage increases. Reducing the output voltage by

0.7V (point B in Figure 14) results in 7.7mA of LED current,

only a 5 percent reduction. Maximum power dissipation is

only 7.7mA x 2.5 V x 24 = 462mW, a reduction of 26%. An

output voltage reduction of 1 volt (point C) reduces LED

current by 10% (7.3mA) but power dissipation by 38%!

(7.3mA x 2.2V x 24 = 385mW).

V– = –3.3V

Figure 12. Generating Negative Supply From +5 V

+5 V

LED

DRIVE

1

V+ VR+EF 36

VR– EF 35

32

COM

TC7107A

VI+N 31

VIN

VI–N 30

+

10µF

8

2

5

4 TC7660

V– GND 21

26

(–5 V)

3

+ 10µF

*3-1/2 DIGIT ADC

Figure 13. Negative Power Supply Generation with TC7660

3-196

Figure 14. TC7107A Output Current vs Output Voltage

Reduced power dissipation is very easy to obtain.

Figure 15 shows two ways: either a 5.1 ohm, 1/4 watt resistor

or a 1 Amp diode placed in series with the display (but not in

series with the TC7107A). The resistor will reduce the

TC7107A output voltage, when all 24 segments are “ON,” to

point “C” of Figure 14. When segments turn off, the output

voltage will increase. The diode, on the other hand, will result

in a relatively steady output voltage, around point “B.”

In addition to limiting maximum power dissipation, the

resistor reduces the change in power dissipation as the

display changes. This effect is caused by the fact that, as

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