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 a 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, electrocardiograms (ECGs), pulse oximeters, and bedside patient monitors.

Specifications

Design Notes

1. Determine the 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 connects 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:
     Equation 1. $\frac{{\mathrm{n}}_{\mathrm{p}}}{{\mathrm{n}}_{\mathrm{s}}}=\frac{{\mathrm{V}}_{\mathrm{IN}}}{{\mathrm{V}}_{\mathrm{OUT}}}=\frac{5\mathrm{V}}{6.5\mathrm{V}}=\frac{1}{1.3}$
   • 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.
   1. Select the LDO to meet the output current requirement of the system and output voltage noise requirement of the system.
   2. 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 160µV_RMS 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.

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: 1.12mV_RMS.

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).

Design Featured Devices