ACPL-785E-200 データシートの表示（PDF） - Broadcom Corporation
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ACPL-785E-200 Datasheet PDF : 16 Pages
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ACPL-785E, HCPL-7850, HCPL-7851, 5962-97557
Data Sheet
Shunt Resistor Selections
Shunt Resistor Selections
The current-sensing shunt resistor should have low resistance
(to minimize power dissipation), low inductance (to minimize
di/dt induced voltage spikes that could adversely affect
operation), and reasonable tolerance (to maintain overall
circuit accuracy). The value of the shunt should be chosen as a
compromise between minimizing power dissipation by
making the shunt resistance smaller and improving circuit
accuracy by making it larger and utilizing the full input range of
the HCPL-785x. Broadcom recommends four different shunts
that can be used to sense average currents in motor drives up
to 35A and 35 hp. Table 1 shows the maximum current and
horsepower range for each of the LVR-series shunts from Dale.
Even higher currents can be sensed with lower value shunts
available from vendors such as Dale, IRC, and Isotek
(Isabellenhuette). When sensing currents large enough to
cause significant heating of the shunt, the temperature
coefficient of the shunt can introduce nonlinearity due to the
signal dependent temperature rise of the shunt. Using a heat
sink for the shunt or using a shunt with a lower tempco can
help minimize this effect. The Application Note 1078, Designing
with Avago Technologies Isolation Amplifiers, contains additional
information on designing with current shunts.
The recommended method for connecting the isolation
amplifier to the shunt resistor is shown in Figure 24. Pin 2 (VIN+)
is connected to the positive terminal of the shunt resistor, while
pin 3 (VIN–) is shorted to pin 4 (GND1), with the power-supply
return path functioning as the sense line to the negative
terminal of the current shunt. This allows a single pair of wires
or PC board traces to connect the isolation amplifier circuit to
the shunt resistor. In some applications, however, supply
currents flowing through the power-supply return path may
cause offset or noise problems. In this case, better performance
may be obtained by connecting pin 3 to the negative terminal
of the shunt resistor separate from the power supply return
path. When connected this way, both input pins should be
bypassed. Whether two or three wires are used, it is
recommended that twisted-pair wire or very close PC board
traces be used to connect the current shunt to the isolation
amplifier circuit to minimize electromagnetic interference to
the sense signal.
The 68Ω resistor in series with the input lead forms a low-pass
anti-aliasing filter with the input bypass capacitor with a
200 kHz bandwidth. The resistor performs another important
function as well: it dampens any ringing that might be present
in the circuit formed by the shunt, the input bypass capacitor,
and the wires or traces connecting the two. Undampened
ringing of the input circuit near the input sampling frequency
can alias into the baseband producing what might appear to
be noise at the output of the device. To be effective, the
damping resistor should be at least 39Ω.
PC Board Layout
In addition to affecting offset, the layout of the PC board can
also affect the common-mode rejection (CMR) performance of
the isolation amplifier, due primarily to stray capacitive
coupling between the input and the output circuits. To obtain
optimal CMR performance, the layout of the printed circuit
board (PCB) should minimize any stray coupling by
maintaining the maximum possible distance between the
input and output sides of the circuit and ensuring that any
ground plane on the PCB does not pass directly below the
HCPL-785x. Using surface-mount components can help
achieve many of the PCB objectives discussed in the preceding
paragraphs. An example through-hole PCB layout illustrating
some of the more important layout recommendations is
shown in Figure 26 and Figure 27. See Application Note 1078,
Designing with Avago Technologies Isolation Amplifiers, for more
information on PCB layout consideration.
Figure 28 Operating Circuit for Burn-In and Steady State Life Tests
1
VDD
2
+
1 k VIN+
3
VIN–
–
4
GND
CONDITIONS: ICC = 17.5 mA
TA = +125° C
27 :
8
VDD 1 k
+
7
VOUT+ 1 k
–
6
VOUT–
5
GND
0.1 PF
(+) VDD
(–) 5.5 VDC
Broadcom
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