TC7106 Microchip, TC7106 Datasheet - Page 16

• Manufacturer Part Number: TC7106
• Description: 3-1/2 Digit Analog-to-Digital Converters
• Manufacturer: Microchip

TC7106/A/TC7107/A
9.0
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:
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
• 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:
DS21455B-page 16
of the Common mode range of the converter.
10µF
TC7107A
+
POWER SUPPLIES
V+
V-
2
4
OSC1
OSC2
OSC3
GND
TC7660
+5V
V+
LED
DRIVE
8
3
+
5
10µF
GENERATING NEGATIVE
SUPPLY FROM +5V
NEGATIVE POWER
SUPPLY GENERATION
WITH TC7660
TC7107A
(-5V)
V- = -3.3V
V+
V-
1
26
CD4009
V
V
COM
REF
REF
V
GND
V
IN
IN
+
+
-
-
36
35
32
31
30
21
0.047
µF
1N914
1N914
µF
10
V
IN
+
–
9.1
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:
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-
10.000
9.000
8.000
7.000
6.000
TC7107 Power Dissipation
Reduction
2.00
C
2.50
B
Output Voltage (V)
TC7107 OUTPUT
CURRENT VS. OUTPUT
VOLTAGE
©
2002 Microchip Technology Inc.
3.00
A
3.50
4.00