HCPL7520 Datasheet, PDF (13/15 Pages) Agilent(Hewlett-Packard) – Isolated Linear Sensing IC

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-7520; 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-7520
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-7520 to the
current sensing resistor is shown
in Figure 17. VIN+ (pin 2 of the
HPCL-7520) is connected to the
positive terminal of the sense
resistor, while VIN- (pin 3) is
shorted to GND1 (pin 4), with the
powersupply 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-7520 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 GND1 of the
HCPL-7520 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-7520 circuit and the gate
drive circuit should be the positive
power supply line.
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