question about VBUS on 4200L running at 3.3. volts (smoke involved!)

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mish_288636
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Hi, we're working on a design using the cy8c4248azi-L485, powered by a 3.3. volt LDO being powered by the USB connector, and the VBUS from it also goes directly to the PSoC.  On a few occasions now, the PSoC has released its magic smoke when plugging it in.  The datasheet doesn't mention much about power sequencing or whether it's safe to apply VBus if the VDD rail isn't powered. (the 3.3 volt regulator takes a few microseconds to come up after the USB cable is plugged in).  I looked at the reference designs, but they seem to all just power the chip with the USB voltage.

   After the chips fail, the VBUS pin typically measures 1-3 ohms to ground.

  Are there any additional documents or recommendations about how to do this or if it's possible or what.

Thanks!

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Vasanth
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Hi Mike,

Our kits have similar configuration. I could not see any problem in the schematic. One observation (not quite sure) is that this may be due to 3.3V rail itself, which is connected to the motor driver which is likely to have some spikes and all (VM pins are connections for H-Bridge, which is the same voltage rail for PSoC) . Can you investigate in that direction too?

Best Regards,
Vasanth

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Vasanth
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Hi,

As per PSoC datasheet, there is no constraint on VBUS with respect to VDDD. Let me confirm this internally and get back to you whether there is any sequencing requirement.

Best Regards,
Vasanth

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Hi, thanks for the info, and just wanted to see if you've heard anything back yet?

Thanks!

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Hi, just pinging again to see if there's any new information?

Thanks,

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Vasanth
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Hi Mike,

Apologies for the delay here. The USB supply is independent of VDD and the sequencing should not affect the device. Are you giving any other pins in the same power domain as input to the PSoC when it is not powered ?

Best Regards,
Vasanth

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Hi, thanks so much for the information.

There should not be any pins being powered as all the power is coming from the USB connector.

We're attaching a schematic, would be be able to review it to make sure we're not missing some other requirement?

Thanks,

-Mike

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Vasanth
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Hi Mike,

One additional question regarding your settings, Is VBUS is configured as 3.3 V in the project? In that case fitter would bypass the VBUS regulator and connect VBUS directly to power the USB block – which could result in damage.

Best Regards,

Vasanth

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Hi, thanks for asking, VBUS is set to 5 volts.  Did you see any potential issues with the schematic?

Thanks!

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Hello.

My $0.02 worth...

Are you using Kitprog or Miniprog for programming the device?

If using Kitprog, remove diode D1 from Kitprog pcb.  If you don't, it will apply Vbus to the target from its USB connection.  With D1 removed, Kitprog will correctly apply SWD signals at 3.3V.

If using Miniprog, you don't want to give Miniprog the opportunity of back-powering your target.  I would insert a diode between the target +3.3V and debug connector, with the diode's cathode pointing at debug connector.

Adding a diode at debug connector does not solve issue of Kitprog D1.  Kitprog measures Vtarg when connected.  If Kitprog reads its Vtarg via its own USB Vbus, Kitprog will apply SWD at 5V.  This is bad news for target pcb since it is being back-powered.  I don't know what the 3.3V regulator (or other hardware chips) will do in this case.

Good luck with your project.

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Vasanth
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250 sign-ins 500 solutions authored First question asked

Hi Mike,

Our kits have similar configuration. I could not see any problem in the schematic. One observation (not quite sure) is that this may be due to 3.3V rail itself, which is connected to the motor driver which is likely to have some spikes and all (VM pins are connections for H-Bridge, which is the same voltage rail for PSoC) . Can you investigate in that direction too?

Best Regards,
Vasanth

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