40
35
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
the HCPL-7510 circuit to the
sense resistor. By referencing
the input circuit to the
negative side of the sense
resistor, any load current
440
380
220
120
30
25
20
15
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 connection between
the HCPL-7510 circuit and the
gate drive circuit should be
the positive power supply line.
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.
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
recommended.
Sense Resistor Connections
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
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
14
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
