LTM4651 simulation issue

Hi there!

Does the Spice model of LTM4651 work?
I am using 4651 to make -12V from +24V but can't get the desired output.
The output is -0.6V with 0.5Vpp ripple and freq is about 15kHz but the switching freq is set to 1.1MHz.
All external parts have selected according to the datasheet.
Pls check the attached sch.


And how the output of 4651 can be controlled by DAC voltage like the VDAC pin of LTC2974 or similar?

Thanks

4651-12Vout.asc

  • 0
    •  Analog Employees 
    on Jan 26, 2021 8:17 PM 1 month ago

    Hi Alexey,

    It looks like Fset resistor should be tied to -Vout, not GND.  Also why do you have a 50 load on Extvcc?

    I'll get back on adding the 2974 DAC. Do you want +/-5% control of Vout?

    Mike

  • +1
    •  Analog Employees 
    on Jan 28, 2021 1:33 AM 29 days ago in reply to mpeters

    Hi Alexey

    I have modified your LTspice file.  See attached.

    The 2974 DAC can be connected to a PNP transistor that adds/subtracts current to the ISET node. There's a pre-bias current that needs to be sourced from the PNP collector.  It adds to the ISET current from the LTM4651.  The ISET resistor is chosen such that Vout/(prebias+ISET).  In the example file, the prebias current is 10uA.  200k*60uA=12V.  With prebias current nominally set to 10uA, this means that you can add another 10uA or take away 10uA, which allows this current go from 0uA to 20uA. The simulation is set up for approx +/-5% of Vout margin.

    The 2974 DAC configuration bits needs to be set to high range and non-inverting.  Soft-connect will work if the DAC is set to high range.

    Mike

    4651-12Vout_mtp.asc

  • Hi Mike

    Thanks for your help. It was my mistake that Fset was connected not to -Vout.

    50R at Extvcc is according to the datasheet (Para: INTVcc and EXTVcc Connection)

    The last example regarding to margin -Vout works as I expected to get.
    Thank you again!

    Alexey