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LM5085: LM5085 Drop when I connect the load

user5163386
Prodigy 100 points
Part Number: LM5085


I did the following design :

when I connect the next solenoid:

Manufacturer Part Number   -----> ROB-11015

V= 5V          R dc= 4.5Ohm  so the current should be ---->I= 1.11Amp

But the PMIC drop to 0V.

  • 0
    TI__Mastermind 40160 points
    Hello,

    Please can you share your schematic along with your input voltage range?

    Thanks.

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    Hi David, this is the schematic and BOM:

    I perform tests with a swiching source of 42V@ 2Amps.

    BOM

    LM5085MY/NOPBCT-ND
    LM5085MY/NOPB
    IC REG CTRLR BUCK 8MSOP
    ZXMP10A18KTCCT-ND
    ZXMP10A18KTC
    MOSFET P-CH 100V 3.8A DPAK
    SRR1208-120MLCT-ND
    SRR1208-120ML
    FIXED IND 12UH 4.8A 25 MOHM SMD
    1655-1469-1-ND
    MBRD10200TR
    DIODE SCHOTTKY 200V DPAK
    399-15154-1-ND
    C1210C155K1RAC7800
    CAP CER 1.5UF 100V X7R 1210
    1276-2183-1-ND
    CL10C122JB8NFNC
    CAP CER 1200PF 50V C0G/NP0 0603
    399-15154-1-ND
    C1210C155K1RAC7800
    CAP CER 1.5UF 100V X7R 1210
    478-3085-1-ND
    TPSB476K010R0250
    CAP TANT 47UF 10% 10V SMD
    587-1282-1-ND
    EMK212B7474KD-T
    CAP CER 0.47UF 16V X7R 0805
    311-1.82KHRCT-ND
    RC0603FR-071K82L
    RES SMD 1.82K OHM 1% 1/10W 0603
    311-3.32KHRCT-ND
    RC0603FR-073K32L
    RES SMD 3.32K OHM 1% 1/10W 0603
    RR12P10.0KDCT-ND
    RR1220P-103-D
    RES SMD 10K OHM 0.5% 1/10W 0805
    CSR1206FK10L0CT-ND
    CSR1206FK10L0
    RES 0.01 OHM 1% 1/2W 1206
    311-68.1KHRCT-ND
    RC0603FR-0768K1L
    RES SMD 68.1K OHM 1% 1/10W 0603
    490-11430-1-ND
    GRM1885C1E621JA01D
    CAP CER 620PF 25V C0G/NP0 0603

    If you need I could upload the pcb.Thanks

  • 0 in reply to user5163386
    Prodigy 100 points
    Some advances,
    when I use Vin=25 V the PMIC respond correctly.
  • 0 in reply to user5163386
    TI__Mastermind 40160 points
    Hello,

    A couple of things I would change with your design to get this working.

    1. You will need to place a series resistor in series with C30. I would place at least 25mR to 50mR? This is a COT architecture and it needs a ripple voltage presented at the feedback node, that is in phase with the inductor current. Using a ceramic, with an ESR<5mR is not enough for stable operation. For more information please review ripple injection section in the datasheet

    2. looks like you are switching at a high frequency. I would reduce the frequency to ~450kHz, modify R41 from 68k to 415k.

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    Good morning David,

    I implemented both suggestions and I obtein those results:

    1) I place a series resistor with C30 and the value that I can use 42V was 1 Ohm. I could move the solenoid and its consume 10W....but the ripple is too big.

    With 500mOhm doesn't work.

    2) Doesn't work. The R41 is funtion that the times of PFET and obtein Noise in Vout.

    Any other advice for improve Vout . thanks David

  • 0 in reply to user5163386
    TI__Mastermind 40160 points

    Hello,

    1/0.5 ohms is too large.  Can you try 50mR please?

    If it still does not work can you show me somce scope captures of Vout and Switch?

    Thanks.

    David.

  • 0 in reply to David Baba
    Prodigy 100 points
    I don't have this value of resistor. Ican try with O ohm %20 error or have 10mOhm that I use for R sense. This is cause that I live in Argentina and I don't have way to obtain some components.
  • 0 in reply to user5163386
    TI__Mastermind 40160 points
    I would place 3x10mR in series, just to try. If it works, then I would obtain a Cout with adequate ESR< or find the right resistor value?

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    David,

    I place and take scope's screen that you suggest.

    Scope_outputs_30mR.pdf

    and

    Scope_outputs_1_Ohm.pdf

    While with 1 Omh the ripple at the output is almost 1V the signal SW is stable, while that does not happen with 30mR.
    I hope it is useful, thanks David

  • 0 in reply to user5163386
    TI__Mastermind 40160 points
    Hello,

    looks like you have ~500mV of feedback ripple with 1ohm. you only need ~50mV. can you place a 100mR in series with the output.

    Please understand this is a hysterestic controller that needs feedback ripple voltage to be in phase with the inductor current, the recommended amount of ripple in the datasheet is 25 to 50mV and its detailed in the datasheet.. Too much Cout with low ESR shifts the ripple towards 90 degrees and will cause instability.

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    Hi,

    I had to travel and leave the project for a few weeks.
    I tell you yesterday and today resume the tests. Because I do not have a wide availability of components, I told you what I was doing.

    The output capacitor (Cout) is not low ESR, it has 250m.

    TPSB476K010R0250
    CAP TANT 47UF 10% 10V SMD

    It is the one that recommends webench.

    1) At the output I put in series 30m resistor. But when I placed the 5V load it fell to 3v.

    2) Change Cout for 100uF of Aluminum. It generated noise.

    3) Change the topology to get less ripple. I did not get good results.

    4) Place a trimmer in Rt and probe with 68.5k - 70k - 90.9k - 243k and did not get good results.

  • 0 in reply to user5163386
    TI__Mastermind 40160 points
    Suggest following WeBench design.

    Please make sure you have used the same components WeBench suggested.

    If this does not resolve your issue after implemented the suggestions above, please post your layout so I can review.



    Thanks.

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    I attach the webench report and the component list. The main components (L, D, Cout, Ic,Rsen) are the same.

    I will attach the layout. Thanks

    Component List.pdfwebench_esim_4814812_15_2_497811592.pdf

  • 0 in reply to user5163386
    TI__Mastermind 40160 points
    Thanks for posting.

    Can you confirm that you are using the exact components recommended?

    Look forward to seeing your layout.

    Thanks.

    David.
  • 0 in reply to David Baba
    Prodigy 100 points

    I resolved the problem.

    Parts:

    The inductive load mentionated in the first post was always the load.

    steps:

    I modified the Cfb for obtein a more representative Vfb. 620pF to 1200pf. This inside directly in the equation:

    Cff = (3 * ton) / (Rfb1 // Rfb2)

    Another change was the Radj:

    1.82k to 2.2k

    Icl = 40uA * (Radj / Rsen)

    Thanks for the assistance and I hope it will be useful.