TC7117CPL datasheet(13/22 Pages) MICROCHIP

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DS21457B-page 13

TC7116/A/TC7117/A

6.0

TYPICAL APPLICATIONS

The TC7117/TC7117A sink the LED display current,

causing 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 TC7117/

TC7117A package power dissipation is reduced.

Figure 6-1 is a curve tracer display showing the rela-

tionship between output current and output voltage for

typical TC7117CPL/TC7117ACPL devices. Since a

typical LED has 1.8V across it at 8mA and its common

anode is connected to +5V, the TC7117/TC7117A out-

put is at 3.2V (Point A, Figure 6-1). Maximum power

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

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 6-1) results in

7.7mA of LED current, only a 5% reduction. Maximum

power dissipation is now only 7.7mA x 2.5V x 24 =

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

tion of 1V (Point C) reduces LED current by 10%

(7.3mA), but power dissipation by 38% (7.3mA x 2.2V

x 24 = 385mW).

FIGURE 6-1:

TC7117/TC7117A OUTPUT

VS. OUTPUT VOLTAGE

Reduced power dissipation is very easy to obtain.

Figure 6-2 shows two ways: either a 5.1

Ω,1/4W resis-

tor, 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 6-1. 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 seg-

ments (a “111” display) to worst case (a “1888” display),

the resistor circuit will change about 230mW, while a

circuit without the resistor will change about 470mW.

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 dis-

play brightness remaining steady is very important to

the designer, a diode may be used instead of the

resistor.

FIGURE 6-2:

DIODE OR RESISTOR

LIMITS PACKAGE POWER

DISSIPATION

6.000

7.000

8.000

9.000

10.000

2.00

2.50

3.00

3.50

4.00

Output Voltage (V)

TP2

TP5

100

Ω

TP1

24k

Ω

0.1

µF

TP3

0.01

µF

–

0.22

µF

Display

100

+5V

Ω

-5V

150k

Ω

0.47

µF

TC7117

TC7117A

Ω

1N4001

1.5W, 1/4

Ω

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