TC510CPF datasheet(10/17 Pages) TELCOM

10 / 17 page

3-28

TELCOM SEMICONDUCTOR, INC.

PRECISION ANALOG FRONT ENDS

TC500

TC500A

TC510

TC514

APPLICATIONS

Component Value Selection

The procedure outlined below allows the user to arrive

at values for the following TC5xx design variables:

(1) Integration Phase Timing

(2) Integrator Timing Components (RINT, CINT)

(3) Auto Zero and Reference Capacitors

(4) Voltage Reference

Select Integration Time

Integration time must be picked as a multiple of the

period of the line frequency. For example, tINT times of

33msec, 66msec and 132msec maximize 60Hz line rejec-

tion.

DINT and IZ Phase Timing

The duration of the DINT phase is a function of the

amount of voltage stored on the integrator during TINT, and

the value of VREF. The DINT phase must be initiated imme-

diately following INT and terminated when an integrator

output zero-crossing is detected. In general, the maximum

number of counts chosen for DINT is twice that of INT (with

VREF chosen at VIN (max)/2).

Calculate Integrating Resistor (RINT)

The desired full-scale input voltage and amplifier output

current capability determine the value of RINT. The buffer

and integrator amplifiers each have a full-scale current of

µA.

The value of RINT is therefore directly calculated as

follows:

RINT(in MΩ) =

VIN MAX

where: VIN MAX = Maximum input voltage (full count voltage)

RINT =Integrating Resistor (in MΩ)

For loop stability, RINT should be ≥ 50kΩ.

Select Reference (CREF) and Auto Zero (CAZ) Capacitors

CREF and CAZ must be low leakage capacitors (such as

polypropylene). The slower the conversion rate, the larger

the value CREF must be. Recommended capacitors for CREF

and CAZ are shown in Table 1. Larger values for CAZ and

CREF may also be used to limit roll-over errors.

Table 1. CREF and CAZ Selection

Conversions Typical Value of Suggested *

Per Second

CREF, CAZ (

µF) Part Number

0.1

WIMA MK12 .1/63/20

2 to 7

0.22

WIMA MK12 .22/63/20

2 or less

0.47

WIMA MK12 .47/63/20

*WIMA Corp. listing on the last page of this data sheet.

Calculate Integrating Capacitor (CINT)

The integrating capacitor must be selected to maximize

integrator output voltage swing. The integrator output volt-

age swing is defined as the absolute value of VDD (or VSS)

less 0.9V (i.e.,

 VDD– 0.9V or V

SS +0.9V ). Using the 20µA

buffer maximum output current, the value of the integrating

capacitor is calculated using the following equation.

CINT = (in µF) =

where: tINT = Integration Period

VS = Applied Supply Voltage

It is critical that the integrating capacitor has a very low

dielectric absorption. Polypropylene capacitors are an ex-

ample of one such chemistry. Polyester and Polybicarbonate

capacitors may also be used in less critical applications.

Table 2 summarizes recommended capacitors for CINT.

Table 2. Recommend Capacitor for CINT

Value

Suggested Part Number*

0.1

WIMA MK12 .1/63/20

0.22

WIMA MK12 .22/63/20

0.33

WIMA MK12 .33/63/20

0.47

WIMA MK12 .47/63/20

*WIMA Corp. listing on the last page of this data sheet.

Calculate VREF

The reference deintegration voltage is calculated

using:

VREF (in Volts) =

(VS – 0.9) (CINT) (RINT)

2(tINT)

(tINT) (20 x 10 –6)

(VS – 0.9)

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