TC7107 datasheet(16/26 Pages) MICROCHIP

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TC7106/A/TC7107/A

DS21455B-page 16

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 gen-

erate -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 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

TC7107A package power dissipation is reduced.

Figure 9-3 is a curve tracer display showing the rela-

tionship 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 connected

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

Figure 9-3). Maximum power dissipation is 8.1mA x

3.2V x 24 segments = 622mW.

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 con-

stant as output voltage increases. Reducing the output

voltage by 0.7V (point B in Figure 9-3) results in 7.7mA

of LED current, only a 5 percent reduction. Maximum

power dissipation is only 7.7mA x 2.5V 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).

Reduced power dissipation is very easy to obtain.

Figure 9-4 shows two ways: either a 5.1 ohm, 1/4 watt

resistor or a 1 Amp diode placed in series with the dis-

play (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.”

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 fewer segments are “ON,” each “ON” output drops

more voltage and current. For the best case of six seg-

TC7107A

V+

OSC1

OSC2

OSC3

GND

V-

V+

CD4009

0.047

µF

1N914

µF

–

V- = -3.3V

GND

VIN-

VIN

VREF+

VREF-

COM

+5V

LED

DRIVE

V+

V-

TC7660

µF

(-5V)

TC7107A

VIN+

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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