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LM3478: Inconsistency in Datasheet & Webench

Adam Campbell
Prodigy 20 points

Part Number: LM3478

Tool/software:

Hello, 

I am in the process of re-designing a boost converter with the LM3478 and noticed something in the documentation that didn't match WEBENCH, which left me a bit confused. The datasheet outlines the following configuration for the Isen pin -- citing that if there is current above 343mV at the pin, it will start short circuit protection, decreasing the operating frequency by a factor of 5.  

However, WEBENCH seems to recommend an RC LPF into the Isen pin (below). What are the pitfalls/advantages of either option? 

The datasheet cites slope compensation with the non-filter configuration, how does the filter configuration suggested by WEBENCH account for the slope compensation? 

Please advise where possible. 

  • +1
    TI__Mastermind 33510 points

    Hi Adam,

    Thanks for using the e2e forum.

    Depending if and where filters are place, a different effect is achieved.

    In theory, the design can function properly without RC filter or slope compensation resistor. However, to stabilize the system and reduce risk of noise/oscillation the following components are recommended:

    - RC filter (as per webench): If the internal blanking time is not sufficient in filtering the voltage spike at turn-on of the MOSFET, is it recommended to place an RC filter. This filter is generally recommended, as it also reduces noise at the current sense pin.
    (The datasheet only uses one capacitor for filtering. I would recommend the complete RC filter and not only one C)

    - slope comensation resistor: If the internal current slope is too small in comparison to the inductor current falling ramp, there is a risk of subharmonic oscillation. This can occur at duty cycles of >50%. In such case, it is recommend to place an external resistor to increase the slope ramp artificially and avoid oscillation.
    Even if no subharmonic oscillation is expected, it is still recommended to place a 0 Ohm resistor at the current sense pin, so slope compensation can still be added later on if necessary.

    The schematic in the quickstart calculator tool show an example where both RC filter and external slope compensation resistor are place.
    This is the recommended implementation for most designs.

     

    Please let me know if there are any questions on this.
    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 20 points

    Hi Niklas,

    Thank you this points me in the right direction! Regarding the slope compensation, the ratio they mention in the datasheet, is this an absolute value, or should we design for Sf to be larger than Se? 

  • +1 in reply to Adam Campbell
    TI__Mastermind 33510 points

    Hi Adam,

    There some kind of threshold for the ramps, from where on subharmonic oscillation can be avoided.
    Here I would refer to this article which summarizes some TI application notes:
    https://www.digikey.de/de/articles/the-role-of-slope-compensation-in-current-mode-controlled-voltage-regulators

    "For perfect compensation, the compensation ramp must have a slope equal to exactly half the downslope of the voltage waveform at the other comparator input, which is the voltage analog of the inductor-current downslope as seen across the current-sense resistor."

    Best regards,
    Niklas