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LM2621: Converting 2V ... 3,3V to 5V with LM2621

jens kapmeyer
Prodigy 30 points
Part Number: LM2621

Tool/software:

Hi all,

I have built a PBC where the LM2621 is used as step up converter according to Chapter 8.2.1 / Fig 13 of the data sheet.

The circuit works fine down to an input voltage of 2.2 V, then when it comes lower,  the output voltage drops from 5V to around 3V, and the "SW" output shows some continuous pulse pattern instead of the regulating "bursts".

The input current then rises from 17mA to around 40mA.

My load is a small micro controller and 1 or 2 LEDs, each drawing 5mA or so.

Is there a possible explanation for this behaviour?

I had no problem if the LM2621 just would go into shutdown, and stopped drawing current.

Also, can the C1/C2 tantals be replace by ceramic capacitors ? (I tried both types) or is this a no go ?

best regards,

Jens Kapmeyer

  • 0
    TI__Genius 17450 points

    Hi Jens,

    Could you upload your schematic for me to review? Thanks.

  • 0 in reply to Shipeng Cheng
    Prodigy 30 points

    Hi Shipeng, thanks for asking !

    Unfortunately, I am able only to insert a screenshot via copy+paste, which is basically not readable, when I tried to upload the pdf directly, I got a message ("upload of this file not allowed"). But I found my problem I guess: My LM2621 circuit is supplied via a P-Channel MOSFET, and this MOSFET has an inpedance of up to 20 Ohms, depending on U(GS) and temperature. When I bypass the MOSFET by a mechanical switch, the desdcribed effect disappears.

    Anyway, I thank you for your response, and mark the issue as solved !

    Best regards, Jens