TC820CPL datasheet(16/22 Pages) TELCOM

16 / 22 page

3-164

TELCOM SEMICONDUCTOR, INC.

3-3/4 A/D CONVERTER WITH

FREQUENCY COUNTER

AND LOGIC PROBE

TC820

Digital Ground (DGND)

Digital ground is generated from an internal zener diode

(Figure 14). The voltage between VDD and DGND is the

internal supply voltage for the digital section 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 operation at high fre-

quency, 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.

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.

Figure 13. Powering the TC820 From a Low-Voltage Battery

VDISP

The VDISP input sets the peak-to-peak LCD drive volt-

age. In the 40-pin package, VDISP is connected internally to

DGND, providing a typical LCD drive voltage of 5 VP-P. The

44-pin package includes a separate VDISP input for applica-

tions requiring a variable or temperature-compensated LCD

drive voltage. See the applications information for sug-

gested circuits.

APPLICATIONS INFORMATION

Power Supplies

The TC820 is designed to operate from a single power

supply such as a 9V battery (Figure 12). The converter will

operate over a range of 7V to 15V. For battery operation,

analog common (COM) provides a common-mode bias

voltage (see analog common discussion in the theory of

operation section). However, measurements cannot be

referenced to battery ground. To do so will exceed the

negative common-mode voltage limit.

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 13. The voltage doubler uses the TC7660

and two external capacitors. With this configuration mea-

surements can be referenced either to Analog Common or

to battery ground.

Figure 12. Powering the TC820 From a Single 9V Battery

TC820

VIN

V –

REF

VDD

REF

COM

V –

VSS

–

VDD

REF

–

REF

TC820

3.5V to 6V

TC7660

TC04

–

µF

COM

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