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2007 Microchip Technology Inc.
DS21433C-page 15
TC530/TC534
6.0
SELECTING COMPONENT
VALUES FOR THE
TC530/TC534
1.
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 20 A. The value of
RINT is therefore directly calculated as follows:
EQUATION 6-1:
2.
Select Reference (CREF) and Auto Zero (CAZ)
Capacitors.
CREF and CAZ must be low leakage capacitors
(such as polypropylene). The slower the conver-
sion rate, the larger the value CREF must be.
Recommended capacitors for CREF and CAZ are
shown in Table 6-1. Larger values for CAZ and CREF may also be used to limit rollover errors.
TABLE 6-1:
CREF AND CAZ SELECTION
6.1
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) less 0.9V (i.e.,IVDD – 0.9VI or IVSS +0.9VI).
Using the 20 A buffer maximum output current, the
value of the integrating capacitor is calculated using
EQUATION 6-2:
It is critical that the integrating capacitor have a very
low dielectric absorption. PPS capacitors are an exam-
ple of one such dielectric.
Table 6-2 summarizes
various capacitors suitable for CINT.
TABLE 6-2:
RECOMMENDED CAPACITOR
FOR CINT
6.2
Calculate VREF
The reference de-integration voltage is calculated
using the following equaton:
EQUATION 6-3:
6.3
Serial Port
Communication
with
the
TC530/TC534
is
accomplished over a 3-wire serial port. Data is clocked
into DIN on the rising edge of DCLK and clocked out of
DOUT on the falling edge of DCLK. R/W must be HIGH
to read converted data from the serial port and LOW to
write the LOAD VALUE to the TC530/TC534.
Conversion
Per Second
Typical Value
of CREF, CAZ
(F)
Suggested (1) Part
Number
>7
0.1
SMR5 104K50J0IL
2 to 7
0.22
SMR5 224K50J2L
2 or less
0.47
SMR5 474K50J04L
Note 1: Manufactured by Evox-Rifa, Inc.
Where:
VIN(MAX)
=
Maximum Input Voltage (full
count voltage)
RINT
=
Integrating Resistor (in M
Ω)
For loop stability, RINT should be ≥ 50 kΩ.
RINT MΩ
()
VINMAX
20
-------------------
=
Value (F)
Suggested Part Number(1)
0.1
SMR5 104K50J0IL
0.22
SMR5 224K50J2L
0.33
SMR5 334K50J03L4
0.47
SMR5 474K50J04L
Note 1: Manufactured by Evox-Rifa, Inc.
Where:
TINT
=
Integration Period
VS
=IVDDI
CINT
=
Integrated Capacitor Value (F)
CINT μF
()
TINT
() 20 10
6
–
×
()
VS 0.9
–
()
---------------------------------------------
=
VREF
VS 0.9
–
() C
INT
() R
INT
()
2RINT
()
-----------------------------------------------------------V
=