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LM25575 function block

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Hi,

This is Ben. I want to discuss some function block about LM25575 with you.

1. When I connect two LM25575 by SYNC pin. Can they become a muti-phase power source?

2. In my  opinion, because of OP –Vsupply is connect to SW pin and the gate signal which generate by flip-flop should match OP Vin range. So the level shift is required.

Bootstrap Cap(C7) is needed to boost gate drive voltage above VIN to keep it on next time. Is it right? Does other power ICs need this level shift function?

  • Ben,

    Thank you for posting your question on the forum.

    If you want to operate two LM25575 in parallel, the key point is to tie the COMP pin together. So that the load current can be evenly shared by the two LM25575. Tie SYNC pin together will keep the switching action in phase. But two-phase operation usually prefer the switching actions 180deg out of phase, so that input voltage ripple can be smaller. To have out of phase operation, you need two 180deg out of phase clock inputs to the two ICs.  

    What is the goal to use two LM25575s to power one load? LM25575 is a 1.5A part. If you need 3A total load current, I’d recommend using a newer part: LM43603, 36V 3A part.

    http://www.ti.com/product/LM43603?keyMatch=LM43603&tisearch=Search-EN-Everything

    It is the same package size of the LM25575. It is a fully synchronous regulator, meaning you don’t need a diode on the board. The solution size should be much smaller than two LM25575. Easier to design as well. With two LM25575, you need two inductors, two power diode and two sets of Cin. With a single LM43603 to provide 3A load current, you only need one inductor, no diode, one set of Cin. 

    Regarding the question of level shifter, yes, it is in every buck converter when it has a high side N-FET. N-FET requires the gate voltage to be higher than the source voltage to be ON. The source voltage of the HS FET is about VIN when it is ON. So level shifting circuitry is needed to boost HS gate drive voltage above VIN to keep it ON. Since the FET, the driver and level shifting circuitry are all integrated inside the IC (both LM25575 and LM43603), no effort is needed from customer.

    Best regards,

    Yang

    TI Wide Vin Buck Converter & Controller Applications Engineer

  • Hello Ben,
    The SYNC functionality of the device allows to synchronize many devices to a highest frequency together. But it does not allow multi phase power capability.
    The bootstrapping circuit is needed for any buck regulator that employs N type MOSFETs as the switching elements. For P type MOSFET the bootstrapping circuit is not needed.

    Hope this helps.
    Regards,
    Akshay

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