TC7129
3.5
Voltage Reference
(D
REF
, R
REF
, R
BIAS
, C
RF
)
FIGURE 4-1:
POWERING THE TC7129
FROM A ±5V POWER
SUPPLY
The reference potentiometer (R
REF
) provides an
adjustment for adjusting the reference voltage; any
value above 20kΩ is adequate. The bias resistor
(R
BIAS
) limits the current through D
REF
to less than
150µA. The reference filter capacitor (C
RF
) forms an
RC filter with R
BIAS
to help eliminate noise.
+5V
TC7129
24
V+
0.1µF
REF HI
REF LO
35
34
3.6
Input Filter (R
IF
, C
IF
)
For added stability, an RC input noise filter is usually
included in the circuit. The input filter resistor value
should not exceed 100kΩ. A typical RC time constant
value is 16.7msec to help reject line frequency noise.
The input filter capacitor should have low leakage for a
high-impedance input.
36 DGND
28
COMMON
0.1µF
33
IN HI
0.1µF
IN LO
V-
23
32
V
IN
+
–
3.7
Battery
The typical circuit uses a 9V battery as a power source.
Any value between 6V and 12V can be used. For oper-
ation from batteries with voltages lower than 6V and for
operation from power supplies, see Section 4.2,
Powering the TC7129.
-5V
4.4
Low Voltage Battery Source
4.0
4.1
TYPICAL APPLICATIONS
TC7129 as a Replacement Part
The TC7129 is a direct pin-for-pin replacement part for
the ICL7129. Note, however, that part requires a
capacitor and resistor between Pins 26 and 28 for
phase compensation. Since the TC7129 uses internal
phase compensation, these parts are not required and,
in fact,
must be removed
from the circuit for stable
operation.
A battery with voltage between 3.8V and 6V can be
used to power the TC7129, when used with a voltage
doubler circuit, as shown in Figure 4-2. The voltage
doubler uses the TC7660 DC-to-DC voltage converter
and two external capacitors.
FIGURE 4-2:
POWERING THE TC7129
FROM A LOW VOLTAGE
BATTERY
24
4.2
Powering the TC7129
36
V+
REF HI
34
While the most common power source for the TC7129
is a 9V battery, there are other possibilities. Some of
the more common ones are explained below.
+
3.8V
to
6V
DGND
REF LO
COMMON
35
28
33
32
4.3
±5V Power Supply
8
2
TC7129
IN HI
IN LO
V-
+
V
IN
Measurements are made with respect to power supply
ground. DGND (Pin 36) is set internally to about 5V
less than V
+
(Pin 24); it is not intended as a power sup-
ply input and must not be tied directly to power supply
ground. It can be used as a reference for external logic,
as explained in Section 4.6, Connecting to External
Logic (see Figure 4-1).
–
+
TC7660
10µF
4
5
3
10µF
23
+
2002 Microchip Technology Inc.
DS21459B-page 9
©
MICROCHIP [ MICROCHIP TECHNOLOGY ]
