LM3671: LM3671

Hi Andre,

I cannot open the images attached. Could you please upload again or use another format.

Regards,

Febin

I'm sorry . Here follows a file with schematic and PCB design. It is a 4 layer PCBDesign circuit.docx7608.PCB.pdf

Adding more information I measured the voltage output goes to 3.3 v but almost instantly temperature increases to high value in inductor and IC as well. I suppose that the current inductor is saturating . Less than a second, voltage drops to 0 V and seems that it was caused by the high temperature and IC seems to shutdown because of it. 

I don't know if some component is already damaged or the PCB has some influence in this behaviour.

Regards 

Andre 

Another important thing is that in the documentation < Common Mistakes in DC/DC Converters and How to Fix Them> , there is a equation  4 bellow 

L = (VIN(MAX) −VOUT/ K *IOUT)* (Vout /(Vin_max * fsw))

Assuming that Vin_Max is 4.1 V ( a Io lit battery), Vout  is 3.3V , K=0.3 , Iout = 130mA,  fsw = 2MHz , we have an inductor of 8.2 uH instead of 2.2uH indicated in the IC documentation. 

Which is the adequate value ??

Hi Andre,

Could you please turn on the component naming in the layout and resend the file?

In general, inductor's heating is caused due to saturation or poor layout.

Can you please share the inductor current and all voltage waveforms?

Did you also try to swap the IC? 

Do you have the possibility to use an EVM instead?

Could you please share the App note you are referring to?

Regards,

Febin

Dear Febin. Thanks for your quick answer. I appreciate it.

Please follows my answers followed by '>>>'

Could you please turn on the component naming in the layout and resend the file?

Please, see in the attached file , the component naming and see if it is ok for you.

5556.Design circuit.docx

In general, inductor's heating is caused due to saturation or poor layout.

>>> That's what I suspect. As the PCB was manufactured in another country, it seems to be working as soon as arrived but we didn't pay attention if the components were cooking or not. We measured the voltage over the capacitor C14 and was correctly measured. Than , I passed to test the battery charge which worked as well when I connected it to an external 5V DC power supply. It took sometime to charge and it worked. The problem appeared exactly after doing this. I suspect that the reverse current protection used with the trasistor AO3407 may have affected something with the downstep converter. As you can see, there is a switch that turns on the battery directly to the transistor and at the same time a +5V applied to the gate. But I think this could not have affected the IC.

Can you please share the inductor current and all voltage waveforms?

>>> I wish I could but I have to open the circuit and it is very tiny and I don't have a way to probe it.

Did you also try to swap the IC? 

>>> this is the next step. I'll have to purchase a new one

Do you have the possibility to use an EVM instead?

>>>  DO you mean evaluation module ? What model do you recommend ?

Could you please share the App note you are referring to?

>>> Sure.  I found it in https://www.ti.com/seclit/ml/slup384/slup384.pdf    

Common Mistakes in DC/DC Converters
and How to Fix Them

There you can see on page 6 the formula that I cited before. So if you use the values before, the calculation is different from the value recommend in the LM3671 documentation. Is it correct ?

Also, what is the normal operation temperature of this circuit ? Is it ambient temperature or higher ?

Thanks for your help!

Best regards

Andre

Hi Andre,

Thank you for all the details.

--My guess here is that the IC is already damaged. I would suggest to replace with a new one as a next step.

--I would keep the recommended inductor as mentioned in the datasheet.

--I meant EVM for LM3671. So, to remove LM3671 IC and use the EVM externally, this will help to understand if it's due to any layout issues as well.

--Can you ensure the voltage and current ratings are not exceeding recommendations?

-- As long as the temp is within the recommendations, it should not be a problem.

Regards,

Febin

Dear Febin. Thanks for your quick answer.

I'll replace the LM3671 chip.

I'll check if the current and voltages are ok. I'll check.

But I still have few questions.

In general description (https://www.ti.com/lit/ug/snva122f/snva122f.pdf?ts=1598987671802&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM3671)

the paragraph states that:

The LM3671 converts high input voltages to lower output voltages with high efficiency through an inductorbased switching topology. Automatic intelligent switching between PWM low-noise and PFM low-current mode offers improved system control. LM3671 is available in both fixed output voltage options and adjustable voltage options range from 1.1 V to 3.3 V in SOT23-5, DSBGA-5 package or 6-pin USON packages. The fixed output voltage options available in a SOT 23-5 package are 1.2 V, 1.25 V, 1.375 V, 1.5 V, 1.6 V, 1.8 V, 1.875 V, 2.5 V, 2.8 V, and 3.3 V. The fixed output voltage options available in the DSBGA-5 package are 1.2 V, 1.5 V, 1.8 V, 1.875 V, 2.5 V, 2.8 V, and 3.3 V. The fixed output voltage options available in the 6-pin USON package are 1.2 V, 1.3 V, 1.6 V and 1.8 V.

My question is: Just for my understanding, is there a way to have a fixed output voltage using the fixed output diagram if Vin is a Lit battery which voltage can vary from 2.7 till 4.2 ? What are the right settings for a 3.3 v output voltage ?

 The other question is: what is the operating temperature of the circuit in the documentation ? What does it depend on ?

Warmest regards.

Hi Andre,

For a step-down converter the input voltage should be higher than the output voltage at all times.

For a fixed output version, Vout value is set internally and you need to simply choose the right part number in SOT 23-5 or DSBGA-5 package (for 3.3V Vout).

For a ADJ version, you can set the feedback divider values such that Vout is 3.3V, then Vin must be higher than 3.3V always.

As seen from the User's guide : 

Ambient temperature (TA) range: -30C to +85C

Junction temperature (TJ ) range: -30C to +125C

Regards,

Febin