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Currently I'm using a buck switching regulator AOZ1284PI of Alpha & Omega Semiconductor to generate a 5V supply from a 12V input of a lead-acid sealed battery, and I plan to use AOZ1284PI again on the project below.

I plan to design some vehicle tracker that can be used in cars and bus/truck, so my input voltage range to be considered is from 10 to 30V (maybe 32?) steady. Of course I will use a schottky diode at the input to prevent reverse connection of the supply input (battery power of the vehicle). And I plan to use a TVS diode SMAJ30CA right at the supply input (it is bidirectional TVS diode), before the protection diode.

According to the datasheet of AOZ1284PI, the maximum supply voltage recommended is 36V, and absolute maximum is 40V. According to specs of TVS diode SMAJ30CA, Voltage Clamping (Max) @ Ipp = 48.4V.

Then, considerer voltage spikes are occuring at the battery and the TVS diode is clamping them at the input of my circuit. Clamping voltage would be 48.4V, and this is greater than the maximum of 40V of AOZ1284PI. Of course such voltage peaks will probably have a very short time duration in practice.

I would like to know what are your observations for that situation.

Would the AOZ1284PI be prone to be easily damaged in such situations? In a vehicle environment? That's my main question.

Or in this case we have to consider the ESD rating of the IC?

Datasheet of AOZ1284PI: http://aosmd.com/res/data_sheets/AOZ1284PI.pdf SMAJ30CA specs on Digikey: https://www.digikey.com/product-detail/en/littelfuse-inc/SMAJ30CA/SMAJ30CALFCT-ND/762507

Regards.

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    \$\begingroup\$ There are surge compliance specifications for your application. You should adhere to those and design your protection scheme accordingly. \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2020 at 9:47
  • \$\begingroup\$ @Andyaka What is the "surge compliance specifications" you mean? Do it have a code/standard name? \$\endgroup\$
    – abomin3v3l
    Commented Mar 1, 2020 at 11:06
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    \$\begingroup\$ You will need to find that out. I use EN61000-4-5 a lot but it relates to AC power systems. You need to find an automotive spec and that may include such things as load dumps where the peak voltage can rise to over 100 volts. \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2020 at 11:08
  • \$\begingroup\$ do you have idea if such buck controller IC (AOZ1284PI) will be prone do get damaged easily in a vehicle environment? (consider that I will use a TVS diode or the MOV, and a fuse...) \$\endgroup\$
    – abomin3v3l
    Commented Mar 1, 2020 at 11:25
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    \$\begingroup\$ Pretty much all regular chips and devices will get damaged if not protected. Automotive is a somewhat violent place to put chips and care has to be taken. Personally I'd be considering devices that can "take" a much bigger voltage but, it all boils down to choosing a TVS that can handle the surges without generating a clamping over-voltage that exceeds the chips maximum rating (by some clear margin). \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2020 at 11:29

1 Answer 1

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Normally, VCL (clamping voltage) of the TVS should not exceed the abs max input voltage of the device to be protected. Note that VCL is given for a maximum IPP (peak pulse current) value. For higher peak currents that VCL may not be valid. Also, VRM (standoff voltage) should be higher than the nominal input voltage. For two-battery vehicles (coach, truck, etc) the nominal battery voltage is 28.7VDC. According to these considerations, SMAJ30CA may not be suitable for you.

As a former designer for automotive electronics, I've never used TVS diodes anywhere but sensitive analog and digital inputs. Instead, I had put a varistor (a.k.a. VDR or MOV) following a fuse (if not present on the power line externally) in my designs.

schematic

simulate this circuit – Schematic created using CircuitLab

The advantage of using a VDR is that the VDR can absorb the excess energy created by the overvoltages or spikes and it can lead the fuse to blow, so the end-user can replace the fuse and the device can work again. This may not be possible when using a TVS since the TVS can get damaged even before the fuse to blow.

VDR serves good also for Load Dump. For better protection, Texas Instruments has a greatly detailed application note.

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  • \$\begingroup\$ Nice answer. Could you supply me a part number for the MOV you refer? Better if I can find it at Digikey. \$\endgroup\$
    – abomin3v3l
    Commented Mar 1, 2020 at 11:10
  • \$\begingroup\$ And more important, do you have idea if such buck controller IC (AOZ1284PI) will be prone do get damaged easily in a vehicle environment? (consider that I will use a TVS diode or the MOV you said...) \$\endgroup\$
    – abomin3v3l
    Commented Mar 1, 2020 at 11:13
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    \$\begingroup\$ @abomin3v3l For the VDR, I can recommend this one. About the possibility of the IC to get damaged, I can't say anything because I don't know the chip and I've never used it. But, just to compare, I have used MP2467DN for years. It survived even the Load Dump test based on the requirements of Mercedes Benz/EvoBus. \$\endgroup\$
    – Rohat Kılıç
    Commented Mar 1, 2020 at 11:31
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    \$\begingroup\$ @abomin3v3l I can't remember the exact part number but the specs are the same. \$\endgroup\$
    – Rohat Kılıç
    Commented Mar 1, 2020 at 11:48
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    \$\begingroup\$ @abomin3v3l Ah yes, I thought that it was unrelated. Anyway, here it is: The nominal supply voltage is 28.8VDC and the operating range is 18..36VDC. The required operating temperature range is -40..+80°C. The device should also operate without problems at a 50°C environment for one hour with 36VDC supply. The last one is called "Overvoltage" in ISO-16750. It may not be required to be 50°C ambient temperature but Mercedes Benz and MAN were dictating it to be. \$\endgroup\$
    – Rohat Kılıç
    Commented Mar 1, 2020 at 12:40

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