MCP3909-I/SP Microchip Technology, MCP3909-I/SP Datasheet - Page 18

• Manufacturer Part Number: MCP3909-I/SP
• Description: Power Meter with SPI and Active Power Pulse Output & Internal Oscillator., -40C to +85C, 28-SPDIP, TUBE
• Manufacturer: Microchip Technology

MCP3909
be the difference between the internal and external
voltage. The allowed input range for the external
voltage source goes from 2.2V to 2.6V for accurate
measurement error. A V
will cause additional heating and power consumption
due to the source resistor, which might affect measure-
ment error.
4.5
The MCP3909 contains an internal POR circuit that
monitors analog supply voltage AV
This circuit ensures correct device startup at system
power-up and system power-down events. The POR
circuit has built-in hysteresis and a timer to give a high
degree of immunity to potential ripple and noise on the
power supplies, allowing proper settling of the power
supply during power-up. A 0.1 µF decoupling capacitor
should be mounted as close as possible to the AV
pin, providing additional transient immunity (see
Section 6.0 “Applications Information”).
The threshold voltage is typically set at 4V, with a
tolerance of about ±5%. If the supply voltage falls
below this threshold, the MCP3909 will be held in a
Reset condition (equivalent to applying logic ‘0’ on the
MCLR pin). The typical hysteresis value is approxi-
mately 200 mV in order to prevent glitches on the
power supply.
Once a power-up event has occurred, an internal timer
prevents the part from outputting any pulse for
approximately 1s (with MCLK = 3.58 MHz), thereby
preventing potential metastability due to intermittent
resets caused by an unsettled regulated power supply.
Figure 4-3
power-up and a power-down event in the typical
conditions.
FIGURE 4-3:
4.6
The active real-power value is extracted from the DC
instantaneous power. Therefore, any DC offset
component present on Channel 0 and Channel 1
DS22025A-page 18
DEVICE
MODE
AV
4.2V
5V
4V
0V
DD
Power-On Reset (POR)
High-Pass Filters and Multiplier
RESET
illustrates the different conditions for a
PULSE
OUT
NO
1s
Power-on Reset Operation.
REF
OPERATION
PROPER
value outside of this range
DD
during operation.
RESET
Time
DD
affects the DC component of the instantaneous power
and will cause the real-power calculation to be
erroneous. In order to remove DC offset components
from the instantaneous power signal, a high-pass filter
has been introduced on each channel. Since the high-
pass filtering introduces phase delay, identical high-
pass filters are implemented on both channels. The
filters are clocked by the same digital signal, ensuring
a phase difference between the two channels of less
than one MCLK period. Under typical conditions
(MCLK = 3.58 MHz), this phase difference is less than
0.005°, with a line frequency of 50 Hz. The cut-off
frequency of the filter (4.45 Hz) has been chosen to
induce minimal gain error at typical line frequencies,
allowing sufficient settling time for the desired
applications. The two high-pass filters can be disabled
by applying logic ‘0’ to the HPF pin.
FIGURE 4-4:
(MCLK = 3.58 MHz).
The multiplier output gives the product of the two high-
pass filtered channels, corresponding to instantaneous
real power. Multiplying two sine wave signals by the
same ω frequency gives a DC component and a 2ω
component. The instantaneous power signal contains
the real power of its DC component, while also
containing 2ω components coming from the line
frequency multiplication. These 2ω components come
for the line frequency (and its harmonics) and must be
removed in order to extract the real-power information.
This is accomplished using the low-pass filter and DTF
converter.
4.7
For the active power signal calculation, the MCP3909
uses a digital low-pass filter. This low-pass filter is a
first-order IIR filter, which is used to extract the active
real-power information (DC component) from the
instantaneous power signal. The magnitude response
of this filter is detailed in
the instantaneous power signal has harmonic content
(coming from the 2ω components of the inputs), and
-10
-15
-20
-25
-30
-35
-40
-5
0
0.1
Active Power Low-Pass Filter and
DTF Converter
1
HPF Magnitude Response
Frequency (Hz)
© 2006 Microchip Technology Inc.
Figure
10
4-5. Due to the fact that
100
1000