HCPL7520 Datasheet, PDF (12/15 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 -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 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.
40
440
35
380
220
30
120
25
20
15
10
5
0
0 5 10 15 20 25 30 35
MOTOR PHASE CURRENT – A (rms)
Figure 18. Motor output horsepower vs. motor
phase current and supply voltage.
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