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Inquiries regarding LM2596-3.3

Atsushi Okui
Genius 4015 points

Hello, all

Now we have some inquiries regarding LM2596-3.3 from our customer.

Please refer to the items below, and feedback us with your comment.

When referring the E2E thread below, we could understand that the minimum input voltage for this device is approximately 0.9V when Iout=1A and the junction temperature is 25 degree C. Plus need to have consideration for additional dropout depending on DCR of inductor.

e2e.ti.com/.../310613

However, we could not understand the description of "VOUT = VREG - 50mV" on Figure 8. 

Please let us clarify about the detail of VREG on this formula. Also, please let us clarify about the tolerance of this characteristic (without considering DCR of inductor).

We thank you in advance for your information.

Best regards, 

  • 0
    TI__Genius 16025 points

    Hi Atsushi

    We've received your query and will get back to you shortly.

    Thanks,
    Anston

  • 0 in reply to Anston Lobo
    TI__Genius 16025 points

    Hi Atsushi,

    The 50mV is 1% of 5V output voltage. This means that the part is considered to be in dropout when the output voltage drops by 1% from the nominal regulating voltage. If a 1% tolerance is too high, this can be adjusted in calculation by reducing the tolerance to 0.5%.

    This will ensure drop-out operation using the indicated graph values.

    Let me know if this answers your question or if you have any more questions.

    Thanks,
    Anston

    EDIT: This post was simplified to explain the tolerance with respect to drop-out and the previous explanation was modified to make this point clear.

  • 0 in reply to Anston Lobo
    Genius 4015 points

    Anston-san,

    Thank you for your prompt reply on the inquiries regarding LM2596-3.3 from our customer.

    We have some additional inquiries regarding this device from them.

    Please refer to the items below, and feedback us with your comment.

    When referring "LM2596-3.3 Electrical Characteristics" on datasheet page 3, the output voltage range of this device is described as "4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A". However, this description makes us confused because this device has fixed 3.3V output.

    Please let us clarify how we could understand this description.

    Also, we could not find out any description regarding minimum input voltage level. Only we could find out was Figure 11. however we still could not find out our case.

    Please let us clarify how much would be this level. For your reference, our typical input level is 5V and typical output current level is 1A.

    We thank you in advance for your information. 

    Best regards,

  • 0 in reply to Atsushi Okui
    TI__Genius 16025 points

    Atsushi-san,

    Please note I've revised my earlier answer to offer a more simpler and clear explanation of your earlier question.

    It is not the output voltage range, instead it is the input voltage range for correct 3.3V operation. Put another way, The Output voltage of 3.3V is ensured only when Vin for the fixed voltage part is between 4.75V and 40V with a load range of 0.2A to 3A.

    For 3.3Vout, the minimum ensured input operating voltage is 4.75V as per the datasheet. The example used in Figure 11 is for a Vout of 1.23V.
    It is possible that under certain elevated temperature conditions, the part will still regulate at 3.3V even if Vin <4.75V, however this is not guaranteed by the datasheet.

    Thanks,
    Anston

  • 0 in reply to Anston Lobo
    Genius 4015 points

    Anston-san,

    Thank you for your continuous support towards the inquiries from our customer.

    We have one additional inquiry regarding Delayed Startup on this device.

    Please refer to the item below, and feedback us with your comment.

    We have made our design for 3.3V and 5V outputs with this device, on which 3.3V output is modified as Delayed Startup based on Figure 33 of the datasheet page 27.

    However, we found the overshoot while starting up this device on 3.3V line.

    Please find the scope below for the detail.


    CH1: Vin (12V)

    CH2: Vout1 (5V)

    CH3: Vout2 (3.3V)

    Please let us clarify whether we could find any solution to resolve this issue.

    We thank you once again for your information.

    Best regards, 

  • 0 in reply to Atsushi Okui
    TI__Genius 16025 points
    Atsushi-san,

    Only way to limit this overshoot over and above softstart is to increase minimum loading. This can be accomplished via one of three ways:
    a) Decrease feedback resistor value to increase current draw on the output through the feedback network.
    b) Use a dummy resistor on the output start with 200Ohms and decrease resistance until overshoot is mitigated.
    c) Increase output capacitor capacitance. This is less likely to work compared to other two options above.

    Thanks,
    Anston