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Texas Instruments A Reinforced-isolated Analog Input Chain for Space-constrained Applications Application notes
A Reinforced-Isolated Analog Input Chain for SpaceConstrained Applications
Lars Lotzenburger
The need for higher performance and higher channel
density for PLC I/O modules is still unbroken. Higher
density means less space per channel in all three
dimensions. Requirements in application height are
important, as more modules can be installed to a cap
rail with given length. This is contrary to the need of
isolation of power and data in such a module. While
data isolation is under control with digital multichannel
isolators like the ISO7741, the isolation of power
requires a bulky transformer massively limiting the
module height. Power isolation can be removed by
supplying the isolated (field) side from the 24-V field
power supply, but at the cost of additional screw
terminals on the front-side, more installation effort
(cabling), and a high input voltage power stage,
including protection. In the past, thin, isolated solutions
have been developed using a PCB transformer at the
cost of board space. See TIDA-00688 for an example.
This application note shows how both requirements
(isolation and thinness) can be easily combined using
the UCC12050, an integrated, reinforced-isolated
DC/DC converter in a SOIC16 package.
The UCC12050 accepts an input voltage of 5 V. The
output voltage is 3.3 V or 5 V. Two additional outputs
of 3.7 V and 5.4 V can be used to feed an LDO, which
outputs 3.3 V or 5 V (the 400 mV are used for the LDO
dropout voltage). See Figure 1 for an example
schematic targeting a 3.3 V output voltage either with
or without the LDO LP2985-33. The total output power
of 500 mW enables applications with a reinforcedisolation of up to 5 kVRMS. The peak efficiency of the
UCC12050 is 60%. The integrated thermal shutdown
and tolerance to short-circuit at the output enables
very robust applications.
GNDS
GNDP
GNDS
15
VINP
VISO
14
4
SYNC
SEL
13
5
SYNC_OK
NCS
12
6
NCP
NCS
11
7
NCP
NCS
10
NCP
GNDS
10uF
8
100k
10uF
1
3
5
7
5.4V
5.0V
3.7V
3.3V
2
4
6
8
1
VIN
3
ON/OFF
VOUT
5
BYPASS
4
GND
2
EN
2
3
VCC
3
2
1
VCC_ISO
16
1
0.01uF
2.2uF
LP2985-33DBVT
9
UCC12050DVE
GND
GND
GND_ISO
GND_ISO
GND_ISO
GND_ISO
Figure 1. UCC12050 Schematic
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GND_ISO
GND_ISO
In the following, the performance of the UCC12050 in
a PLC analog input module is investigated. See TIDA01434 for schematics of the analog front end used for
the test.
The ISO7741 in SOIC-16 package is selected to have
at least the same isolation rating as the UCC12050. If
a basic isolation is sufficient, and power consumption
is important, the ISO7041 is a good option.
The UCC12050 output drives an ADS124S08, a
precision, 24-bit delta-sigma converter. The
ADS124S08 has separate power pins for the analog
(AVDD) and digital (DVDD) section. Both portions are
3.3-V compatible, and driven from the UCC12050
output. A rudimentary analog front end connected to
the ADS124S08 mimics a 20-mV input, a 2.435-V
input, and a radiometric 3-wire Resistance
Temperature Device (RTD) input. It is suitable for a
operating temperature range of -40°C to +125°C.
Figure 2 shows the test setup.
Vin (5V)
Microcontroller
SPI
DC/DC
UCC12050
Isolator
ISO7041
LDO
LP2985
ADC
ADS124S08
AFE
Figure 2. Test Setup
The test setup is compared against the ADS124S08
data sheet performance numbers. The measurement
results in Figure 3 follow the ADS124S08 setup in
Table 6 of the data sheet, and presents the influence
of the data rate to the effective resolution in bits. Both
analog input pins are shorted for the measurement.
The gain is 1 V/V to use the PGA. The digital filter is
set to SINC3 for the best noise performance.
Chopping mode is enabled to remove input DC offset,
and 512 samples are taken for every data point (data
rate). To investigate the need for a successive LDO,
the measurement was performed with LDO and LDO
bypassed.
A Reinforced-Isolated Analog Input Chain for Space-Constrained
Applications Lars Lotzenburger
Copyright © 2019, Texas Instruments Incorporated
1
www.ti.com
PARAMETER
UCC12050
ISOW7841
PCB
TRANSFORMER
STANDARD
TRANSFORMER
Application
height
+++
+++
++
-
25
Datasheet
UCC12050_withLDO
UCC12050_noLDO
Effective Resolution (bits)
24
Board space
++
+++
-
+
23
Efficiency
++
+
++
+++
Electromagnetic
emissions
+++
++
+/++/+++
+/++/+++
22
Temperature
range
+++
+++
+++
+++
21
Isolation rating
+++
+++
+/++/+++
+/++/+++
20
Table 1. Alternative Device Recommendations
19
18
1
10
100
Data Rate (SPS)
1000
DEVICE
OPTIMIZED
PARAMETERS
PERFORMANCE
TRADE-OFF
ISOW7841
Power and data isolation
Efficiency
SN6501
Efficiency
Height
10000
D001
Figure 3. Test Results
As Figure 3 shows, there is no noticeable difference
between the data sheet and whether the setup uses
an LDO. The UCC12050 internally works with a
DC/DC converter clock of 8 MHz. The LDO cannot
reject ripple at the frequency anyway, explaining the
alike curves. For good EMI performance, it is good
practice to place a high-voltage cap between the two
isolated grounds of the UCC12050. If space permits,
the cap can be designed with the PCB layers obeying
the targeted isolation ratings.
The ADS124S08 has a power supply good rejection
ratio (AVDD > 90 dB and DVDD > 100 dB), which
helps suppress remaining switching noise from the
UCC12050.
ISO7741
Cost
Basic isolation
ISO7041
Power consumption
Speed, basic isolation
Table 2. Adjacent Documentation
LINK
TITLE
TIDA-00688
1-W Isolated Power Supply With Planar
Transformer Reference Design
TIDA-01434
Isolated, Transformerless, Bipolar Supply
for 24-Bit ADCs Reference Design
SBAS660
ADS124S0x Low-Power, Low-Noise, Highly
Integrated, 6- and 12-Channel, 4-kSPS, 24Bit, Delta-Sigma ADC with PGA and
Voltage Reference
SNVSB38
UCC12050 500 mW, High-Efficiency, 5
kVRMS Isolated DC-DC Converter
This application note has shown that with the
UCC12050, an integrated, low-height isolation power
device, the performance of the analog signal chain is
not influenced, and space-constrained applications can
be served with this new technology. The 500 mW
output power of the UCC12050 can support channelto-channel and group-isolated analog input modules.
Visit the analog input modules page to learn more
about this application.
2
A Reinforced-Isolated Analog Input Chain for Space-Constrained
Applications Lars Lotzenburger
Copyright © 2019, Texas Instruments Incorporated
SLUA977 – September 2019
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