TC7129 datasheet(9/28 Pages) MICROCHIP

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DS21459D-page 9

TC7129

3.5

Voltage Reference

(DREF, RREF, RBIAS, CRF)

The reference potentiometer (RREF) provides an

adjustment for adjusting the reference voltage; any

value above 20 k

Ω is adequate. The bias resistor

(RBIAS) limits the current through DREF to less than

150

μA. The reference filter capacitor (CRF) forms an

RC filter with RBIAS to help eliminate noise.

3.6

Input Filter (RIF, CIF)

For added stability, an RC input noise filter is usually

included in the circuit. The input filter resistor value

should not exceed 100 k

Ω. A typical RC time constant

value is 16.7 msec to help reject line frequency noise.

The input filter capacitor should have low leakage for a

high-impedance input.

3.7

Battery

The typical circuit uses a 9V battery as a power source.

However, any value between 6V and 12V can be used.

For operation from batteries with voltages lower than

6V and for operation from power supplies, see

Section 4.2 “Powering the TC7129”.

4.0

TYPICAL APPLICATIONS

4.1

TC7129 as a Replacement Part

The TC7129 is a direct pin-for-pin replacement part for

the ICL7129. Note, however, that the ICL7129 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.

4.2

Powering the TC7129

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.

4.2.1

±5V Power Supply

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 to be a power supply

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 4.3 “Connecting to External

Logic”, (see Figure 4-1).

Figure 4-1:

Powering the TC7129 From

a ±5V Power Supply.

4.2.2

Low Voltage Battery Source

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.

V–

V+

REF HI

REF LO

IN HI

COMMON

IN LO

DGND

VIN

–

-5V

0.1 µF

+5V

0.1 µF

TC7129

0.1 µF

V–

TC7129

V+

REF HI

REF LO

IN HI

COMMON

IN LO

DGND

3.8V

10 µF

–

TC7660

VIN

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