TC500ACPE datasheet(11/28 Pages) MICROCHIP

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DS21428B-page 11

TC500/A/510/514

6.0

TYPICAL APPLICATIONS

6.1

Component Value Selection

The procedure outlined below allows the user to arrive

at values for the following TC5XX design variables:

Integration Phase Timing

Integrator Timing Components (RINT,CINT)

Auto Zero and Reference Capacitors

Voltage Reference

6.2

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

rejection.

6.3

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 initi-

ated immediately following INT and terminated when

an integrator output zero-crossing is detected. In gen-

eral, the maximum number of counts chosen for DINT

is twice thatofINT (with VREF chosen at VIN(MAX) /2).

6.4

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

currentof20

µA.

The value of RINT is therefore directly calculated in the

following equation:

EQUATION 6-1:

6.5

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 capac-

itors for CREF and CAZ are shown in Table 6-1. Larger

values for CAZ and CREF mayalsobe usedtolimit

rollover errors.

TABLE 6-1:

CREF AND CAZ SELECTION

6.6

Calculate Integrating Capacitor

(CINT)

The integrating capacitor must be selected to maximize

integrator output voltage swing. The integrator output

voltage swing is defined as the absolute value of VDD

(or VSS)less0.9V (i.e., IVDD - 0.9VI or IVSS +0.9VI).

Using the 20

µA buffer maximum output current, the

value of the integrating capacitor is calculated using the

following equation.

EQUATION 6-2:

It is critical that the integrating capacitor has a very low

dielectric absorption. Polypropylene capacitors are an

example of one such dielectic. Polyester and Polybicar-

bonate capacitors may also be used in less critical

applications. Table 6-2 summarizes recommended

capacitors for CINT.

TABLE 6-2:

RECOMMENDED CAPACITOR

FOR CINT

6.7

Calculate VREF

The reference deintegration voltage is calculated using

the following equation:

EQUATION 6-3:

VIN(MAX)

RINT(in MΩ)=

Where:

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

RINT = Integrating Resistor (in MΩ)

For loop stability, RINT should be ≥ 50kΩ.

Conversions

Per Second

Typical Value of

CREF,CAZ (µF)

Suggested* Part

Number

0.1

SMR5 104K50J01L4

2 to 7

0.22

SMR5 224K50J02L4

2 or less

0.47

SMR5 474K50J04L4

Note:

Manufactured by Evox-Rifa, Inc.

Value

Suggested Part Number*

0.1

SMR5 104K50J01L4

0.22

SMR5 224K50J02L4

0.33

SMR5 334K50J03L4

0.47

SMR5 474K50J04L4

Note:

Manufactured by Evox-Rifa, Inc.

CINT =

(VS -0.9)

(TINT)(20 x 10

-6)

µF

Where:

TINT = Integration Period

VS =IVDDIorIVSSI, whichever is less (TC500/A

VS =IVDDI (TC510, TC514)

(VS –0.9) (CINT)(RINT)

VREF =

2(RINT)

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