Purpose of Schottky diode in this schematic

Question

I have come across the power supply management section used in the Verdin Development Board, addressing overvoltage, undervoltage, overcurrent, and reverse voltage protection.

While I understand how the circuit is functioning, I am unsure about the necessity of the Schottky diode MBRB735PBF connected in parallel across the current path. It seems to be in a constant conducting state. Can you clarify the purpose of the lower circuitry when the diode already provides a parallel current path? How will it cut off the supply in the event of any occurring fault?

Answer 1

Relevant documents:

https://docs.toradex.com/109463-verdin_development_board_datasheet_v1.1.pdf - looks like power input is on X57/X58, 6.3-26.4V input.

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4368.pdf - under/over voltage, overcurrent protection device. Fancy, but then why would...

https://www.vishay.com/docs/96396/vs-mbrb735-m3.pdf - it's a 35V 7A diode, nothing special.

https://developer.toradex.com/hardware/verdin-som-family/carrier-boards/verdin-development-board/#design-resources - schematic download, Altium or PDF of interest here.

Looking at the PDFs, X57 and X58 are indeed on the sheet (p.3), cropped just outside of the image given. D21 and D30 are shown X'd out, indicating a do-not-populate directive via the Assembly Variants mechanism.

There is also a text box missing from the given image:

WARNING: Hot-plugging a 26.4V power supply to the carrier board may trigger the protection IC (IC25) and prevent the product from powering up. This behaviour can be fixed by populating a 4.7nF 50V 0805 capacitor on the D30 footprint. (see Errata #13: HAR-9004 in the related errata document).

I think there are two root causes here:

1. When loading an Altium design, always compile the project, check for SCH/PCB differences, check the Variants, and check the BomDoc and other supporting files, if used. It also doesn't hurt to check Project Options (particularly if parameters are in use), and the OutJob (if used) (to see if any parameters or variants in particular are being selected for output).

   After these steps, with a variant selected, you can select compile view (tab at the bottom of SCH viewing area) to visually see what components are DNP or otherwise varied.

   You can also check the BOM output (if provided), to see what has been omitted (admittedly, a nontrivial task on a BOM of this length), or if they are listed as 0 quantity / DNP.

2. They are probably doing an exclusive placement option, between D21 (reverse protection only, modest forward voltage drop), and all the OV/UV support components (T19, T33, IC25, etc.). I haven't loaded the schematic to confirm, but this should be sufficient information for you to do so.

Answer 2

The MOSFETS have some turn on delay due to the MCU startup time. The purpose of the diode D21 is to gate power to the output at power-on immediately.

As soon as the MCU switches the MOSFETs on, they provide a power path with much less impedance.

So you can say that the diode is kind of a quick starter for the power transfer path.