AM3352: Short circuit on custom board

Patch Tept4400.pdfThank you for the answer!

The fault is most probably caused by our construction or process related.  When making the PCB or PCBA.

We have a light sensor onboard. We had to change the one we designed from the beginning because this one contained RoHS substances. The new one we patch in like attached document shows. Here the 1V8 is directly connected to the ADC more or less. Can this be a problem?

//Per

Visio-Ritning1.pdfSorry for all mails :)

Here is another one!

See attached schematics of the USB part of the board.

Worried about R174 when supplying from USB.

When plugging in the USB connector in it takes about 50mS before all supplies are on.

Can we supply U1 backwords through R174 and the internal clamps in U1? During this start-up phase?

Is it dangerous to only have 0ohm here on R174? should we have 10k or 100k?

Can this be the cause of the internal U1 shortcut on 1V8?

Regards,

Per

Based on the ADC schematic you provided, it appears the power source for analog inputs AIN1, AIN2, and AIN6 is the same as the one that powers VDDA_ADC. If so, I would not expect an issue with the light sensor change. I'm more concerned with the power sources for analog inputs AIN0 and AIN3. The ADC inputs are not fail-safe so they should not have any potential applied before VDDA_ADC is valid. Is there any chance a potential is applied to AIN0 or AIN3 before the ADC is powered?

The USB0_VBUS input is fail-safe, so apply VBUS potential to this input before power is applied should not cause an issue. However, you can increase the value of this resistor up to 1k ohm without creating any functional issue. This would limit current into the pin if some ESD energy is getting past the ESD clamp.

Regards,
Paul

Ok i will add 1k instead on 0R R174 USB_SENSE!

Concerning the AIN0 and AIN3 see attached schematics.

//PerAnswer to question ADIN.pdf

It looks like AIN3 will have 3.3V applied when the hall switch is not driven low.  This violates the absolute maximum input voltage to the ADC, [Steady state max voltage at all I/O pins = –0.5 V to (I/O supply voltage + 0.3 V)].  The internal ESD protection circuit is very likely turned on in this condition, but the current will be limited by the 3.3V pull-up R90.  The ESD protection circuit was not designed to be turned on for extended periods of time.  I'm also concerned with the peak current that can be sourced by capacitor C90 when power is turned off.  If the hall switch is not driving low when power is turned off, the capacitor will be charged and source current to ADC supply rail through the ESD circuit.

I realize the current sourced to AIN0 will be limited by R29, but any potential that exceeds the absolute maximum value defined in the data sheet has the potential of damaging the device.

Regards,
Paul

0654.Schematics.pdfChange ADIN.pdfHow it looks at Scanfil test-eq.pdfHow it looks with Zigbee on and off.pdfThanks for the reply!

I have change it according attached file: Change ADIN.pdf

After that i have been investigating a little bit more and found something that doesnt look good at all.

See file: How it looks with Zigbee on and off.pdf

In production it looks worse see file: How it looks at Scanfil test-eq.pdf

When 3v3_wifi goes on other voltage rises 0.2V.

I add some schematics also so you can get a better picture of it.

It looks that this can be the reason for the shortcut.

Thanks in advance!

/Per

I do not see how the change you made to the door sensor circuit works. Based on your latest schematic the BAT54S diode connected between R87 and ground will always be conducting and T2 will always be on. The other BAT54S diode will always be reverse biased, so R90 will hold the Door Sensor signal low.

It appears you are only concerned with the 1V to 3.3V transition on 3V3_WiFi. Why does 3V3_WiFi go up to about 1V when the 1.8V supply is turned on? This indicates there is a leakage path between the two supplies which needs to be understood.

It is not obvious from the schematics you sent where the path of this leakage. You may need to remove components until you find the path. Once the path is understood we will be in a better position to determine if this is a problem.

I can’t tell from your scope capture where the strange things happening on SPI signals occur relative to power sequencing. Does this occurring during power sequencing or is the event completely independent of power sequencing?

I would like to confirm a few details. Power pin shorts only recently began? The only things that changed on the board were the light sensor connection to AIN2 and the value of R174? The power sequencing concerns being discussed now have existed for a long time? If so, you should continue looking at usage differences in parallel just in case the issue causing shorts is unrelated to these power sequencing concerns.

Regards,
Paul

Reply to answer Paul.pdfThanks again for the answer!

Think i should leave the 1V8 area it seems ok here. See attached document of how the connections are on the board with trouble.

The grounding of ADIN is ok? there are three directly grounded.

Have received one more module where it was 3,3V out on the AIN3 (Doorsensor pin) the BAT54S was on fire :).

We havent changed the 0R of USB yet. So you mean that this good be a problem not to have some kind of current limiter here?

BR

Per

You are correct, the base to emitter path through Q1 will be forward biased and this will limit the voltage applied to AIN3. I'm sorry for any confusion with respect to this circuit. However, you still need to be concerned with the ADC inputs not being fail-safe. For example, when +3V3_ONOFF is on while VDDA_ADC is off.

The advisory previously discussed, recommends any unused pins should be connected to ground through resistors. These resistors will limit current through the unexpected paths that occur during power-up. However, our analysis shows internal resistance of the analog multiplexers should limit current such the device would not be damaged if the condition is not allowed to exist for long periods of time. Therefore, I’m not expecting this to be causing power supply shorts. The external resistor recommendation provides additional margin.

I do not understand how it is possible for an ADC input pin to develop a short to 3.3V. I’m fairly sure there are no 3.3V supply rails inside the ADC, but will discuss this with our ADC designer to confirm.

The VBUS pin has an ESD protection circuit inside the device. This circuit is only designed to protect the device during product assembly and handling. It is not adequate for system level protection. So it is good that your product has external ESD protection devices to provide protection for system level ESD events. However, some energy from an ESD event may get past the external protection device. Inserting a 1K ohm series resistors will limit the current that flows into the VBUS ESD protection circuit when this occurs.

Regards,
Paul