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LM2623: LM2623ALD - About behavior when VDD is less than 0.8V(VIN_OP)

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Replies: 9

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Part Number: LM2623

Hi Team,

 

Our customer would like to know the behavior of LM2623ALD when VDD is less than 0.8V(VIN_OP).

Could you please let us know if you have any information?

 

[Q1]

If VDD is less than 0.8V(VIN_OP), Does this device stop switching?

 

[Q2]

After Q1 case, If VDD rise to 0.8V, does this device restart switching as the current limit is working?

 

Regards,

Kanemaru

  • I am checking with the original designer about the VDD UV threshold. According to the datasheet, VDD is required to be 1.1V to guarantee the startup. It is not clear in the datasheet how low the VDD can go to maintain operation. However, if follow the datasheet and tie VDD to the BOOT pin, the operation can continue even when the VIN drops to 0.8V after startup. Your customer should follow the datasheet recommendations and such questions become irrelevant.
  • In reply to Youhao Xi:

    Hi Youhao-san,

     

    Thank you for the information.

    According to our customer, a big load transient occurs, and the input voltage may be reduced(around 0.8V) in their application.

    Therefore, they would like to know about Q1 and Q2.

     

    I understand that this device does restart the switching as the current limit is working.

    Is my understanding correct?


     


     

    ---------

    [Their concern point]

    For example, if the input voltage reaches around 0.8V by a big load transient.

    Then, LM2623A stops by UVLO and can’t restart the switching from the input voltage is around 0.8V.

    Because, LM2623A need the input voltage 1.1V to start-up.

    ---------

     

    I’d greatly appreciate your verification.

     

    Regards,

    Kanemaru

  • In reply to hideto kanemaru:

    Hi Kanemaru-san,

    Like I said in the other e2e post of yours, I am still waiting for the original designer's reply. Depending on your input battery condition as well as the load transient condition, the input voltage can be pulled very low, and the device may stop switching (needing designer to confirm this). However, once the load transient terminates, the battery voltage should restore and the device can restart. If the large load transient heavily discharged the input battery and the battery voltage cannot restore back to >1.1V, the restart is not guaranteed. Note that 1.1V is the higher limits on the required min VIN level.
  • In reply to Youhao Xi:

    Hi Youhao-san,

     

    Thank you for your response.

    Please contact me when you receive the information from designer’s.

    And, with my understanding, I guess that it will be as written below.

      

     

     

    I’d greatly appreciate your verification.

     

    Regards,

    Kanemaru

  • In reply to hideto kanemaru:

    Hi Youhao-san,

     

    Would you tell me about the progress on this matter?

    I would appreciate your prompt reply.

     

    I’m sorry to bother you despite your business, but I would appreciate your consideration.

     

    Regards,

    Kanemaru

  • In reply to hideto kanemaru:

    Hi Kanemaru-san,

    First, here is some background info:
    (1) VIN must be 1.1V plus the diode forward drop to guarantee a start-up. It is about 1.6V if a Schottky diode is used.
    (2) The VDD voltage must be between 3V and 5V for normal operation. This is why its recommended to be tied to Vout if Vout is in the range of 3 to 5V. Yes VDD startup is 1.1V but it needs to be raised up . Refer to the Datasheet Section 8.2.2.4.
    (3) When VDD is reduced, the converter may stop switching.

    Now answer to your question:
    (1) When Vin is reduce to 0.8V, the converter may stop switching regardless the VDD and Vout conditions. I did a test on one EVM and it stop switching at Vin=1.0V under load. It can operate down to 0.6Vin under no load, being consistent with the Datasheet EC-table.
    (2) VDD supplies the driver and a very low VDD may not able to successfully turn on the FET, therefore the switching is not guaranteed. However, there is not VDD UV threshold implemented inside the IC. It will continue to switch until it's below the internal FET Vth.
    (3) During a large load transient, unless the pk current limit is tripped, nor Vin and VDD pulled too low, the converter will continue to operate.
    (4) Once it stop switching, it requires Vin to be greater than (1.1V+VF) to re-start.


    Hope this clarifies the issues.

    Best Regards,
    Youhao
  • In reply to Youhao Xi:

    Hi Youhao-san,

     

    Thank you for the detail information.

    And, I have some questions.

     

    ----------

    [Q3]

    Could you please let us know the measurement condition of LM2623A EVM?

     

    <Conditions>

    -VOUT=??V

    -Under load = ??A

    ----------

     

    [Q4]

    We would like to know the current limit when the low voltage startup.

    I understand that if VDD is more than 1.1V, On start-up, the control circuitry switches the N-channel MOSFET continuously until the output reaches VDD is 3V.

    Then, can the cycle-by-cycle peak current limit function?

    ----------

     

    [Q5]

    If we can use this device at the following condition.

    Then, we don't have to care “VDD_ST=1.1V”.

    So, LM2623 operates(switching) unless VIN reaches around 0.8V.

    Is my understanding correct?

     

    ----------

     

    Regards,

    Kanemaru

  • In reply to hideto kanemaru:

    Hi Kanemaru,

    [A3] I just borrowed an EVM different from the datasheet schematic, and modified it to test in a particular condition that you are interested (Vout=5V). When Vin>2V, the load can go up 400mA.

    [A4] Our designer has tested one device and confirmed that the peak current limit is greater than >2.5A over temperature. It verifies the 2.2A min of the peak current limit published in the datasheet. And, this peak limit is independent of the input voltage.

    [A5] Yes you can start with VIN=0.8V with the external bias at VDD of 3.0V. However, you cannot have much load. The board cannot support >50mA when starting with 0.8V. Please refer to the datasheet Figure 7 for the loading capability vs Vin.

    Thanks,
    Youhao
  • In reply to Youhao Xi:

    Hi Youhao-san,

     

    Thank you always for your kind support and response.

    I will report this information to our customer.

     

    Regards,

    Kanemaru

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