LTC1967
W U U
U
APPLICATIO S I FOR ATIO
currents. The power delivered to the load depends on the
firing angle, as well as any parasitic losses such as switch
“ON” voltage drop. Real circuit waveforms will also typi-
cally have significant ringing at the switching transition,
dependent on exact circuit parasitics. For the purposes of
this data sheet, “SCR Waveforms” refers to the ideal
chopped sine wave, though the LTC1967 will do faithful
RMS-to-DC conversion with real SCR waveforms as well.
the lowpass filter. The input to the LPF is the calculation
from the multiplier/divider; (VIN)2/VOUT. The lowpass
filter will take the average of this to create the output,
mathematically:
2
V
(
)
IN
VOUT
=
,
VOUT
The case shown is for Θ = 90°, which corresponds to 50%
of available power being delivered to the load. As noted in
Table 1, when Θ = 114°, only 25% of the available power
is being delivered to the load and the power drops quickly
as Θ approaches 180°.
Because VOUT is DC,
2
2
V
(
)
IN
V
(
)
IN
=
, so
VOUT
VOUT
2
With an average rectification scheme and the typical
calibration to compensate for errors with sine waves, the
RMS level of an input sine wave is properly reported; it is
only with a non-sinusoidal waveform that errors occur.
Because of this calibration, and the output reading in
V
(
)
IN
VOUT
=
, and
VOUT
V
OUT
2 = V 2, or
(
)
(
)
IN
V
RMS, the term True-RMS got coined to denote the use of
2
VOUT
=
V
IN
= RMS V
(
)
(
)
IN
an actual RMS-to-DC converter as opposed to a calibrated
average rectifier.
2
V
(
)
IN
VOUT
V
I
LOAD
+
–
+
–
×
÷
V
V
LPF
V
IN
OUT
LOAD
THY
+
AC
MAINS
V
LINE
–
CONTROL
1967 F03
1967 F02a
Figure 2a
Figure 3. RMS-to-DC Converter with Implicit Computation
Unlike the prior generation RMS-to-DC converters, the
LTC1967 computation does NOT use log/antilog circuits,
which have all the same problems, and more, of log/
antilogmultipliers/dividers,i.e.,linearityispoor,theband-
widthchangeswiththesignalamplitudeandthegaindrifts
with temperature.
V
LINE
Θ
V
LOAD
V
THY
I
LOAD
1967 F02b
Figure 2b
How the LTC1967 RMS-to-DC Converter Works
TheLTC1967usesacompletelynewtopologyforRMS-to-
DC conversion, in which a ∆Σ modulator acts as the
divider, and a simple polarity switch is used as the multi-
plier1 as shown in Figure 4.
How an RMS-to-DC Converter Works
MonolithicRMS-to-DCconvertersuseanimplicitcompu-
tation to calculate the RMS value of an input signal. The
fundamental building block is an analog multiply/divide
used as shown in Figure 3. Analysis of this topology is
easy and starts by identifying the inputs and the output of
1Protected by multiple patents.
1967f
8
Linear Systems [ Linear Systems ]
