[ADAU1451] The PNP transistor drive by VDRIVE pin for ADAU1451 DVDD

Dear Sir,

Our customer met an issue, their product(speaker) using ADAU1451 had finished development stage and MP for a while, but their end customer returned the failures set to them (2 sets in this Feb, 3 sets in May of end, total 5 failures sets) from end user.

RD checked the root cause, found due to ADAU1451 DVDD pin can't get the 1.2V supply voltage from the transistor, so the whole set can't work. The five sets all have the same problem.

RD found the external PNP transistor (ST STD2805, all circuit followed the ADAU1452 EVB's reference circuit) near by ADAU1451 was no working, and the resistance value between BC junction have about 600 ohm.

RD try to short the Emitter and Collector pin of transistor directly, then the speaker can work normally; And RD removed the short circuit between the Emitter and Collector pin, the speaker can't work ,so they think the STD2805 was the root cause.

Our customer sent the transistor (STD2805) to ST to do FA analysis, but according to the FA result, the failures part is OK.

The FA samples returned to our customer, RD re-mounted the transistor into the speaker PCBA board, the speaker can work again.

Our customer want to know :

1. The internal 1.2V reference voltage how to be generated? Is by IOVDD 3.3V or others?

Is it the possible that the internal reference voltage cause the issue happened?

2. About the transistor, the part number follow ADAU1452 EVB, and have the issue happen.

What is the condition let the PNP transistor working? Power on sequence or others?

Above the problems, please kindly to help us advised it.

Regards,

Thank you very much!!

Sam

  • 0
    •  Analog Employees 
    on Jun 16, 2017 6:19 PM over 3 years ago

    Hello Sam,

    Answer 1A)

    The internal voltage reference is generated from the IOVDD using a bandgap reference.

    Answer 1B). It is possible that something may have gone wrong so I cannot say it can never happen. We have not seen any issues with this part of the IC. I will ask more questions below...

    Answer 2) I have not experimented personally with bringing the part up incorrectly to see if I could induce a failure. I have experimented with adding significant amount of capacitance to try to make the regulator unstable and was not able to.  I have operated the part with an external DVDD and found that I had to remove the 1K resistor and the PNP transistor to prevent the part from drawing some current in the regulator circuit. It was not a problem but if the internal regulator is not being used then usually you do not want it drawing current for no reason. So again, there was not a problem with the circuit.

    I am troubled that they shorted the emitter to collector on the PNP pass transistor exposing the core to an over-voltage condition. Absolute MAX of the DVDD is 1.4V so even and IOVDD of 1.8 is too high. 3.3V would be really bad!

    Measuring the resistance of the BC junction in-circuit will not yield too much information. The measurement current will go through the PMOS device to ground and then through the DVDD load to return to the meter. 600-ohms seems reasonable but I have not measured it myself.

    My Questions:

    So what voltage is IOVDD in their system?

    Are they using any external sources for PVDD or AVDD? Usually it is all off of the IOVDD. If they are using external PVDD then I would like to see the timing of startup with relation to IOVDD.

    I need to see their schematics.

    I would really love to see a failed unit. If that is possible it would be great.

    Please email me directly with any of this information that may be confidential.

    Thanks,

    Dave T