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LMR51420: The reason LMR51420YF starts up in DisContinuous Mode

Shinichi Inoue
Guru 21566 points
Part Number: LMR51420

Tool/software:

Dear Specialists,

My customer is evaluating LMR51420EVM and has questions.

I would be grateful if you could advise.

---

When the LMR51420YFDDCR is turned on with a light load (about 100mA), the inductor current goes into discontinuous mode.

The LMR51420YFDDCR is  FPWM version that maintains a constant frequency even at light loads.

I believe it will start up in PWM mode even at start up.

So why does it go into discontinuous mode after power is turned on, and then switch to PWM mode about 40ms later?

Vin: 24V, Vout:5V, Iout:0.1A

Schematic 

0385.Schematic.pdf

Start up DCM then CCM

03_DCM to CCM.pdf

Power ON ~ 1ms

01_1ms.pdf

Power On ~5ms

02_5ms_1.pdf

Voltage and Current at DCM

The output voltage is 5.057V

Voltage and Current at DCM.pdf

Voltage and Current at CCM

The output voltage is 5.052V

Voltage and Current at CCM.pdf

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 23685 points

    Hi Shinichi,

    Something in the setup is triggering the over current blanking time.

    Is the customer testing the raw evm or did they modify to the schematic you sent?

    Thanks,

    Andrew

  • 0 in reply to Andrew Kutzler
    Guru 21566 points

    Hi Andrew,

    Thank you for your reply.

    I understand that the reason why the LMR51420YF goes into discontinuous mode immediately after startup is because the overcurrent protection blanking time, tOCP_BLK (33 ms typical) is activated.Is this correct?

    I’ll share this information with the customer.

    The customer modified the EVM to check its characteristics.
    The components used are as shown in the connection diagram.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 23685 points

    Hi Shinichi,

    Yeah charging the 100uF output capacitor at startup could be causing the hiccup to occur. Especially if they are using it in a system that can Input caps to power other devices downstream.

    Let me know if you have any other questions.

    Thanks,

    Andrew

  • 0 in reply to Andrew Kutzler
    Guru 21566 points

    Hi Andrew,

    Thank you for your reply.

    I understand.

    In this case, the customer uses 100uF output capacitor, this causes hiccup operation at start up.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi 

  • 0 in reply to Shinichi Inoue
    Guru 21566 points

    Hi Andrew,

    I just received an additional request from a customer.

    Could you please advise?

    ---

    I confirmed the datasheet.

    It says that LMR51420 controls the average inductor current at the overcurrent protection blanking time, but that's not enough to understand that it operates in discontinuous current mode at startup.

    Please tell me more details.

    How does the overcurrent blanking function actually work?

    Looking at the operating waveforms, it does not seem to simply disable overcurrent protection temporarily.

    Why does it operate in discontinuous current mode at startup when the output capacitor is 100 μF?

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 23685 points

    Hi Inoue-san

    I will check with design.

    Thanks,
    Andrew

  • 0 in reply to Andrew Kutzler
    Guru 21566 points

    Hi Andrew,

    Thank you for your reply.

    I'm looking forward to waiting your answer.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi 

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 23685 points

    Hi Inoue-san,

    Does the customer have a way to enable EN independently from VIN? I believe the answer may be that VIN ramp is strange causing OCP mode.

    If it still does it then likely the high COUT is causing the OCP to trigger. If that's the case, the only way to have the FPWM after softstart that would be to use a lower COUT value.

    Thanks,
    Andrew