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LM76002: Does any current flow from PVIN to internal LDO when 5V DC is supplied to BIAS terminal from external?

Kazuya Nakai59
Guru 20600 points
Part Number: LM76002

 Hello guys,

 One of my customers is evaluating LM76002 using the device EVM for their next products.

 Their evaluation conditions are follows.

 - EVM parts were not changed from default values.

 - VIN=48V.

 - VOUT=3.3V

 - IOUT=0A

 - in Forced PWM mode

 They have the following questions about LM76002. Could you please answer the questions?

 Q1. Does any current flow from PVIN to internal LDO when 5V DC is supplied to BIAS terminal from external?

 Q2. They found that VCC voltage changed from 3.1V to 3.27V when VOUT reached to the target voltage (3.3V) at starting up.

        They think LM76002 is in AUTO (PFM) mode during starting up. Is it correct?

        If the answer is yes, why does LM76002 start in AUTO mode in spite of FPWM mode setting?

 Q3 When the EVM started up with 0A load, about 50mV overshoot was observed at the end of start up.

       Is this a normal phenomenon? If yes, is there any counter-measure against the overshoot?

 Q4. Do you dhave any data of the device each terminal open/short test?

 Your reply would be much appreciated.

 Best regards,

 Kazuya.

  • 0
    TI__Expert 4720 points

    Hi,

    We will examine it and reply you tomorrow.

  • 0 in reply to Neal Zhang
    Guru 20600 points

     Hi Neal,

     Thank you very much for the prompt reply.

     I'm looking forward to receiving your reply.

     Thank you again and best regards,

     Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Expert 4555 points

    Hi  Kazuya,

    Answering your questions:

    Q1. Does any current flow from PVIN to internal LDO when 5V DC is supplied to BIAS terminal from external?

    Ans: No, there will be some leakage current going from PVIN to Internal. The higher the difference between the input and output voltages of the LDO, the more losses occur to supply the same LDO output current. The BIAS input is designed to reduce the difference of the input and output voltages of the LDO to improve efficiency, especially at light load. . When used, TI recommends a 1-µF to 10-µF high-quality ceramic capacitor be used to bypass the BIAS pin to ground. If there is high-frequency noise or voltage spikes present on VOUT (during transient events or fault conditions), TI recommends connecting a resistor (1 to 10 Ω) between VOUT and BIAS.

    Q2. They found that VCC voltage changed from 3.1V to 3.27V when VOUT reached to the target voltage (3.3V) at starting up.

    Ans: This happens because of the transistion between Internal LDO to External Bias Pin. It  is an expected behavior and should not cause any disturbance VOUT and at whole system.

    Q3. They think LM76002 is in AUTO (PFM) mode during starting up. Is it correct? If the answer is yes, why does LM76002 start in AUTO mode in spite of FPWM mode setting?

    Ans: Yes, this is correct. The device will operate in Auto mode with the frequency increasing during soft start.  This will limit the inrush current and should eliminate voltage overshoot on the output.

    Q4. When the EVM started up with 0A load, about 50mV overshoot was observed at the end of start up. Is this a normal phenomenon? If yes, is there any counter-measure against the overshoot?

    Ans: Try slowing down the softstart via using larger Cap value on SS Pin, and see if the overshoot on VOUT gets improve.

    Q5. Do you dhave any data of the device each terminal open/short test?

    Ans: Data which can be shared by TI with customers can be found on the Datasheet and User-Guide in the product folder. link 

    I hope my response answered all your questions. Please let me know if you have more questions.

    Thanks!!

    Best Regards,

    Ankit Gupta

    Application Engineer.

  • 0 in reply to Ankit Gupta95
    Guru 20600 points

     Hello Ankit,

     Thank you very much for your reply. Your answers are very helpful for the customer and me.

     Could I ask you the following questions?

    Q6. Regarding to Q4, the customer changed SS capacitor from 0.022uF to 0.1uF.

           But the overshoot (50mV with no load) could not be reduced.

           The customer thinks that the device internal reference voltage may be changed when the operation mode is changed from AUTO to FPWM.

            Is their thought correct?

            I think that the device internal reference voltage is same between AUTO mode and FPWM mode.

            I think the overshoot (output voltage gap between  AUTO mode and FPWM mode) is caused by feedback method. I think AUTO mode uses a comparator to sense the output voltage and the comparator senses the bottom of the output ripple voltage.

            (As a new information, the overshoot was disappeared when Io=0.2A)

      If you have any idea, could you pelase tell me it?

     Q7, When is LM76002 overshoot finished and the mode is changed from AUTO to FPWM?

     Q8. When EN terminal level is changed from H level to L level in LM76002 operation for constant voltage generating, then on time become longer like form 160ns to 200ns. Due to the on time changing, Inductor current is increase just after EN = H -> L.

     Do you know why the on duty become longer?

     Thank you very much again and best regards,

     Kazuya..      

  • 0 in reply to Kazuya Nakai59
    TI__Expert 4555 points

    Regarding Q6:

    Ans: Actually I misunderstood your question 4 on first place. I thought you are seeing 50mV overshoot during the start of softstart operation. But it seems you were concerned about 50mV of overshoot when softstart finishes. Which is an expexted behavior and happening due to Auto to FPWM transition.

    Q7When is LM76002 overshoot finished and the mode is changed from AUTO to FPWM? 

    Ans: Yes.

     Q8. When EN terminal level is changed from H level to L level in LM76002 operation for constant voltage generating, then on time become longer like form 160ns to 200ns. Due to the on time changing, Inductor current is increase just after EN = H -> L.

    Ans: I think I know what is happening. Can you please share some waveforms?

    Best Regards,

    Ankit Gupta

  • 0 in reply to Ankit Gupta95
    Guru 20600 points

     Hello Ankit,

     Thank you very much for your reply.

     Could I exchange e-mails with you off line because the customer doesn't want to disclose their waveforms on E2E?

     If it is O.K, could you please tell me your e-mail address?

     Thank you again and best regards,

     Kazuya Nakai.

  • 0 in reply to Kazuya Nakai59
    Guru 20600 points

     Hello Ankit,

     Could I have your e-mail address?

     Thank you and best regards,

     Kazuya.

     

  • 0 in reply to Kazuya Nakai59
    TI__Expert 4555 points

    Yes Sure Kazuya. We can discuss this offline over emails. My email Id: a-gupta1@ti.com.

    I am closing this thread here, so that TI won't monitor this discussion on E2E anymore.

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    Guru 20600 points

     Hello Ankit,

     Thank you for your kind reply.

     I've just sent the customer questions with waveform to your email.

     Could you give me your answers to the customer questions?

     Thank you again and best regards,

     Kazuya.