Alternative of LDFM50DT-TR LDO

[me_manan001]

Hi, 

In my current design, I am using LDFM50DT-TR LDO for 5V output which acts as a source for RFID transceiver circuitry. LDFM50DT-TR is preceded by MEV3S0505SC (Digikey link) whose output goes to LDFM50DT-TR, please check the attached image for more of the circuit diagram.

I am planning to replace LDFM50DT-TR with another suitable LDO, please suggest a viable replacement for LDFM50DT-TR. 

Thank You,

Manan Mehta

[Peter BENSCH]

Welcome @me_manan001, to the community!

Well, an LDO is not necessarily a good choice for at least two reasons, firstly because the theoretical difference of 0V is a bit tight even for LDOs with an extremely small drop-out. SVR or PSRR of a normal LDO is rarely more than 30...40dB at the relatively high switching frequency of the preceding DCDC (here 60kHz), which does little to suppress interference. So if you want the LDO to filter the interference frequencies, you should choose an LDO with a corresponding PSSR at the switching frequency, or alternatively an LC-Pi filter.

Regards
/Peter

[me_manan001]

Hi Peter, 

I agree with what you are suggesting, but I have the following constraints:

1. the activity I am doing is of component obsolescence, hence the alternative components package in my scenario has to be TO-252-3, DPAK (2 Leads + Tab), SC-63 & from an online search I did, I did not find LDO with SVR or PSRR in 60kHz range.

2. Cannot add an LC-Pi filter, since I cannot modify the schematic & PCB Layout. 

Can you please suggest an LDO in the current STM portfolio that will suit my needs? 

Do you think, LF50CDT-TR can work as a drop-in alternative for LDFM50DT-TR, I can carry out some measurement tests in the lab if you want, to give you more data.

Thank you,

Manan Mehta

 

[Peter BENSCH]

Incidentally, I had previously overlooked the LC filter 1µF+22µH in your screenshot, whose cut-off frequency of 33.9kHz is actually still too high. If the layout is fixed, you will hardly be able to replace L251 with one with 220µH or more, because the thicker wire results in a larger package of the inductor. But perhaps you can at least increase C1073 from 1µF to 10µF or more, as this offers better damping of the switching frequency of 60kHz (assuming a good layout).

When it comes to the LDFM50DT-TR, you do not need to replace it with an alternative as it has not been discontinued. On the contrary, all LDFM controllers are guaranteed product longevity until June 2033.
Although the LF50CDT-TR is pin-compatible, you should be aware of its limitations:

• it's not listed on the Product Longevity, so no guarantee of availability can be given
• it's based on a pnp bipolar transistor as control element, while the LDFM is based on a DMOS transistor, which results in a quiescent current up to 15 times higher for the LF50

Hope that helps?

Regards
/Peter

[me_manan001]

Hi Peter, 

Thank you for your valuable suggestion, of replacing the 1uF capacitor in the LC filter post-DC-DC converter, I will follow it through, and replace the 1uF with a 10uF capacitor which will shift the cut-off frequency to 10.73kHz which will be in the range of the PSSR bandwidth of all the TO-252-3 package LDOs, and update you with the measurement test results. 

Also on the subject of LDFM50DT-TR not being discontinued and having guaranteed product longevity until June 2033, on distributor websites like Digikey, Mouser, Element 14 & Arrow LDFM50DT-TR is being shown as obsolete or discontinued, please refer to the attached images of these distributors websites for LDFM50DT-TR.

Mouser:

https://www.mouser.sg/ProductDetail/STMicroelectronics/LDFM50DT-TR?qs=3q62CFVeiIqFagnq1CM2Mg%3D%3D

Element 14:

https://sg.element14.com/c/semiconductors-ics/power-management-ics-pmic/voltage-regulators/ldo-voltage-regulators?st=ldfm50dt-tr&showResults=true

Arrow:

https://www.arrow.com/en/products/ldfm50dt-tr/stmicroelectronics?q=LDFM50DT-TR

Digikey:

https://www.digikey.pl/en/products/detail/stmicroelectronics/LDFM50DT-TR/3678673?s=N4IgTCBcDaIAQBkAiAxAsgVgAxICoFpcAlEAXQF8g

 

Can you please let me know why are all of these distributors showing LDFM50DT-TR discontinued or obsolete?

And meanwhile, if we want to procure LDFM50DT-TR how do we go ahead with it?

Thank You,

Manan Mehta

 

[Peter BENSCH]

I am sorry, I had previously given incomplete information and did not check it sufficiently: the Product Longevity only applies to the ADJ and 3.3V versions, the 5V versions are indeed actually discontinued. It is with deepest regret that I apologise for any confusion.

Regards
/Peter

[Peter BENSCH]

I can suggest the L4931CDT50-TR as an alternative in your application, but I can't say anything about the other manufacturers.

Good luck!
/Peter