LMZ12001: FB pin sitting at 0.65 V and not at 0.8 V

Hi Martin,

1. What is your FB resistor? Per datasheet section 8.2.2.3, the FB resistors should be 1k to 10k and make sure they are in the recommended range. 

2. Make sure the converter has 10-20mA mini load for properly operation.

3. Please deep clean the FB pin/FB resistors and all converter area to rule out flux contamination.

Hope this helps,

Thanks,

Nancy  

Hi Martin,

The feedback resistors look good. Could you try item#2 and #3? Could you please remove C3 (22nF) feedforward cap to see if any improvement?

1. Did all the boards have this issue or only one board?

2. What is input voltage and output current?

3. Increase output cap C4/C5 from 10uF to 47uF each.10uF is pretty small after 3.3V derating.

Thanks,

Nancy

We are receiving more boards in the next few days. I will confirm if it is just this board or not.

The input voltage is 12 V and the expected output current changes between 350 mA and 800 mA.

We added a 30 mA mini-load on the output and increased the output capacitance to above 100 uF. No change in the output voltage or the feedback pin. We will remove the feed-forward capacitor now.

We removed the feed-forward capacitor and the output voltage dropped to 1.956 V. Should we try a larger feedforward capacitor?

Hi Martin!

Nancy will reply you tomorrow!

Shuai

Hi Martin,

I run the simulation for your schematic and it is working properly.

1. Double check R20/R9/R15 to make sure they are not  wrong parts.

2. Make sure 12Vin is correct without any dip.

3. Double check 12Vin current limit to make sure it can supply enough current

4. Double check if you pcb layout is same as Figur41 in the datasheet

5. Removing feed-forward capacitor should not affect FB voltage and output voltage. Cff only affects the load transient performance. I think it is something wrong with the device or soldering if above 4 items are good. Is it possible re-flow LMZ12001?  De-solder the part and check if there is any solder or flux residue underneath the part. Deep clean with alcohol and solder the part back. Then check FB voltage and it should be 0.8V.

Thanks,

Nancy 

Hi Nancy,

We have done all the above and can confirm that all is correctly implemented as stated in the datasheet. I have just received a second assembled PCB. Upon powering up we noticed that the enable pin stayed at 0.6 V and the output stayed off. Removing the EN resistors resulted in no change. We applied external 3V3 to the EN pin and the regulator starts up and works. 

Can I send you the batch number of the LMZ12001 that we purchased from Digi-Key and could I please ask you to verify the age of the components?

Hi Martin!

Nancy will reply you next Monday!

Shuai

Thank you Shuai, can I ask you if the LMZ12001 is washable?

Hi Martin,

Yes, LMZ12001 is washable. You can clean it with water. Do you mean the converter could not start up with resistor divider? If you apply 12Vin, the EN pin voltage should be 3.2V. Why is it 0.6V? Are R10/R28 correct value?  Please send me batch number.

Thanks,

Nancy

Hi Martin,

Please make sure R10/R28 are correct. use water or alcohol to brush those pin area (FB pin/EN pin), Then blow to dry the board. It should be flux contamination to make EN pin or feedback pin resistor lower value.

Thanks,

Nancy

Batch number/ Lot trace code is: 18AXT8G3

Hi Martin,

Will check with the team and may get you back next week.

Thanks,

Nancy

Hi Martin,

From the report, TI already improved that process and it should be no issue now. I will send it to QC team for checking and get you back next week.

Meanwhile, could you please check if there is any solder extrusion on your part after de-soldering? Also check if the manufacturer follows TI power module SMT guideline including solder paste, reflow profile and moisture handling?

Thanks,

Nancy

We have emphasized the guidelines to our assemblers now. I will collect the de-soldered parts and see if we can open them.