TPS55340: doesn't boost after activating ENABLE pin

Doc18.docx

Attached file

Hi Helen

yes, I'm sure. We tested removing R218 and populating a 20K pull-up just to have PIN EN with fixed +24v. Still doesn't start-up.

Thank you

Antonio

Hello Helen

answering your questions:

What is the EN frequency?

It has a ON/OFF signal only use in few times.

Are you sure the IC is still OK and not damaged? 

I don’t know, it is the behavior in this pcb and other 3, but the question is Why a good pcb turns to this behavior?

Please remove the Vin and then add Vin back  again to check whether the IC could startup successfully.

Yes, if I remove the Vin the IC startup correctly until activate EN signal

Thanks

Antonio.

Hi Helen

• Yes, I tried to connect EN pin through a 20K resistor to +24V, and didn’t work.

• I measured the rise up in EN signal and it was <2uS, but what will be the point if that circuit doesn’t work?

• No, I don’t try to enable/disable with 200Hz because for my real situation I only have 1 enable or two for each power on.

thank you

antonio

Hi Helen

we will try. In the meantime  attached a new sch in better resolution. I hope you could find anything that explain this problem.

thank you.

Antonio

Helen

thank you very much for your help. Customer will test your recomendations. In the meantime:

We substitute a broken TPS55340 with a new device and we did the following tests removing R218 to get more than 1,3v in PIN EN. We mounted 3 different configurations:

1. Original configuration:

– R70= 73K2.

– R71= 2K23

– C12= 22K

– C9= 47K

– C11= 39p

Behavior is always good, boost up to 33v, even with 1hz square signal in PIN EN.

1. Excel values configuration:

– R70= 124K.

– R71= 2K94

– C12= 68K

– C9= 47K

– C11= 270p

Behavior is always good, boost up to 33v, even with 1hz square signal in PIN EN.

1. Webench values configuration.

– R70= 60.4K.

– R71= 3K32

– C12= 68K

– C9= 22K

– C11= open

Behavior is always good, boost up to 33v, even with 1hz square signal in PIN EN.

Voltage in PIN EN is always over 26v.

Voltage in PIN COMP is always over 1.2v.

We all these configurations working OK we deduct that the problem was the chip (TPS55340). Why this device is broken after operating in field?

What’s the best configuration we described above?

antonio

Hellen

I don't have yet Vin value. In the meantime. Following comments of my customer:

If I remove Q22, I couldn’t control En signal and I need to control it. If I never turn off En signal, the Ic works fine but I want to control it.

I don’t know Why when I turn off/on EN signal the IC don’t work again until I remove +24V. That’s the problem I have.

Did you revise the sch? All values are correct?

Antonio

not yet sent. We started this week the bureaucratic process to send pieces for failure analysis. It's long.

antonio

Hi Helen:

I'm working with Antonio, and he asked to me for reply your questions. I attached the pcb desing, if have any question tell me and I answer ASAP. Also, I gonna to compare the wrong IC with another one, through measure the impedance between pin 6 and the rest of them, and between pin 11 and the rest of them; If I detect something strange I tell you.

Thanks for your help, Regards

TPS55340.zip

Hi Helen:

I saw your recomendations, and yesterday when I saw your post requesting the layout, I started to review our layout and comparing it with the example in the datasheet. I realized that the connections in the top layer wasn't do it and I wrote in my book to improve in a future redesing.

I want to ask you a few questions:

If we do this changes that problem won't happen in the future?

Are you completly sure, if we change the layout the problem will be gone forever?

When we redesing the board, could I send you the new layout for a review?

We don't have to connect PGND and AGND on internal layers, correct?

We have to create a thermal plane on internal layers to radiate IC heat, but this plane don't have to connect with AGND or PGND, only with thermal pad throuhg vias, correct?

We could try to change in an actual PCB the layout with your recomendations, Do a damaged IC works well again? what do you think?

Regards

I’m a contractor, hired by MP to try and help them with their boost converter. They have shipped me several of their PCBs, both non-working and working, and I’ve seen all of the same issues that you, Antonio and Juan discussed. I’ve done extensive startup and shutdown testing as well as cycling of the EN pin with a signal generator, and I haven’t seen any waveforms that look like they could damage the IC.

At the end of the chain you were discussing the PCB layout, noting that connection of the AGND, PGND and PowerPAD goes through vias to internal layers instead of connecting on the top layer (where the IC is.) I took one of the working boards and put in some low-inductance test fixtures:

While I agree that the layout could be improved, the waveforms I see don’t look bad. These were taken at the maximum output current of 330 mA.

     I noticed that you recommended trying different slew rates for the voltage at the EN pin, and that you tested that on a TPS55340 EVM. Is it possible that the added inductance of the vias between the AGND pin and the PowerPAD are causing a latchup in the IC? Since this happens during shutdown, I suppose a fast, negative transient voltage could exist between AGND and the PowerPAD. One thing I did find was that on some of the damaged TPS55340 ICs, if the EN pin was pulled to 24V via the 20k resistor (R66) upon startup, then the TPS would start switching again.

       Many thanks,

Chris Richardson

Hi Helen,

      My customer sent me three boards on which the TPS55340 was not responding to the EN pin signal. First I checked to make sure that the EN pin voltage was always above 1.3V, and it was. Then I removed R128, the bottom divider resistor, and applied a voltage to the EN pin with a lab supply. None of the TPS55340's turned on, even when I applied the full 24V (equal to the input voltage). However one of the boards did turn on if I powered it on with R66 in place but R128. This led me to believe that the slew rate was important.

       I will check to see if my customer has sent in any damaged parts.

Thanks,

Chris