LTC1966CMS8 Linear Technology, LTC1966CMS8 Datasheet - Page 10

• Manufacturer Part Number: LTC1966CMS8
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966CMS8

• Current - Supply: 155µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1966
RMS-TO-DC CONVERSION
Definition of RMS
RMS amplitude is the consistent, fair and standard way to
measure and compare dynamic signals of all shapes and
sizes. Simply stated, the RMS amplitude is the heating
potential of a dynamic waveform. A 1V
will generate the same heat in a resistive load as will 1V DC.
Mathematically, RMS is the “Root of the Mean of the
Square:”
Alternatives to RMS
Other ways to quantify dynamic waveforms include peak
detection and average rectification. In both cases, an
average (DC) value results, but the value is only accurate
at the one chosen waveform type for which it is calibrated,
typically sine waves. The errors with average rectification
are shown in Table 1. Peak detection is worse in all cases
and is rarely used.
Table 1. Errors with Average Rectification vs True RMS
WAVEFORM
Square Wave
Sine Wave
Triangle Wave
SCR at 1/2 Power,
SCR at 1/4 Power,
10
= 90
= 114
V
RMS
1V (AC + DC) RMS
V
2
1V AC
1.000
1.000
1.000
1.000
1.000
V
RMS
1V DC
U
RMS
Figure 1
+
–
RECTIFIED
AVERAGE
U
1.000
0.900
0.866
0.637
0.536
(V)
R
R
R
1966 F01
W
ERROR*
11%
*Calibrate for 0% Error
–3.8%
–29.3%
–40.4%
SAME
HEAT
RMS
AC waveform
U
The last two entries of Table 1 are chopped sine waves as
is commonly created with thyristors such as SCRs and
Triacs. Figure 2a shows a typical circuit and Figure 2b
shows the resulting load voltage, switch voltage and load
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 LTC1966 will do faithful
RMS-to-DC conversion with real SCR waveforms as well.
The case shown is for = 90 , which corresponds to 50%
of available power being delivered to the load. As noted in
Table 1, when
is being delivered to the load and the power drops quickly
as
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
an actual RMS-to-DC converter as opposed to a calibrated
average rectifier.
RMS
approaches 180 .
, the term True-RMS got coined to denote the use of
MAINS
AC
V
V
I
= 114 , only 25% of the available power
V
LOAD
LOAD
LINE
THY
V
LINE
+
–
Figure 2a
Figure 2b
+
CONTROL
V
I
LOAD
LOAD
–
1966 F02b
+
–
1966 F02a
V
THY
sn1966 1966fas