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LM5022: Can one chip provide multiple simultaneous voltage outputs?

Daniel Miller
Prodigy 40 points
Part Number: LM5022
Other Parts Discussed in Thread: TPS43060EVM-199

Using Webench, I've created a pair of power supply circuits. One boosts 24VDC to 32VDC, the other boosts 24VDC to 40VDC. Obviously with same controllers and similar voltages many of the components are the same. The design is intended to output a maximum of 3A at each voltage.

The use of these two power supplies is to drive a single circuit - the load will be switched between them so only one voltage will be active at any given time. The "standard" voltage will be 40VDC - it will switch to 32VDC for up to 20ms and back to 40VDC for pulse signaling.

My question - is it possible to combine these two into one power supply to reduce component count and board space?

  • 0
    TI__Genius 10385 points

    Hi Daniel,

    Thank you for using E2E forums, regarding your question what do you mean combine them? each output voltage would need a separate and different FB divider to give out different output voltages. 

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Prodigy 40 points

    I'm not sure myself - but some possibilities I've wondered about include:

    1. Two separate power supplies but sharing input capacitors. Probably not enough savings to be worth it.

    2. One power supply, where the FB divider is switched to a different value to change the output voltage. My question is can the switching be done fast & consistent enough - ideally I'm wanting to have a square wave with at least 10ms frequency.

  • 0 in reply to Daniel Miller
    TI__Genius 10385 points

    Hi Daniel,

    I see, so LM5022 should be able to handle switching every 10ms to 20ms from the two output voltages, but you would have to make sure how the FB dividing resistor switches between the two values that would give you the two output voltages, and how immediate is that change as this would seem like a load transient to the device and the compensation and stability needs to be arranged accordingly so allow the transition to occur smoothly and without causing any damage. 

    Hope this answers your question.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Prodigy 40 points

    So is it possible to do this - either with the LM5022 or another controller? The TPS43060EVM-199 example board actually looks almost perfect for this - I can just switch out resistor R9 on that board but the concept of changing the dividing resistor would probably apply to any of the controllers. So the question is either what controller would be best suited for this switching - and/or what would be required in external components to get the necessary smooth switching? It is not a problem if there is a delay in output following the resistor change - as long as that delay is consistent and predictable and still allows for a total of less than 20ms between high/low states.

  • 0 in reply to Daniel Miller
    TI__Genius 15785 points

    Hello Daniel,

    It is possible to change the output voltage during operation by changing the feedback divider.

    Bot you must take care that the power stage of your converter can follow the change.

    - Please make sure that you will use the lower resistor of the voltage divider as the adjustable element (NOT the upper one)
    This way the compensation does not get messed up.

    - You can e.g. leave one resistor on the low side always connected and put other switchable resistor in parallel

    - When the output voltage gets changed immediately / in one step during operation, it appears to the controller like an immediate load change
    Therefore:
    Change the output voltage in very small steps by using
    - a DAC,
    - creating a switchable resistor with FETs and resistors
    (This is close to the original idea, but allows for finer (digital) steps by using more switches and resistors)
    - or maybe using a programmable resistor (but no experience with using such a device for this purpose).

    Do not put a capacitor in parallel to the feedback divider to slow down the change.
    This would break the behavior of the compensation network.

    You can try to make the compensation slow, but this would also slow down the reaction on a change on the load or on the supply side.


    The Power Stage Designer tool even supports such a concept.
    Select Tools -> Output Voltage Scaling and you will get a new window.

    Dynamic Digital Output Voltage Scaling makes use of the switchable resistors:
    Rbit0 .. Rbitn create an n bit controllable resistor divider with 2^n settings.
    This would need to be switched in a digital sequence to allow for a smooth transition.

    Dynamic Analog Output Voltage Scaling assumes a DAC which gets connected to R3.
    Then you can program a slow ramp with fine steps from one voltage to the other.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Genius 15785 points

    Hello Daniel,

    We have not seen an update from you for nearly two weeks, so I assume the question has been answered.
    I close this thread now. If there is still something open, please reply and the thread will get opened again.
    If you have any other question or of the thread has been locked, please open a new one.

    Clicking the Resolved Button also helps us to maintain this forum.

    Best regards,
    Harry