TC820 datasheet(17/28 Pages) MICROCHIP

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DS21476B-page 17

TC820

5.0

TYPICAL APPLICATIONS

5.1

Power Supplies

The TC820 is designed to operate from a single power

supply such as a 9V battery (Figure 5-1). The converter

will operate over a range of 7V to 15V. For battery oper-

ation, analog common (COM) provides a Common

mode bias voltage (see analog common discussion in

the theory of operation section). However, measure-

ments cannot be referenced to battery ground. To do so

will exceed the Negative Common mode voltage limit.

FIGURE 5-1:

POWERING THE TC820

FROM A SINGLE 9V

BATTERY

A battery with voltage between 3.5V and 7V can be

used to power the TC820, when used with a voltage

doubler, as shown in Figure 5-2. The voltage doubler

uses the TC7660 and two external capacitors. With this

configuration, measurements can be referenced either

to analog common or to battery ground.

FIGURE 5-2:

POWERING THE TC820

FROM A LOW VOLTAGE

BATTERY

5.2

Digital Ground (DGND)

Digital ground is generated from an internal zener

diode (Figure 5-3). The voltage between VDD and

DGND is the internal supply voltage for the digital sec-

tion of the TC820. DGND will sink a minimum of 3mA.

DGND establishes the low logic level reference for the

TC820 mode select inputs, and for the frequency and

logic probe inputs. The DGND pin can be used as the

negative supply for external logic gates, such as the

logic probe buffers. To ensure correct counter opera-

tion at high frequency, connect a 1

µF capacitor from

DGND to VDD.

DGND also provides the drive voltage for the LCD. The

TC820 40-pin package internally connects the LCD

VDISP pin to DGND, and provides an LCD drive voltage

of about 5VP-P. In the 44-pin package, connecting the

VDISP pin to DGND will provide a 5V LCD drive voltage.

FIGURE 5-3:

DGND AND COM

OUTPUTS

5.3

Digital Input Logic Levels

Logic levels for the TC820 digital inputs are referenced

to VDD and DGND. The high level threshold is

VDD – 1.5V, and the low logic level is DGND + 1.5V. In

most cases, digital inputs will be connected directly to

VDD with a mechanical switch. CMOS gates can also

be used to control the logic inputs, as shown in the logic

probe inputs section.

5.4

Clock Oscillator

The TC820 oscillator can be controlled with either a

crystal, or with an inexpensive resistor capacitor com-

bination. The crystal circuit, shown in Figure 5-4, is rec-

ommended when high accuracy is required in the

Frequency Counter mode. The 40kHz crystal is a stan-

dard frequency for ultrasonic alarms, and will provide a

1-second time-base for the counter or 2.5 analog-to-

digital conversions per second. Consult the crystal

manufacturer for detailed applications information.

TC820

VIN

VREF-

VDD

VREF+

COM

VIN+

VIN-

VSS

–

VDD

VSS

VREF+

VREF-

TC820

3.5V to 6V

TC7660

VIN+

VIN-

–

VIN

10µF

COM

–

12µA

TC820

Logic

Section

3.2V

VDD

VSS

COM

DGND

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