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LM76003-Q1: Adjust RT by switched resistors possible?

Will Brown
Intellectual 451 points
Part Number: LM76003-Q1
Other Parts Discussed in Thread: LM76003

I am looking to use the LM76003 part over a wide input voltage range (20V - 60V), with an MCU/DAC trimmed output voltage that could range from 5V to 48V.  I haven't evaluated all design permutations, but it appears that it may ideal to change the operating frequency, depending on the input voltage and output voltage requested, to maintain operation in a reasonable zone for the inductor that's in the circuit.

The point of the question, is can the switching frequency be adjusted by placing, let's say 4 resistors (in parallel), with one end of each resistor tied to RT (all four resistors tie to RT) and the other (ground) side of each resistor goes through it's own N-Channel MOSFET acting as a switch, which is either shorted to ground or open-circuit.  The effective resistance presented to the RT pin can then be adjusted via MCU control, closing the MOSFET switches to adjust the overall resistance presented to the RT pin, therefore adjusting the operating frequency of the buck regulator.

I see two possible issues:
1) Something about the RT voltage precludes using a MOSFET to ground the bottom side of the resistor (doesn't seem likely to me)
2) The buck regulator may not take kindly to abrupt changes in RT, and therefore switching frequency, while in operation.  I can control that the regulator is disabled during the time that RT is changed.

Thanks for any input,

Will

LM76003

  • 0
    TI__Mastermind 22301 points

    Hi Will, 

    The device will not take kindly to changes in RT while running, it will be in an indeterminate state.

    You could disable the device by pulling EN low and then switching RT.

    However if your goal is to change output voltage, why dont you just do the same thing with the FB resistors? Those resistors set the output voltage, and you should be able to change those without disabling the device. You could keep the top resistance fixed and select between different bottom resistors.

    Please see 7.3.3 in the datasheet, let me know if you have questions.

    -Orlando

  • 0 in reply to Orlando Murray
    Intellectual 451 points

    Thanks for the reply Orlando,

    The issue does not revolve around changing the output voltage.  I was asking about changing the switching frequency, in order to perhaps optimize efficiency and/or losses, since my Voltage Input Range spans a 5:1 ratio.  The same device may be installed into a 20V system, or a 60V system  If I could optimize device efficiency by altering the switching frequency, that was what I was thinking

    Will

  • 0 in reply to Will Brown
    TI__Mastermind 22301 points

    Will,

    Yes you can change the RT as you are thinking, but you must first disable the device by pulling EN low.

    However you will essentially be trading losses from MOSFET switching and gate charge for RMS ripple current conduction losses, and your efficiency gain will probably be marginal.

    WEBENCH models these losses, so you should try your different permutations there and see how much the efficiency varies.

    http://www.ti.com/lit/an/slyt664/slyt664.pdf has a written overview of the losses I'm referring to.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    Intellectual 451 points

    Thanks for the confirmation, and the additional information regarding losses.