HCPL-7520-300 Datasheet, PDF (13/16 Pages) AVAGO TECHNOLOGIES LIMITED – 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 par-
ticular value for the resistor is usually a compromise
between minimizing power dissipation and maximiz-
ing accuracy. Smaller sense resistance decreases power
dissipation, while larger sense resistance can improve
circuit accuracy by utilizing the full input range of the
HCPL -7520.
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 requirements of a specific design.
When sensing currents large enough to cause sig-
nificant heating of the sense resistor, the temperature
coefficient (tempco) of the resistor can introduce non-
linearity 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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