TC7106CLW datasheet(18/34 Pages) MICROCHIP

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TC7106/A/TC7107/A

DS21455D-page 18

9.0

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

capacitors, and an inexpensive IC (Figure 9-1).

FIGURE 9-1:

Generating Negative Supply

From +5V.

In selected applications a negative supply is not

required. 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 -5V from +5V (Figure 9-2).

FIGURE 9-2:

Negative Power Supply

Generation with TC7660.

9.1

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

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 9-3 is 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 7 mA, and its common anode is

connected to +5V, the TC7107A output is at 3.2V (point

A on Figure 9-3). Maximum power dissipation is

8.1 mA x 3.2V x 24 segments = 622 mW.

FIGURE 9-3:

TC7107 Output Current vs.

Output Voltage.

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 9-3) results in

7.7 mA of LED current, only a 5 percent reduction.

Maximum power dissipation is only 7.7 mA x 2.5V x 24

= 462 mW, a reduction of 26%. An output voltage

reduction of 1 volt (point C) reduces LED current by

10% (7.3 mA) but power dissipation by 38% (7.3 mA x

2.2V x 24 = 385 mW).

Reduced power dissipation is very easy to obtain.

Figure 9-4 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 TC7107A). The resistor will

reduce the TC7107A output voltage, when all 24

segments are “ON,” to point “C” of Figure 9-4. 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”.

TC7107A

V+

OSC1

OSC2

OSC3

GND

V-

V+

CD4009

0.047

µF

1N914

µF

–

V- = -3.3V

GND

REF +

REF -

COM

+5V

LED

DRIVE

V+

V-

TC7660

10 µF

(-5V)

TC7107A

IN+

6.000

7.000

8.000

9.000

10.000

2.00

2.50

3.00

3.50

4.00

Output Voltage (V)

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