HCPL-7510 Datasheet, PDF (13/16 Pages) Agilent(Hewlett-Packard) – Isolated Linear Sensing IC

Current Sensing Resistors
The current sensing resistor should have low resistance
(to minimize power dissipation), low inductance (to
minimize di/dt induced voltage spikes which could
adversely affect operation), and reasonable tolerance (to
maintain overall circuit accuracy). Choosing a particular
value for the resistor is usually a compromise between
minimizing power dissipation and maximizing accuracy.
Smaller sense resistance decreases power dissipation,
while larger sense resistance can improve circuit accuracy
by utilizing the full input range of the HCPL-7510.
The first step in selecting a sense resistor is determining
how much current the resistor will be sensing. The graph
in Figure 18 shows the RMS current in each phase of a
three-phase induction motor as a function of average
motor output power (in horsepower, hp) and motor
drive supply voltage. The maximum value of the sense
resistor is determined by the current being measured
and the maximum recommended input voltage of the
isolation amplifier. The maximum sense resistance can
be calculated by taking the maximum recommended
input voltage and dividing by the peak current that the
sense resistor should see during normal operation. For
example, if a motor will have a maximum RMS current
of 10 A and can experience up to 50% overloads during
normal operation, then the peak current is 21.1 A (=10
x 1.414 x 1.5). Assuming a maximum input voltage of
200 mV, the maximum value of sense resistance in this
case would be about 10 mΩ. The maximum average
power dissipation in the sense resistor can also be easily
calculated by multiplying the sense resistance times the
square of the maximum RMS current, which is about 1 W
in the previous example. If the power dissipation in the
sense resistor is too high, the resistance can be decreased
below the maximum value to decrease power dissipation.
The minimum value of the sense resistor is limited by
precision and accuracy requirements of the design. As
the resistance value is reduced, the output voltage across
the resistor is also reduced, which means that the offset
and noise, which are fixed, become a larger percentage
of the signal amplitude. The selected value of the sense
resistor will fall somewhere between the minimum and
maximum values, depending on the particular require-
ments of a specific design.
When sensing currents large enough to cause significant
heating of the sense resistor, the temperature coefficient
(tempco) of the resistor can introduce nonlinearity due
to the signal dependent temperature rise of the resistor.
The effect increases as the resistor-to-ambient thermal
resistance increases. This effect can be minimized by
reducing the thermal resistance of the current sensing
resistor or by using a resistor with a lower tempco.
Lowering the thermal resistance can be accomplished
by repositioning the current sensing resistor on the PC
board, by using larger PC board traces to carry away more
heat, or by using a heat sink. For a two-terminal current
sensing resistor, as the value of resistance decreases, the
resistance of the leads become a significant percentage
of the total resistance. This has two primary effects on
resistor accuracy. First, the effective resistance of the
sense resistor can become dependent on factors such
as how long the leads are, how they are bent, how far
they are inserted into the board, and how far solder
wicks up the leads during assembly (these issues will be
discussed in more detail shortly). Second, the leads are
typically made from a material, such as copper, which
has a much higher tempco than the material from which
the resistive element itself is made, resulting in a higher
tempco overall. Both of these effects are eliminated
when a four-terminal current sensing resistor is used.
A four-terminal resistor has two additional terminals
that are Kelvin-connected directly across the resistive
element itself; these two terminals are used to monitor
the voltage across the resistive element while the other
two terminals are used to carry the load current. Because
of the Kelvin connection, any voltage drops across the
leads carrying the load current should have no impact
on the measured voltage.
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