HCPL-7840-300E Datasheet, PDF (17/19 Pages) AVAGO TECHNOLOGIES LIMITED – Isolation Amplifi er

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-7840
to the current sensing resistor is shown in Figure 18. V
IN+
(pin 2 of the HPCL-7840) is connected to the positive
terminal of the sense resistor resistor, while V (pin 3) is
IN-
shorted to GND1 (pin 4), with the power-supply return
path functioning as the sense line to the negative termi-
nal of the current sense resistor. This allows a single pair
of wires or PC board traces to connect the HCPL-7840
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 impor-
tant that the connection from GND1 of the HCPL-7840
to the sense resistor be the only return path for supply
current to the gate drive power supply in order to elimi-
nate potential ground loop problems. The only direct
connection between the HCPL-7840 circuit and the gate
drive circuit should be the positive power supply line.
Output Side
The op-amp used in the external post-amplifier circuit
should be of sufficiently high precision so that it does not
contribute a significant amount of offset or offset drift
relative to the contribution from the isolation amplifier.
Generally, op-amps with bipolar input stages exhibit
better offset performance than op-amps with JFET or
MOSFET input stages.
In addition, the op-amp should also have enough
bandwidth and slew rate so that it does not adversely
affect the response speed of the overall circuit. The
post-amplifier circuit includes a pair of capacitors (C5
and C6) that form a single-pole low-pass filter; these
capacitors allow the bandwidth of the post-amp to
be adjusted independently of the gain and are useful for
reducing the output noise from the isola-tion amplifier.
Many different op-amps could be used in the circuit,
including: MC34082A (Motorola), TLO32A, TLO52A, and
TLC277 (Texas Instruments), LF412A (National Semicon-
ductor).
The gain-setting resistors in the post-amp should have a
tolerance of 1% or better to ensure adequate CMRR and
adequate gain toler-ance for the overall circuit. Resistor
networks can be used that have much better ratio toler-
ances than can be achieved using discrete resistors. A
resistor network also reduces the total number of com-
ponents for the circuit as well as the required board space.
Please refer to Avago Applications Note 1078 for addi-
tional information on using Isolation Amplifiers.
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