TPS7A4701

Azad Chacko

Hi,

I recently got a few samples of the new TPS7A4701 adjustable LDOs and hooked up a circuit following the TI EVM module:

I wanted to get a nominal 4V output and there chose R1=18.2K and R2=10K (0.1% resistors). From the datasheet this will give Vout = (1 + R1/R2) * Vref. Vref is stated as 1.4V so results in Vout = 3.948V

But upon powering up the circuit with 6V, the output (no load) appeared 'saturated' at close to 6V. This will usually happen if the FB loop is broken but there was none in my setup. However, when I started to increase the input voltage the output eventually stabilised at 8.5V and continued to hold steady right up to 30V input. This was rather surprising!

I was suspecting that Vref is not really 1.4V. To check this case I removed R2 so that Vout=Vref and when powered Vout registered 3.0V. So with this new Vref of 3V will give Vout= (1 + 18.2K/10k) * 3 = 8.46V which is close to the actual result above

Hope I have not missed out anything with this LDO. Please check and advise.

over 11 years ago

0 Ryan Eslinger over 11 years ago

TI__Guru 55940 points

Hi Azad,

This certainly is not expected behavior. Please verify that there is no short to ground on any of the ANYOUT pins as this would bias the output. Could you also confirm any changes and jumper settings made to the EVM?

Very Respectfully,

Ryan

0 Azad Chacko over 11 years ago in reply to Ryan Eslinger

Prodigy 70 points

Hi Ryan,

Thank you very much for the prompt response.

Actually, just to clarify I did not use (or have) the EVM board. I have gone straight to roll out a 3-term regulator based on this chip and referenced the EVM schematic as shown:

Of course the board layout is different but I have checked and verified the trackwork and all the pin connections. Also, the "Anyout" pins are all unconnected and there are no shorts to ground or anywhere else. I built up another board and this is also behaving the same as the earlier. There are no faults or other issues and quiescent current is <1mA. Output is stable but not as expected.

Happy to email the actual board schematic and pcb layout for you to check further.

0 Ryan Eslinger over 11 years ago in reply to Azad Chacko

TI__Guru 55940 points

Hi Azad,

Thank you for the clarification. It is interesting that a second board is exhibiting the same behavior. Please e-mail the schematic and layout to support@ti.com and reference my name.

Very Respectfully,

Ryan

0 Azad Chacko over 11 years ago in reply to Ryan Eslinger

Prodigy 70 points

Thank you, Ryan

I have emailed accordingly.

Kind regards,

Azad Chacko

0 Azad Chacko over 11 years ago in reply to Azad Chacko

Prodigy 70 points

Sorry, forgot to update but the problems were actually due to bridging of pins and once cleared all worked well

Thanks for all the support

0 Naomi McFarland over 10 years ago

Prodigy 10 points

Hi Azad it's Naomi. Would you kindly email me your telephone number naomi.mcfarland70@gmail.com or text it to me on 0428699415.

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TPS7A4701