LM46002: Vout stays at ~ 1.7V after a random number of reboots.

HI Mickael:

below is some of my idea:

1. can you remove the LED circuit  from 5V, the R40 value is out of our datasheet recommend for PGOOD pull up resistor value, it should be between 10K-100K

2. remove C30 , 

3. in the same time, pls check the BOOT voltage, I suspect the boot to SW voltage may less than 2V

if the issue stil exist, can you help show the Vin, VO and SW waveform? 

Thanks

Hi Daniel,

Thank you for these inputs.

1. I removed the LED circuit and replaced R40 with a 12k1 resistor. So there is no LED and the PGOOD pin is pulled high as recommended in the datasheet, but nothing changed. I still only measure 1V level on RT pin. I'd like to point out the fact that I replaced the PGOOD resistor with a 560R on the EVM board, the issue does not replicate there.

2. Removing C30 is an interesting idea. I had some stability issues when touching this capacitor when following the we bench design values : 47pF for Cff, 1M and 249k for RFBT and RFBB. The output would start to strongly fluctuate (from 0 to 8V ) and which would cause overheating to the chip it was supposed to feed, destroying it in the meanwhile. I suspected the human's finger capacity was to close to the 47pF and that would cause a strong interference so I replaced Cff with a higher value. It didn't change much so i reduced the feedback resistor value which got me rid of this issue. (The instability is present on the EVM)

Still I completely removed C30 and I see no change at the output, nor at the RT pin.

3. The BOOTvoltage seems fine here are some screenshots :

4. Here is the waveform right at the SW pin :

Here is the Vin pins signal as seen today :

I don't recall it being as noisy before but I would usually probe at C29; 5cm from the pin, so there is a lot of room for interferences to have an impact.

Here is Vin at shutdown (nothing strange but the board still worked after reboot) :

Here is Vout signal:

Do you have any other suggestion ?

So far, adding a capacitor to the soft start pin seems to provide a sturdiness boost to the design as it's been 1 day with more than 150 reboots (one reboot per 10 minutes).

I'm waiting for your feedback.

Best regards,

Mickael

HI Mickael:

I think may something related to Soft Start, but when you have below issue waveform.

how the Vo raise up to 1.7V when there is no any switching?  cause the output only can get the voltage when high side mosfet turn on , so can you close look at SW VS Vo with very short time zone see whether it has switching when Vo raise up to 1.7V? 

Thanks

Hi Daniel,

When Vo rises to 1.7V there is no switching at all. The voltage on SW is perfectly constant and is equal to 1.7V too.

It kind of look like the combination of some diodes threshold voltage inside the converter. I expect this could be a reason as for why there is still a voltage level to be seen at the output even without switching.

When this happen, the LM46002 only seems to be in a latched state, because replacing the RT resistor then rebooting the board seems to unlock the converter. Is there a known condition that would prevent the DC/DC regulator from enabling the PWM during start up ? Like overvoltage/overcurrent, insufficient load, etc.?

Or is there any internal part that could force the output to 1.7V ?

Best regards,

Mickael

Hi Mickael:

Whether your 5V and 3.3V provide to same ASIC? any other rails is also 1.7V or 1.7V+0.7V(diode votlage drop) at this moment?  

Thanks

Hi Daniel,

I managed to make the board fail after a reboot again, I had a LDO at the output of  the 5V regulator and It was the one responsible for the 1.7V level I was observing. I removed that LDO and now the 5V buck output is 0V instead of 1.7V.

It looks like we're almost there..

Thanks.

Hi,

Quick update now that the chip is cut off from the rest of the circuitry :

Vin : 12V

Vout : 0V

Voltage on SW pin : 0V

Voltage on CBOOT pin :  0.8V

Voltage on VCC pin : 1.2V

Voltage on BIAS pin : 0V

Voltage on RT pin : 0.45V

Voltage on FB pin : 0V

HI Mickael:

Thank you for your feedback, will it be latched after you moved LDO?

Daniel Li

Hi Daniel,

I'm not sure if i understood what you meant. The buck is locked, as it doesn't provide the desired 5V output voltage but It only outputs 0V.

My previous tip about removing RT resistor then booting doesn't work anymore.

Best regards

Mickael

HI 

VCC voltage is not right, is the chip broken or all the chip works same?

Thanks

Hi,

The chip is currently broken.

When it is working, Vcc voltage is >3.2V

Regards,

HI

if replace new chip, is it still latched?

Thanks

Hi,

No, it works perfectly fine, but the issue is not fixed, it latches again after a while.

Regards

Hi

the chip initial statues after power down should be same with 3V3 and 5V output, so if 3V3 is OK, 5V should not be latched, unless there is some unexpected noise interfere the chip.

what's your Vin on/off period and duty?

Hi,

There is no period regarding Vin. Sometimes we have to turn it off and the chip fails. The only relevant information is "the chip must have been working for some time before failing after reboot". Maybe because of heating+noise accumulation ?

Regards

Hi

when you mean "chip fails", it means chip is OK but no output or it is broken? 

if it is only be latched, I think it is noise. 

if it is broken, may improper voltage or signal added on the chip.

Thanks

Hi Daniel,

Thank you for the feedback. When I say "chip fails" It means that there is no output. I don't know if the chip is OK or broken, because tests and trials have shown both happening.

If it is latched because of noise, could you tell me which pin would be the main culprit please ?

What do you mean by "improper votlage or signal added on the chip" ?

Thanks

HI 

I suspect that SS is be interfered.

and for "improper votlage or signal added on the chip", I mean any over voltage at Vin or other external voltage add at other pin after device shutdown. 

you can disconnect the second stage as well as 3V3 , only power on/off 5V , whether output latch will happen again.

Thanks

Hi Daniel,

Thank you for your support. We decided to use a new chip reference for our design. The issue could not be fixed and time was running low.

Thanks for providing us with some of your time and efforts.

Best regards,

Mickael