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AN50-1
Using high side current monitors in DC powerline
signalling
Introduction
The ZXLD1081 is a 60 volt high side current sensing device is used in circuit applications
where current has to be measured without imposing a sense resistor in the ground which
would give a discontinuity in the ground plane .
A connected ground plane gives better and more predictable EMC characteristics which is
also an important requirement of systems which use a DC power rail to signal to remote
addressed sensors by imposing data onto the rail in the form of voltage variations.
Likewise, low power addressable sensors connected to this power rail send data back in the
form of current variations in the supply line at a time slot which is known to be relatively
quiet.
The Zetex current monitors are ideally suited to this application with bandwidths far
exceeding the lower speeds typically used in this type of communication. A range of current
monitor devices is shown at the end of this application note and by suitable circuit
adaptation, the principles discussed in this application note can be used with all of them.
Typical method for DC power signalling
The circuit below shows how a UART transmit output (usually within a microcontroller) is
used to modify the voltage output to a pair of sensor wires via a class B power amplifier
formed by transistors Q1 and Q2 in a feedback loop with an Op Amp. (The ZXTC2045
shown is a complementary pair housed in a single SOT23-6 package) A nominal 24Vdc
supply is present when the UART output is floated . When a transmission is about to take
place, the UART output becomes active high for a time period which increases the supply
rail. This high voltage is detected at the sensors and puts them into receive mode whereby
they expect a data packet which contains an address. The UART output transmits data as
a series of high and low transitions which modulates the supply voltage between high
(typically 30Vdc) and nominal levels.
If a particular sensor is addressed then after a delay for supply line settling , it responds by
modulating its supply current with data . The level of modulation can be whatever is
acceptable but a figure of 20mA is typical in industrial applications but can be as high as
250mA in lighting circuits.
The output from the op amp is limited to 5V by the schottky diode and resistor, some UART’s
and microprocessors have input clamping diodes built in and in such cases all that is needed
is a series resistor.
The output of the current sensor, in this case is a ZXCT1081, is a voltage and by AC
coupling this signal into a comparator , the overall DC power line load current is eliminated
leaving the current variations as the digital data reply which can be fed into the UART
receive input. The exact capacitor value needed depends on the data rate.
Data rates around 2400 baud are often used with this arrangement using None Return Zero
(NRZ) format data.
ZXLD1081
Feb 2007
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AN50-1
Using high side current monitors in DC powerline
signalling
Small data packets using parity and or checksums are advised to ensure data integrity.
The Class B power output stage is slew-rate limited to ensure EMC compatability and for the
same reason , small inductors and capacitors are included in the output power lines.
The ZXCT1081 is chosen in this circuit because of the high rail voltage, but the principle is
the same for other Zetex current sensors on lower DC rails (eg below 20Vdc)
Schottky diodes are used around the sense resistor to protect the ZXLD1081 device when
higher transient currents are flowing during power up and when voltage signaling is in
progress.
Circuit diagram
Micro controllers have the ability to control the three state outputs of the port lines
used for Serial transmission outputs and this feature is used here to control the power supply
output without the need for additional circuitry
NRZ data consists of a high signal during the idle state followed by a low going start bit plus
data bits (high represents one and low is a zero ) followed by a high stop bit . At 2400 baud
the width of these data bits is 416 microseconds .
Sensors can return data at the same speed and change their supply current via a resistor
and transistor fed from a UART port pin similar to the outgoing transmission.
ZXLD1081
Feb 2007
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AN50-1
Using high side current monitors in DC powerline
signalling
A typical outgoing data packet is shown here
The Cyclic Redundancy Check is a robust way of ensuring correct data is received . Slew
rates exist on all edges to reduce EMC issues and can be set to around 10% of the bit width
without reception problems. Return data packets are similar and should contain the address
of the responding sensor as confirmation of where the data originated.
Zetex Current monitor devices
ZXCT 1008 current output
ZXCT 1009 current output
ZXCT 1010 current output
ZXCT 1011 current output
ZXCT 1021 voltage output
ZXCT 1022 voltage output
ZXCT 1030 voltage output
ZXCT 1041 voltage output
ZXCT 1051 voltage output
ZXCT 1081 voltage output
ZXLD1081
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