HCPL-7510 Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – Isolated Linear Sensing IC

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MOTOR PHASE CURRENT – A (rms)
Figure 18. Motor output horsepower vs. motor phase current and supply
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-7510; 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-7510 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-
7510 to the current sensing resistor is shown in Figure
17. VIN+ (pin 2 of the HPCL-7510) 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-
7510 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-7510 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 connec-
tion between the HCPL-7510 circuit and the gate drive
circuit should be the positive power supply line.
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