BCTLC goes to 12V

[manishsangram]

Hi,

We are using S12ZVMC128 which has the ability to generate power supply for CAN chip using an external ballast transistor. we are using BC53PA transistor with 5.6K biasing resistor (also tried 10K). We have lost as many as 20 CAN chips till date because the output of the ballast randomly goes to 12V instead of 5V (on power up).

We have been in touch with the local support group and we can assure you that this is not a software problem because we are using PE to generate the code which also is simply setting the the bit EXTCON right at the beginning.

We are very frustrated with this problem as it is just not getting solved. We are having to use a 5V LDO to test our system.

The important things to note are that this issue is seen when connecting to battery input and not when connecting to a lab power supply. Also note that we do not have reverse protection as we have physical means of ensuring that so no mosfet/diode exists between battery input and VSUP to MCU and to both ballast transistors.

Schematic attached.

[manishsangram]

RadekS​

Very glad to know that we are not the only one who have faced the problem. Your answer is good and helpful but it will not solve the problem for us because all the point you have mentioned have already correct on our board.

I want to ask you to check how the problem was finally resolved for others who had this problem.

Also note that this problem is not seen on the external VDDX BJT with everything being same.

Also the link you have posted for submitting to support is not clickable/working. " modifiedtitle="true" title="How to submit a new question for NXP Support

Can you escalate this issue internally in the company to see if this is not really a bug in the S12Z MCU?

[RadekS]

I am sorry, here is the link again:

https://community.nxp.com/docs/DOC-329745

How to submit a new question for NXP Support

It was caused probably by some issue in the editor.

[manishsangram]

Despite registering for customer support, I just cannot log in. There is some problem with the ticketing system ?

[RadekS]

Hi Manish,

I am not aware of any issue in the ticketing system.

Some screenshot with error message might be helpful. With that, we could report it.

Anyway, I already registered ticket from our FAE in India which refers to this thread content.

In the case of a continuous problem with ticket system, please send non-public data (like screenshots, schematic, layout, …) to my email address radek.sestak@nxp.com.

I hope it helps you.

Have a great day,
Radek

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[iggi]

Hi,

Recently i used one S12ZVM EVB board for some testing and found out that the BCP53 was heating a lot. That was due to damaged TJA1042 CAN transciever and after replacement the ballast transistor does not heat anymore.
Not sure how that happen as it was previously used by one of my colleagues. Usually the power supply used is 12V adapter connected to CON_1.
We would need to check the behavior of the output of the ballast transistor and confirm if the same behavior happens as you stated.
Btw, do you have your custom build board with S12ZVM or it is the EVB of NXP?

Regards,

iggi

[manishsangram]

iggi​

The issue I have described is consistent. I am providing more details below.

1. Yes we have a custom board and not an EVB.

2. This issue is ONLY seen when you connect a 12V (or higher) battery. This problem will NOT be seen with a power supply/ adapter.

3. We have a capacitor (820uF) at the power input. We have seen that to reproduce this issue fairly consistently you have to discharge the capacitor. In other words if we turn off power and turn it on again we will see 5V output but if we discharge the capacitor by keeping it powered off for 10-15 seconds or disconnect the power lead (+ev) and GND it and then try again the output will be 12V.

At this time we are pretty convinced that this could be a bug in the S12Z chip.

I've also attached a photo of the board around the ballast just FYI.

We have tried the following combinations and all have failed to solve the issue.

1. Use of 5.6K, 10K, 15K, 1K as bias resistors.

2. Replacement of 10uF ceramic with 10uF Tantalum at the output

3. Add additional 10uF capacitor between pin 2 (emitter) and GND of ballast transistor right at the pin itself.

4. Remove additional capacitor added in pt. 3

5. CUT power track to emitter (going towards R3 also) and place a diode in order to have a reverse protection diode (as we don't have one at the input).

[RadekS]

Hi Manish,

One of our customers already detected the same/similar issue.

It happened during a very quick power up of the device causing an overshoot of the VDDC regulator and then eventually damaging the external CAN transceiver connected to VDDC.

Unfortunately,  I was not able to found any specification related to maximum slew rate at Vsup the device that can tolerate without causing an overshoot yet.

The generic recommendations are the following:

a)            The grounds VSSA and VSSX MUST be shorted on the PCB, no components nor any “0 ohm” devices allowed.

b)            VDDA and VDDX MUST be shorted on the PCB, again no components allowed between

c)            The recommended bypassing should be used for VDDX/A as well as for VDDC (4.7uF X7R or 10uF tantalum || 100/220nF X7R)

d)            The low beta bipolar pnp with 1kohms base emitter resistance should be used

e)            Also, no coils in the path from emitter of the pnp to the VDDC node to the cap to finally VSSA (as the regulator reference)

f)            The routing from BCTLC to the pnp should be short, not much parasitic cap

Typically the coils between grounds and or VDDX/A may create trouble and confuse the VDDC reg.

Could you please measure slew rate when you connect external battery?

Could you please share (appropriate part of) schematic/layout with us (from a battery to CAN transceiver and VDDX/VDDXA power lines)?

If you do not want to share it in this public space, please create a ticket:

https://community.nxp.com/docs/DOC-329745

I hope it helps you.

Have a great day,
Radek

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