Avago Technologies ACPL-C790-000E User Manual
Isolated Current Sensing Module with ACPL-C79X
User Guide
Introduction
The ACPL-C79x isolated current sense evaluation module,
shown in Figure 1, is pin-to-pin compatible with several
Hall eect current transducers. The evaluation module has
a ±1% (ACPL-C79A) isolation amplier gain accuracy, but
the ACPL-C79x is also oered with a ±0.5% (ACPL-C79B)
or ±3% (ACPL-C790) specication. The ACPL-C79B/C79A/
C790 isolation ampliers were designed for current and
voltage sensing in electronic power converter, motor
drive and renewable energy applications.
For many compact power drive designs, a shunt resistor
plus isolation amplier measurement solution has many
benets over traditional Hall eect current sensors. The
benets include a smaller, lower prole component with
better linearity, lower temperature drift, and lower cost.
As shown in Figure 2, the evaluation module is 20 mm by
16 mm and 24 mm high.
Key components, including the shunt resistor and isolation amplier, are available in small surface mount packages. Some designers who are familiar with current transducers can nd using the shunt resistor plus optically
isolated isolation amplier a challenge. This evaluation
module makes the evaluation quick and very easy.
The evaluation module is pin-to-pin compatible with selected Hall eect current transducers, such as LEM’s LTS
6-NP, LTS 15-NP, LTS 25-NP, CAS 6-NP, CAS 15-NP, and CAS
25-NP models
using the LTS and CAS devices will nd it easy to evaluate
and compare the ACPL-C79X based current sensing solution against their existing Hall eect current transducer
solutions. For example, a designer can simply remove an
LTS 15-NP device and plug in the evaluation module and
start a quick evaluation.
[1]
. With the evaluation module, designers
Figure 1. ACPL-C79x isolated current sense evaluation module
Figure 2. Current sense isolation amplier evaluation module
DC-DC converter
PCB2
ACPL-C79A
Shunt Resistor
PCB1
Note 1. Products of LEM Holding SA
Schematic
The evaluation module circuit schematic is show in Figure
3. The evaluation module is made from two printed circuit
boards (PCBs): PCB1 and PCB2. PCB1 has the shunt resistor and two header connectors to form a current path; a
third header connector is used as an interface. PCB2 has
an ACPL-C79A isolation amplier (±1% gain accuracy) and
an isolated DC-DC converter. The two PCBs are assembled
with header connectors P4 and P5.
AC or DC current through shunt resistor R1 results in a
voltage that is proportional to current. This voltage is ltered by the anti-aliasing lter formed by R2, R3 and C1
and then sensed by the dierential input ACPL-C79A. A
dierential output voltage that is proportional to the input voltage is created on the other side of the optical isolation barrier.
Following the isolation amplier, an OPA237 congured
as a dierence amplier converts the dierential signal
to a single-ended output. This stage can be congured to
further amplify the signal, if required, and form a low-pass
lter to limit the bandwidth.
In the evaluation module, the dierence amplier is designed for a gain of 1 with a low-pass lter corner frequency of 234 kHz. Resistors R6 and R7 can be selected for a
dierent gain. The bandwidth can be reduced by adding
capacitors to the positions of C6 and C8.
With the ACPL-C79A gain of 8.2, the overall transfer function is:
Vout = I × R1 × 8.2 + Vref .
The output signal, Vout, is then connected to the next
stage, such as a signal processor, through the P7 header
connector, pin 3.
U1
5
+Vin
+Vout
C3
7
P1
3
2
1
Header 3
R01, 3W
WSR3R0100F
P2
3
2
1
Header 3
0.47uF
PCB2PCB1 PCB1
VDD1
C2
Vinp
P3
R1
1 2
3 4
Leave Blank
I
Vinn
*
P4
1 2
3 4
MHDR2X2
*
R2
R3
GND1
10R,1%
10R,1%
100nF
GND1
C1
22nF
NKE0505DC
U2
1
VDD1
2
VIN+
3
VIN-
GND14GND2
ACPL-C79X
-Vout
-Vin
VDD2
VOUT+
VOUT-
Shunt Resistor and Current Range
The shunt resistor in this module is xed at 10 milliohm.
The appropriate current measurement range is about 15
A
. This is calculated from the nominal input range of
RMS
±200 mV of the ACPL-C79A. The ACPL-C79x species a
full scale input range of ±300 mV, which allows accurate
overload current detection of up to 21 A
. Because the
PEAK
evaluation module does not provide good heat dissipation for the shunt resistor due to small PCB size, limit current to 10 A
check at 15 A
during evaluation or for a quick functional
RMS
. For a detailed performance evaluation,
RMS
a PCB with proper layout for the shunt resistor and other
components is recommended.
Power Supplies
The evaluation module works on a single 5 V supply,
which can be the same power supply for the signal processor and controller device connected through pin 1 and
2 of P7.
The isolated DC-DC converter (U1 in Figure 3) is included
in the evaluation module to power up the signal input
side of the ACPL C79A; this makes the evaluation an easy
plug-and-play operation. However, to make an ACPL C79A
based solution cost eective in mass production, the 5 V
supply would usually be supplied by a oating power supply, which in many applications could be the same supply
that is used to drive the high-side power transistor. A simple three-terminal voltage regulator will provide a stable
voltage. Another method is to add an additional winding
to an existing transformer to produce a 5 V supply.
4
C4
1
0.47uF
GND2GND1
C5
100nF
8
GND2
7
6
5
GND2
R4
C6
NM
10K,1%
10K,1%R5
C7
68pF
R6
10K,1%
VDD2
R8
Vref
GND2
1K,1%
R9
1K,1%
GND2 GND2
Vout
VDD2
3
4
GND2
C11
1uF
P5
12
MHDR2X2
V+
0V
*
*
P6
1234
Leave Blank
*
NMC8
68pFC9
10K,1%R7
2
3
5
GND2
0.1uFC10
7
U3
8
OPA237UA
6
1
4
P7
1
2
Out
3
Header 3
Vref
Note *: Dotted lines denote header pin connections.
Figure 3. The Evaluation module schematic
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Data subject to change. Copyright © 2005-2012 Avago Technologies. All rights reserved.
AV02-3742EN - August 7, 2012
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