ACPL-C780-500E Avago Technologies US Inc., ACPL-C780-500E Datasheet - Page 15

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ACPL-C780-500E

Manufacturer Part Number
ACPL-C780-500E
Description
IC,Isolation Amplifier,SINGLE,SOP,8PIN,PLASTIC
Manufacturer
Avago Technologies US Inc.
Datasheet

Specifications of ACPL-C780-500E

Amplifier Type
Isolation
Number Of Circuits
1
Output Type
Differential
-3db Bandwidth
100kHz
Voltage - Input Offset
300µV
Current - Supply
11mA
Current - Output / Channel
18.6mA
Voltage - Supply, Single/dual (±)
4.5 V ~ 5.5 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
8-SSOP
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Slew Rate
-
Gain Bandwidth Product
-
Current - Input Bias
-
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
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.
Note: Please refer to Avago Technologies Application Note 1078 for
additional information on using Isolation Amplifiers.
Sense Resistor Connections
The
ACPL-C78A/C780/C784 to the current sensing resistor is
shown in Figure 18. V
C784) is connected to the positive terminal of the sense
resistor, while V
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 ACPL-C78A/C780/C784 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 induc-
tances 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 ACPL-C78A/C780/
C784 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
con nection between the ACPL-C78A/C780/C784 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.
15
recommended
IN-
(pin 3) is shorted to GND1 (pin 4), with
IN+
method
(pin 2 of the ACPL-C78A/C780/
for
connecting
the
In addition, the op-amp should also have enough band-
width 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: TL032A, TL052A,
and TLC277 (Texas Instruments), LF412A (National Semi-
conductor).
The gain-setting resistors in the post-amp should have a
tolerance of 1% or better to ensure adequate CMRR and
adequate gain tolerance 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 compo-
nents for the circuit as well as the required board space.
Frequently asked questions about the
ACPL-C78A/C780/C784
1.
1.1. Why should I use the ACPL-C78A/C780/C784 for sensing current
2.
2.1. Where do I get 10 m: resistors? I have never seen one that low.
2.2. Should I connect both inputs across the sense resistor instead of
The basics
when Hall-effect sensors are available which don’t need an
isolated supply voltage?
Available in an auto-insertable, Stretched SO-8
package, the ACPL-C78A/C780/C784 is smaller than
and has better linearity, offset vs. temperature and
Common Mode Rejection (CMR) performance than
most Hall-effect sensors. Additionally, often the
required input-side power supply can be derived
from the same supply that powers the gate-drive
optocoupler.
Sense resistor and input filter
Although less common than values above 10 :, there
are quite a few manufacturers of resistors suitable
for measuring currents up to 50 A when combined
with the ACPL-C78A/C780/C784. Example product
information may be found at Vishay's web site
(http://www.vishay.com) and Isotek’s web site
(http://www.isotekcorp.com).
grounding V
This is not necessary, but it will work. If you do, be
sure to use an RC filter on both pin 2 (V
(V
IN-
) to limit the input voltage at both pads.
IN-
directly to pin 4?
IN+
) and pin 3

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