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I am developing an electronic circuit that will be installed in a car and get its power from the car battery. The circuit will use around 600mA. The device should be turned on when the car ignition is turned on. To protect the device against voltage transients and reverse battery connection I will use this power supply reference design from TI. I'm planning to only use the smart diode controller (LM74610) and the buck converter (LM53603) part of the design, and leave the supervisory circuit and the boost controller out of the design.

My plan was to connect the car ACC wire directly to the enable pin (EN) of the buck converter, so that my device gets power when the ignition is turned on. My question is whether I also have to protect the EN pin from voltage transients and reverse battery connection?

If yes, do I need the full transient and reverse polarity section of the reference design in front of the EN pin or is there an easier way to safely detect voltage in the ACC wire?

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  • \$\begingroup\$ Yes All car accessories have these requirements including Load dump when connected to Vbat. \$\endgroup\$ – Tony Stewart EE75 Dec 30 '16 at 22:07
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There are two things I would suggest:

  1. If it's an EN signal, it doesn't need much current to function correctly (look at the input bias current specification). Putting a large resistor in series with this pin will help.

  2. As Tony mentioned in his comment, yes, automotive electrical systems can see huge transients as a result of load dumps. Imagine that your vehicle is driving around with your headlights on, and you then decide to turn them off. Your alternator will not respond fast enough to the sudden disappearance of that load that you'll see a huge spike in voltage on what is nominally a 12V system until it recovers. Additionally, the nature of the loads (inductive, etc.) can contribute to this.

The solution here is to find a nice TVS diode that can withstand say 100V, and protect a line around 12V nominal. It looks like that part is already designed with automotive in mind, and EN can tolerate up to 40V, so choosing a TVS diode that starts conducting around say 18V + adding a 10K series resistor should be sufficient from a protection point of view. Choosing a TVS with too low a working voltage may cause it start conducting earlier than expected, burning power / potentially destroying the device.

The other way you could do this as well would be with an optocoupler, but I don't think that is necessary here.

e: You can see here the minimum bias current: enter image description here

So if we back of the envelope some margin on that (pretend it needs a minimum of 10uA), and then pretend our worst case low voltage is 10V, then a 1Mohm resistor would still allow up to 10uA of current to flow. So of course the 10K could work, or you could then use a 100K or 1M resistor in series with that input.

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