New Industry Products

ST’s Twin 5W Isolated Buck Converters Require No Opto-coupler or Ancillary Components

July 18, 2021 by Gary Elinoff

The A69861 and L69861 from STMicroelectronics (ST), are designed for automotive and industrial applications, respectively.

STMicroelectronics’ two new devices operate from an input range of 4 to 38 volts and provide a non-isolated primary voltage and an isolated secondary voltage.

Image courtesy of ST

The secondary side voltages are governed by the turn ratio of the external isolation transformer. The primary side voltage can be adjusted by the choice of eternal resistors.

To regulate the primary output voltage, the units operate in forced-PWM mode with peak-current-mode control and to provide better reliability. For better reliability, the devices also feature a programmable soft-start. Pulse-by-pulse current sensing makes for primary-side constant-current protection. Oscillations caused by the transformer leakage inductance are filtered by a preset current-sense blanking time of 300 ns.

The units also provide thermal shutdown and over-voltage protection. There are external pins for setting the switching frequency and for synchronization.


Operation of the Isolated Buck Converters 

Both of the isolated buck (Iso-buck) converters can be thought of as consisting of three subsections.

The A69861 and the L69861. Image courtesy of A69861 datasheet

These subsections are the primary side, the isolation transformer, and the secondary side.


The Primary Side

The primary side is depicted in blue. It serves as the regulation loop of the peak current mode, constant frequency control architecture that serves to regulate the primary voltage (VOUT_pri). It is the intersection between the error amplifier output and the sensed inductor current that generates the PWM control signal to drive the power switch of the primary side. This serves to define the duty cycle, thus regulating the primary output voltage.

With the MLF pin set to ground, the units operate in forced PWM control. This allows negative currents to flow in the synchronous MOSFET. As such, during off-time energy can flow to the transformer’s secondary coil.


The Isolation Transformer

The isolation transformer, depicted in grey, provides isolation between the primary side and secondary side. As stated, its turns ratio largely determines the isolated output voltage.


The Secondary Side

The secondary side is depicted in green. It serves to generate the secondary side voltage, with the D1 providing rectification. Filtering is provided by an LC filter consisting of the transformer’s secondary winding and capacitor COUT_sec.


Applications for the A69861 Automotive Converter

The 100% duty cycle and the wide range of the A69861 enable it to meet the  load-dump and cold-crank specifications of automotive systems, making them suitable for:

  • Hybrid/Electric-Vehicle (H/EV) traction drives. 
  • On-Board Chargers (OBC)


Applications for the L69861 Industrial Converter

The industrial-grade L69861 will find application in: 

  • Welders
  • Motor drives
  • Uninterruptible power supplies (UPS)
  • Solar power-conditioning systems



  • Both units operate over a junction temperature range of -40 to +150℃
  • The automotive A69861 is AEC-Q100 Grade-1 qualified 


Getting to Market Faster

ST offers two evaluation boards to help designers more quickly develop single and dual output converters.