HCPL788J Datasheet, PDF (15/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

15
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 compro-
mise between minimizing power
dissipation and maximizing ac-
curacy. Smaller sense resistance
decreases power dissipation,
while larger sense resistance can
improve circuit accuracy by
utilizing the full input range of
the HCPL-788J.
The first step in selecting a sense
resistor is determining how much
current the resistor will be sens-
ing. The graph in Figure 28
shows the rms current in each
phase of a three-phase induction
motor as a function of average
motor output power (in horse-
power, 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
40
35
440
380
30
220
120
25
20
15
10
5
0
0 5 10 15 20 25 30 35
MOTOR PHASE CURRENT – A (rms)
Figure 28. Motor Output
Horsepower vs. Motor Phase Current
and Supply Voltage.
the isolation amplifier. The maxi-
mum sense resistance can be
calculated by taking the maxi-
mum 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 reposi-
tioning 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 per-
centage 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 resis-
tive element itself is made, result-
ing 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