Err in haste, repent in leisure... or maybe not.

Other Parts Discussed in Post: LM5069

Today’s average Joe tends to double as assembler, installer, etc, which means (for good or bad) your design needs to be easy-to-use and well protected. To be fair, with all the technology out there, most of us are quite unfamiliar on the technical details. That said, we continue to assemble our new surround sound system, home office network, or even the latest infotainment technomagic in our vehicle of choice. Between this and a growing demand for power density, efficiency, accuracy, and miniaturization... the power protection wizards are quite busy. 

In the last few years, I have seen an increase request for protection solutions to defend against reverse hookup, surge clamping, and reverse current protection. Such requests have come in for both positive GND and negative GND systems, with absolute voltages ranging from 12 to 48-V.

So, what are these scary sounding, polysyllabic threats?!

  • Reverse hookup connects the DC supply backwards. Think plus to minus, minus to plus. Car jumper cables as connected by Uncle Leo after Thanksgiving dinner and the football game(s).  Potential for smoke, sparks, and adrenaline. Reverse hookup can also happen in adapter powered systems when the wrong adapter is used.
  • Surge clamping are large, beyond the maximum-rated voltage of the load, spikes typically caused by stray inductance (wire) and rapidly changing load currents. Ldi/dt strikes in accordance with the laws of physics whether you like it or not. Many systems require specific surge testing before deployment.
  • Reverse current protection are wrong way electrons going from the load to the source. This is a particular concern in redundant supply systems and “last gasp” energy solutions. Solid State Drives (SSDs) require such energy for 30 -40ms to shut off in a sanitary manner. If that energy goes out to the bus instead of to supporting shutdown activities on the SSD it’s “goodbye data” time. 

There are a few simple, effective solutions based on tried and tested protection switch controllers already existing.

Would you like your Hot Swap device with surge clamping, reverse current blocking, or reverse hookup protection?

  Example of a hot swap device with reverse hookup, surge clamping, and reverse current protection.

Particular solution elements have been color coded for easy viewing and review. All the technical details are in the application note, Surge stopping and reverse voltage protection with the LM5069, but the highlights shall be recapped here.

CODE YELLOW - Reverse Current Blocking - FET (Q2) will prevent flow through the invisible body diode of Q1 when VOUT>VIN. This circuit will automatically shut off both FETs when the input voltage drops below the UV level programmed by R1, R2, R3.

CODE BLUE – Clamp That Surge - A Zener diode from the GATE pin to GND will limit VOUT to VZENER – VTH_FET. This slick, one Zener accessory will, unlike simple OV protection that shuts off power completely, continue to provide power to the load during a voltage surge. If this limiting starts to stress out the FET, fear not. The built in FET SOA (Safe operating Area) protection will keep the FET from destruction. Automotive load dump surge specs are often 80 to 100 V for 100 ms.

CODE GREEN – Back words can hurt! – When your uncle Leo hooks up the jumper cables backwards the last thing you want is one of the critical modules in his car succumbing to reverse hookup damage and extending his visit while it is repaired. These simple modifications will protect all downstream circuits from backwards voltage. Diode D1 prevents the LM5069 from getting reverse biased when applying voltage backwards. Unpowered, the LM5069 will pull GATE to GND to turn it off but, in a reverse hookup state, GND and gate are effectively floating. To ensure the GATE is pulled low during such conditions D2 and R11 are connected from GATE to VIN (which is now the lowest potential node in the circuit). 

For applications requiring Automotive Q100 parts there are similar solutions in the LM5060-Q1 datasheet.

And for those of you who have made it this far… a short survey:

  1. I know systems that could use this type of protection.                               T              F
  2. If TI had a test board for this I’d like to test it.                                               T              F
  3. My solution would have to be made of Auto qualified parts (Q100)         T              F
  4. Nobody needs this. Stuff never gets hooked up backwards.                    T              F

Please leave your answers in the comment section below.

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