implemented on a host computer or adapter cable), keeps the request to send (RTS) and data terminal ready
(DTR) lines high when the port is active. As long as the host has the COM port open, these two lines retain
voltage on them. This voltage can vary from 5 V to 12 V, depending on the driver implementation. The 5 V to 12
V is sufficient for the use requirements in this design.
The voltage is put through a diode arrangement to block signals from entering back into the pins. The voltage
charges a capacitor to store energy. The capacitor releases this energy when the barrier and charge pump pull
more current than what is instantaneously allowed. The TPS70933 device is used to bring the line voltage down
to a working voltage for the charge pump and isolation device.
In addition to being used in the RS-232 circuit, an additional TPS709 device is used to regulate the 5-V input
voltage from the 5V_IN rail down to the 3.3 V used to power most of the components on the board.
The TPS70933 linear regulator is an ultra-low quiescent current devices designed for power-sensitive
applications. A precision band-gap and error amplifier provides 2% accuracy overtemperature. A quiescent
current of only 1 µA makes these devices ideal solutions for battery-powered, always-on systems that require
very little idle-state power dissipation. These devices have thermal-shutdown, current-limit, and reverse-current
protections for added safety. These regulators can be put into shutdown mode by pulling the EN pin low. The
shutdown current in this mode goes down to 150 nA (typical).
2.2.9 TVS1800
The TVS1800 device robustly shunts up to 40 A of IEC 61000-4-5 fault current to protect systems from high
power transients or lightning strikes. The device offers a solution to the common industrial signal line EMC
requirement to survive up to ±1 kV IEC 61000-4-5 open circuit voltage coupled through a 42-Ω impedance. The
TVS1800 device uses a unique feedback mechanism to ensure precise flat clamping during a fault, assuring
system exposure below 25 V. The tight voltage regulation allows designers to confidently select system
components with a lower voltage tolerance, lowering system costs and complexity without sacrificing robustness.
In addition, the TVS1800 device is available in a small 2-mm × 2-mm SON footprint which is ideal for space-
constrained applications, offering a 70 percent reduction in size compared to industry standard SMA and SMB
packages. The extremely low device leakage and capacitance ensure a minimal effect on the protected line. To
ensure robust protection over the lifetime of the product, TI tests the TVS1800 device against 5,000 repetitive
surge strikes at high temperature with no shift in device performance. In this design, the TVS1800 device is
placed at the input of the TPS25921L device to protect it from any transient voltages that would occur from the
TPS25921L disconnecting the load during overcurrent, overvoltage, or undervoltage events.
2.2.10 ISO7720
The ISO772x devices are high-performance, dual-channel digital isolators with 5000 V
RMS
(DW package) and
3000 V
RMS
(D package) isolation ratings per UL 1577. These devices are also certified by VDE, TUV, CSA, and
CQC. The ISO772x devices provide high electromagnetic immunity and low emissions at low power
consumption, while isolating CMOS or LVCMOS digital I/Os. Each isolation channel has a logic input and output
buffer separated by a silicon dioxide (SiO2) insulation barrier. The ISO7720 device has both channels in the
same direction while the ISO7721 device has both channels in the opposite direction. In the event of input power
or signal loss, the default output is high for devices without suffix F and low for devices with suffix F. Through
innovative chip design and layout techniques, the electromagnetic compatibility of the ISO772x devices has
been significantly enhanced to ease system-level ESD, EFT, surge, and emissions compliance. The ISO772x
family of devices is available in 16-pin SOIC wide-body (DW) and 8-pin SOIC narrow-body (D) packages.
To test the active energy and reactive energy accuracy of a meter, pulses are output at a rate proportional to the
amount of energy consumed. A reference meter can then determine the accuracy of the electricity meter by
calculating the error based on these pulses and how much energy is provided to the meter. In this reference
design, pulses are output through headers for the cumulative active and reactive energy consumption. Using the
ISO7720 device provides an isolated version of these headers for connection to non-isolated equipment, which
is especially needed when the design is configured for one-voltage operation since the design is referenced with
respect to one of the high-voltage line connections. In this design, the D package of the ISO7720 device is used,
which provides an isolation voltage of 3000 V
RMS
for these signals. These isolated active and reactive signals
can be set to have either a 3.3- or 5-V maximum voltage output by applying the selected maximum voltage
output between the VCC (ISO_VCC) and GND (ISO_GND) of the isolated side.
System Overview
10
High-Accuracy Split-Phase CT Electricity Meter Reference Design Using
Standalone ADCs
TIDUEM8B – MARCH 2019 – REVISED FEBRUARY 2021
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