tc7107a Microchip Technology Inc., tc7107a Datasheet - Page 17

• Manufacturer Part Number: tc7107a
• Description: 3-1/2 Digit Analog-to-digital Converters
• Manufacturer: Microchip Technology Inc.

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:
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
• 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:
Generation with TC7660
© 2006 Microchip Technology Inc.
of the Common mode range of the converter.
10μF
+
POWER SUPPLIES
TC7107A
2
4
TC7660
V+
V-
OSC1
OSC2
OSC3
+5V
GND
LED
DRIVE
8
3
V+
+
5
10μF
Generating Negative Supply
Negative Power Supply
TC7107A
V- = -3.3V
(-5V)
V+
CD4009
V-
1
26
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 internal
voltage reference is used, the changing chip tempera-
ture 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:
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 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 . 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.”
TC7106/A/TC7107/A
10.000
9.000
8.000
6.000
7.000
TC7107 Power Dissipation
Reduction
2.00
C
2.50
Out put V o lt a ge ( V )
B
TC7107 Output Current vs.
3.00
A
DS21455C-page 17
3.50
4.00