HCPL788J Datasheet, PDF (16/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

16
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 GND₁ (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 rela-
tively 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-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 Hewlett-Packard
Applications Note 1078 for
additional information on using
Isolation Amplifiers.