NCV8876 - With Power-Saving Diode Bypass Circuit

This is a reference design for the ON Semiconductor NCV8876 Start-Stop Boost Controller (http://www.onsemi.com/pub_link/Collateral/NCV8876-D.PDF). It is a more complex but higher efficiency implementation than the companion design: https://www.systemvision.com/design/ncv8876-circuit-showing-emc-componen.... This version is intended for use with higher power loads.

This circuit includes a PMOS bypass switch, such that the rectifier diode voltage drop is not present under normal battery voltage conditions. All of the load current flows through the bypass PMOS during normal operation. When the battery voltage falls below the "wake-up" level, the NCV8876 status pin transition provides an early indication that the boost switching is about to begin. This signal is inverted and level-shifted by the BJT circuit. This deactivates the bypass PMOS, forcing the boosted load current through the rectifier diode. When the battery voltage recovers and the output voltage rises, the status re-activates the PMOS bypass and normal operation resumes.

Simulation results show the output to the 20W load during battery drop-out and recovery. Note that in this application, the output voltage (red waveform) is maintained above 5.5V during the drop-out transient, and regulates at 6.8V as the input voltage at the PCB drops to a minimum value of 3V (light blue waveform). The bypass transition can also be observed, with the PMOS gate voltage (purple waveform) and the Ids current (dark green waveform) shown in the lower waveform viewer. Note that no bypass current flows during boost switching. The current transfer to the diode can be observed by moving one of the waveform probes to the diode and choosing to plot "id".

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Mike Donnelly

Joined November 8, 2013
Design added Monday, August 8, 2016 | 5:27 pm PDT