Datasheet

© 2008 Microchip Technology Inc. DS21428E-page 15
TC500/A/510/514
7.0 TYPICAL APPLICATIONS
7.1 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 (R
INT
, C
INT
).
3. Auto-zero and Reference Capacitors.
4. Voltage Reference.
7.2 Select Integration Time
Integration time must be picked as a multiple of the
period of the line frequency. For example, T
INT
times of
33 ms, 66 ms and 132 ms maximize 60 Hz line
rejection.
7.3 DINT and IZ Phase Timing
The duration of the DINT phase is a function of the
amount of voltage stored on the integrator during T
INT
and the value of V
REF
. The DINT phase must be initiated
immediately 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 V
REF
chosen at V
IN(MAX)
/2).
7.4 Calculate Integrating Resistor
(R
INT
)
The desired full-scale input voltage and amplifier output
current capability determine the value of R
INT
. The
buffer and integrator amplifiers each have a full-scale
current of 20 μA.
The value of R
INT
is, therefore, directly calculated in the
following equation:
EQUATION 7-1:
7.5 Select Reference (C
REF
) and Auto-
zero (C
AZ
) Capacitors
C
REF
and C
AZ
must be low leakage capacitors (such as
polypropylene). The slower the conversion rate, the
larger the value C
REF
must be. Recommended
capacitors for C
REF
and C
AZ
are shown in Tab le 7 -1 .
Larger values for C
AZ
and C
REF
may also be used to
limit rollover errors.
TABLE 7-1: C
REF
AND C
AZ
SELECTION
7.6 Calculate Integrating Capacitor
(C
INT
)
The integrating capacitor must be selected to maximize
integrator output voltage swing. The integrator output
voltage swing is defined as the absolute value of V
DD
(or V
SS
) less 0.9V (i.e., IV
DD
- 0.9VI or IV
SS
+ 0.9VI).
Using the 20 μA buffer maximum output current, the
value of the integrating capacitor is calculated using the
following equation.
EQUATION 7-2:
It is critical that the integrating capacitor has a very low
dielectric absorption. Polypropylene capacitors are an
example of one such dialectic. Polyester and poly-
bicarbonate capacitors may also be used in less critical
applications. Tab le 7 -2 summarizes recommended
capacitors for C
INT
.
TABLE 7-2: RECOMMENDED CAPACITOR
FOR C
INT
7.7 Calculate V
REF
The reference de-integration voltage is calculated
using the following equation:
EQUATION 7-3:
Where:
V
IN(MAX)
= Maximum input voltage (full count
voltage)
R
INT
= Integrating Resistor (in M)
For loop stability, R
INT
should be 50 k
R
INT
in M
Ω
()
V
IN MAX()
20
-----------------------=
Conversions
Per Second
Typical Value of
C
REF
, C
AZ
(μF)
Suggested* Part
Number
>7 0.1 SMR5 104K50J01L4
2 to 7 0.22 SMR5 224K50J02L4
2 or less 0.47 SMR5 474K50J04L4
* 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
* Manufactured by Evox Rifa, Inc.
Where:
T
INT
= Integration Period
V
S
=IV
DD
I or IV
SS
I, whichever is less
(TC500/A)
V
S
=IV
DD
I (TC510, TC514)
C
INT
T
INT
()20 10
6
×
()
V
S
0.9()
---------------------------------------------=
V
REF
V
S
0.9()C
INT
()R
INT
()
2T
INT
()
-----------------------------------------------------------
V=