Texas Instruments | Isolated power supply, low-noise circuit: 5V, 100mA (Rev. A) | Application notes | Texas Instruments Isolated power supply, low-noise circuit: 5V, 100mA (Rev. A) Application notes

Texas Instruments Isolated power supply, low-noise circuit: 5V, 100mA (Rev. A) Application notes
Analog Engineer's Circuit: Data
Converters
SBAA268A – February 2018 – Revised March 2019
Isolated power supply, low-noise circuit: 5V, 100mA
Reed Kaczmarek
Power Supplies
AVDD
Vee
Vdd
5.0V
6.5V
5.0V
Design Description
This design shows an isolated power supply using a transformer driver and a low dropout regulator (LDO).
This design is intended to be combined with a digitally-isolated SAR ADC, such as in Digitally-Isolated
ADS8689 Design. Industrial applications that require an isolation interface are the primary application for
this design in combination with a SAR ADC. The transformer driver and LDO can be selected differently
based on the output current requirements and output voltage noise requirements. This power supply was
built and tested on a PCB with the ADS8689 and later in this document the performance of the ADS8689
is shown to prove the effectiveness of the power supply.
This circuit implementation is applicable in applications such as Analog Input Modules, Electrocardiogram
(ECG), Pulse Oximeters, and Bedside Patient Monitors.
SN6505B
D2
GND
Enable
10 µF
EN
Vcc
CLK
D1
LP2981-5.0
IN
OUT
6.5 V
0.1 µF
5V
10 µF
EN
+
GND
5 VDC
10 µF
0.1 µF
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Specifications
Specification
Goal
Measured
LDO Output Current
< 100mA
16mA per channel
LDO Output Voltage Noise
< 1mVRMS
N/A
92dB
92.4dB
–112dB
111.3dB
ADS8689 Signal-to-Noise Ratio
(SNR)
ADS8689 Total Harmonic
Distortion (THD)
Design Notes
1. Determine the required supply current that is needed on the secondary side of the transformer. This
information is used for component selection.
2. Choosing the transformer and transformer driver are very important to creating a correct isolated power
supply.
3. The CLK pin on the SN6505B can be connected to an external clock or left floating to use the internal
420-kHz clock.
Component Selection
1. Select a transformer driver based on the required output current.
• SN6505 will provide up to 1A of output current. The SN6505A has a 160-kHz internal clock and the
SN6505B has a 420-kHz internal clock.
• SN6501 will provide up to 350mA of output current.
2. Select a transformer with the desired turns ratio and current rating.
• This design takes 5-V input and produces 6.5-V output. The turns ratio is determined as shown in
the following:
(1)
•
The 760390014 from Wurth Electronics was used in this design since it has a 1:1.3 turns ratio and
a current rating that meets the 100-mA design specification.
3. Select a low dropout regulator (LDO) to use the transformer output and produce a low-noise supply
voltage.
a. The LDO should be selected to meet the output current requirement of the system and output
voltage noise requirement of the system.
b. The LP2981-5.0 is a fixed output voltage LDO that was selected for this design. This LDO is able
to provide 100mA of output current. Also, the LDO output is accurate with only 160µVRMS of output
voltage noise.
4. Select the rectifier diodes for the fast switching of the SN6505B.
• The SN6505x Low-Noise 1-A Transformer Drivers for Isolated Power Supplies data sheet
recommends using low-cost Schottky rectifier MBR0520L.
• The forward voltage drop will take away from the output voltage of your isolated power supply.
• The diodes must be rated for the expected current level for this supply.
2
Isolated power supply, low-noise circuit: 5V, 100mA
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Measured Transformer Driver Outputs
The following image is an oscilloscope capture of the two transformer drive lines from the SN6505B.
These are 0V to 5V pulses at a frequency of 411kHz.
Measured SN6505B Stage Output Ripple
The following image is an oscilloscope capture of the output voltage following the rectifiers of the
SN6505B power stage. This is the input to the LP2981-5.0 LDO. Result: 1.12mVRMS.
SBAA268A – February 2018 – Revised March 2019
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Measured FFT
This power supply was implemented on a channel-to-channel isolated ADS8689 PCB. Measuring the
AC performance of the ADS8689 proves the effectiveness of this isolated power supply. The AC
performance indicates SNR = 92.4dB and THD = –111.3dB, which matches well with the specified
performance of the ADC: SNR = 92dB and THD = –112dB).
0
-20
10
20
f = 2kHz
40
50
40
50
SNR = 92.4dB
THD = -111.38dB
-40
Amplitude (dB)
30
-60
-80
-100
-120
-140
-160
-180
-200
10
20
30
Frequency (kHz)
4
Isolated power supply, low-noise circuit: 5V, 100mA
SBAA268A – February 2018 – Revised March 2019
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Design Featured Devices
Device
Key Features
Link
Similar Devices
ADS8689
12-bit resolution, SPI, 1-Msps sample rate, single-ended input,
AVDD/Vref input range 1.6V to 3.6V.
www.ti.com/product/ADS8689
www.ti.com/adcs
SN6505B
Low-Noise 1-A, 420-kHz transformer driver
www.ti.com/product/SN6505B
www.ti.com/isolation/transformerdriver/products.html
LP2981
100mA ultra-low dropout regulator with shutdown
www.ti.com/product/LP2981
www.ti.com/ldo
Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
Revision History
Revision
Date
A
March 2019
Change
Downstyle the title and changed title role to 'Data Converters'.
Added link to circuit cookbook landing page.
SBAA268A – February 2018 – Revised March 2019
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Isolated power supply, low-noise circuit: 5V, 100mA
Copyright © 2018–2019, Texas Instruments Incorporated
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