3-1/2 DIGIT A/D CONVERTERS
TC7106
TC7106A
TC7107
TC7107A
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
+
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).
OSC1
OSC2
OSC3
0.047
µF
1N914
10
µF
1N914
+
–
TC7107A
GND
–
V
–
V = –3.3V
Figure 12. Generating Negative Supply From +5 V
+5 V
V
1
+
36
+
VREF
35
–
VREF
32
COM
LED
DRIVE
TC7107A
Figure 14. TC7107A Output Current vs Output Voltage
+ 31
VIN
VIN
– 30
VIN
21
GND
V
8
+
10µF
2
5
4
TC7660
3
+
10µF
(–5 V)
–
26
*3-1/2 DIGIT ADC
Figure 13. Negative Power Supply Generation with TC7660
3-196
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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