Datasheet

18
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.
When laying out a PC board for
the current sensing resistors, a
couple of points should be kept
in mind. The Kelvin connections
to the resistor should be brought
together under the body of the
resistor and then run very close
to each other to the input of the
HCPL-788J; this minimizes the
loop area of the connection and
reduces the possibility of stray
magnetic fields from interfering
with the measured signal. If
the sense resistor is not located
on the same PC board as the
HCPL-788J circuit, a tightly
twisted pair of wires can
accomplish the same thing.
Also, multiple layers of the PC
board can be used to increase
current carrying capacity.
Numerous plated-through vias
should surround each non-
Kelvin terminal of the sense
resistor to help distribute the
current between the layers of
the PC board. The PC board
should use 2 or 4 oz. copper for
the layers, resulting in a current
carrying capacity in excess of 20
A. Making the current carrying
traces on the PC board fairly
large can also improve the sense
resistor’s power dissipation
capability by acting as a heat
sink. Liberal use of vias where
the load current enters and exits
the PC board is also
recommended.
Sense Resistor Connections
The recommended method for
connecting the HCPL-788J to the
current sensing resistor is
shown in Figure 26. V
IN+
(pin 1
of the HCPL-788J) is connected
to the positive terminal of the
sense resistor, while V
IN-
(pin 2)
is shorted to GND
1
(pin 8), with
the power-supply return path
functioning as the sense line
to the negative terminal of the
current sense resistor. This
allows a single pair of wires or
PC board traces to connect the
HCPL-788J circuit to the sense
resistor. By referencing the
input circuit to the negative side
of the sense resistor, any load
current induced noise transients
on the resistor are seen as a
common-mode signal and will
not interfere with the current-
sense signal. This is important
because the large load currents
flowing through the motor drive,
along with the parasitic
inductances inherent in the
wiring of the circuit, can
generate both noise spikes and
offsets that are relatively large
compared to the small voltages
that are being measured across
the current sensing resistor.
If the same power supply is used
both for the gate drive circuit
and for the current sensing
circuit, it is very important that
the connection from GND
1
of the
HCPL-788J to the sense resistor
be the only return path for
supply current to the gate drive
power supply in order to
eliminate potential ground loop
problems. The only direct
connection between the HCPL-
788J circuit and the gate drive
circuit should be the positive
power supply line. Please refer
to Avago Technologies’
Applications Note 1078 for
additional information on using
Isolation Amplifiers.