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LM51521-Q1: Vout vs Iout curve for confirming current limit operation

Part Number: LM51521-Q1
Other Parts Discussed in Thread: TPS61240

Hello Expert,

Our customer is considering to use LM51521-Q1.

Then, they'd like to understand this device's current limit behavior.
For this, they think confirming Vout vs Iout curve as like as this is best way to confirm this behavior.
We don't require to measure it under specific condition due to they just want to confirm device's behavior.

Then, could you please provide this data?

Best regards,
Kazuki Kuramochi

  • Hi Kazuki,

    Thanks for using the e2e forum.
    LM5152x uses peak current control regulation. This requires a sense resistor, which is connected to the CSP and CSN pins to sense the current and device the duty cycle.
    This resistor is also responsible for setting the maximum current limit.
    As this device is a boost controller with external FETs, the maximum current is mostly limited by the external component choice for FETs and inductor.

    The reference voltages at which the overcurrent protection is triggered can be seen from the datasheet:

    Please let me know if there are additional questions.

    Best regards,
    Niklas

  • Hi Niklas-san,

    The concern is relation ship between output current and Vout during current limit operation.

    For example, there are two type of major decreasing types, brick wall and fold back.
    Also, as you can confirm at my example, there may be some specific behavior even if the output decreasing type is classified at above 2 type as like as there is step like behavior and so on...

    Then could you please consider to provide this data?

    Best regards,
    Kazuki Kuramochi

  • Hi Kazuki,

    Thanks for the update.
    LM51521 has a cycle by cycle overcurrent protection. If the voltage at CSP-CSN exceeds the overcurrent threshold, the FET will be turned off until the rest of the cycle. This means the device will keep operation.
    The calculation and description on this can be found within the datasheet as well:

    Best regards,
    Niklas

  • Hello Niklas,

    They concern is how output voltage will be decreasing depends on Iout.
    It is not for the detection method of output current.
    Could you consider to issue Vout and Iout curve as like as below?

    https://www.ti.com/jp/lit/ds/symlink/tps61240-q1.pdf#page=8

    Best regards,
    Kazuki Kuramochi

  • Hi Kazuki,

    Thanks for the update.
    I understand that customer is requesting a comparison curve like from TPS61240, but we do not have any graph like this for LM51521.
    The steady state accuracy of the output voltage is mainly depending on the FB pin voltage.
    The reference voltage for FB has a range from 0.99V - 1.005V

    This margin comes from temperature based variations or other variances between each IC:

    The device is meant to provide the output voltage set via the feedback divider at the FB pin, independent of the steady state load, as long as the device is operated within its specs (no OC trigger, no min on-/off-time violation).

    The system stability is dependent on the external components. An example for load transient behavior can be seen from the data of the LM5152 Boost EVM:
    https://www.ti.com/tool/LM5152EVM-BST

    Best regards,

    Niklas