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LMR33630: Overshoot that might happen on LMR33630

Ryotaro Fukui
Intellectual 2345 points
Part Number: LMR33630
Other Parts Discussed in Thread: LMQ62440-Q1, LM62440-Q1, LMR36506-Q1, LM61460-Q1, LMQ61460-Q1

Hello team,

My customer has a question about overshoot that might happen while using LMR33630.

■ Product name
・ LMR33630BQRNXRQ1
・ LMR33630CQRNXRQ1

■ Usage environment
・ Vin: 14 [V], Vout: 12 [V]
     * Vin varies from 4 [V] to 33 [V]. When the input is 12 [V] or less, "input voltage-(minus) voltage drop in high-side FET" will be the output.
・ The absolute maximum rating of the Vout supply destination is 16 [V]

■ Questions
They saw an article that Vout overshoots when Vin falls below the Vout set voltage at a level where UVLO (3.2 [V] on the data sheet) does not operate, and then Vin suddenly returns.
The same thing can happen with this usage assumption, so we are concerned about whether the absolute maximum rating (16 [V]) of the Vout supply destination can be met.

We would appreciate it if you could give us feedback on whether there is a problem and what to do if there is a problem.

Best Regards,

Ryotaro Fukui

  • +1
    TI__Mastermind 25590 points

    Ryotaro Fukui,

    The LMR33630 has soft start when waking up from a condition where VIN < UVLO (3.2V). This should prevent any overshoot on the output.

    If you'd like more detail, please link the article so I can provide more specific details on how the LMR33630 will not behave as described in the article.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Below is a link to an article about overshoots that customers are concerned about.
    techweb.rohm.com/.../8313

    This article explains that the following phenomena can occur:
    ① First, the device starts normally once and Vout stabilizes.
    (2) Next, Vin falls below the Vout set voltage at a level where UVLO does not operate (the device does not shut down).
    ③ After that, when Vin returns steeply, Vout overshoots.

    They were concerned that the above overshoot would make it impossible to meet the absolute maximum rating (16 [V]) of the Vout supplier.

    Best Regards,

    Ryotaro Fukui

  • +1 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Ryotaro Fukui,

    I see the concern. VIN falls, error amp rails to try to restore VOUT, VIN rises, and the error amplifier causes an overshoot due to the loop bandwidth delay. We do have some parts which address this specifically such as LM61460-Q1, LMQ61460-Q1, LM62440-Q1, LMQ62440-Q1, LMR36506-Q1, LMQ36503-Q1, and a few more with "Graceful Recovery". Other devices may have some overshoot depending on operating conditions.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Thank you for your reply.

    Apparently, Feature: Soft start corresponds to this feature. In addition to the devices introduced, it seems that there are also catalog devices, so we will also propose these as an alternative to our customers.

    Also, is it possible to deal with it with external parts?

    Best Regards,

    Ryotaro Fukui

  • +1 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Ryotaro Fukui,

    Yes it is possible to deal with this with external parts but it will likely be more expensive, more complicated, and take more space than choosing one of the devices with graceful recovery.

    For example you could implement a preregulator-type circuit like in SNVA900 but on the output.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Thank you for your reply about external parts.

    About the overshoot, they would like to know if there is a method to theoretically calculate the amount of overshoot.

    If you have some kind of a calculation formula, please let me know.

    Best Regards,

    Ryotaro Fukui

  • +1 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Ryotaro Fukui,

    A conservative worst case estimate would be something like dv = sqrt(L/C)*di where

    L = Power stage inductance

    C = Output capacitance

    di = IOUT_Current_limit_Avg - IOUT_Steady_State

    dv = overshoot.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Let me confirm some points about the previous answers.

    Question ①
    About the overshoot formula,
    "di = IOUT_Current_limit_Avg-IOUT_Steady_State"
    How can they get each value? Should they use ISC = 5.05 [A] (max) on the data sheet?

    Question ②
    If there are a characteristic table for Vin: 14 [V] and Vout: 12V, they would like to see it.
    (For example, the graph of "7.4.2 Dropout" in the data sheet)

    Best Regards,

    Ryotaro Fukui

  • +1 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Ryotaro Fukui,

    1. I was thinking of using the average current limit (ISC+ILIMIT)/2 but you're right. Worst case should use the max ISC = 5.05. IOUT_Steady_State will be their load current at nominal VOUT.

    2. I don't see "7.4.2 Dropout" in the LMR33630 datasheet but if you're referring to "Figure 8-9. Overall Dropout Characteristic VOUT = 5V" but for VOUT = 12V, I can look for some data from our validation bench which may be slightly worse than reality (the test includes a current loop to monitor inductor current which adds resistance which will reduce VOUT in dropout). What condition are you interested in? FSW? Package? Output current?

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Thanks for the answers. For the 2nd answer, I received the reply from the customer, what condition are they interested in?

    "I hope it will look like the graphs 7-9 and 7-10 (when VOUT = 12V) in the attached data sheet.
    The purpose of the question is to check how much the output voltage Vout drops when the input Vin is below the output set voltage (4V to 12V)".

    Best Regards,

    Ryotaro Fukuilmr33630-q1.pdf

  • 0 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Fukui-san,

    I have sent you an email with the chart.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Intellectual 2345 points

    Hi Sam,

    Thank you for the graph.

    I'm sorry to contact you in a row, but is there any data when the input voltage is even lower?

    They also wants to check the data up to Vin = 4 [V].

    Regards,

    Ryotaro Fukui

    何度も

    連絡して申Regards,

    し訳ございません。

    先ほど頂いた「LMR33630dropoutCurves.png」ですが、入力電圧がさらに低いときのデータはあるでしょうか。Vin=4[V]くらいまでのデータも確認したいそうです。

  • +1 in reply to Ryotaro Fukui
    TI__Mastermind 25590 points

    Ryotaro,

    Extended image sent through email.

    -Sam