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LM25088MH-1EVAL: peak current mode control shows sub-harmonic oscillation

Andreas Peterswerth
Prodigy 20 points
Part Number: LM25088MH-1EVAL

hi there,

I do habe a sample circuit (product, no eval) on desk, using LM25088MH. The problem ist sub-harmonic oscillation from peak current mode control. When examining the datasheet, on page 22, chapter 9.4.4 i come across slope compensation. Now Iam trying to figure the V_RAMP term used in expression No 15.  What is that voltage? 

cheer Andreas 

  • 0
    TI__Mastermind 22301 points

    Andreas,

    Section 8.3.5 describes the ramp generation, and figure 15 shows the RAMP voltage calculation.

    Vramp  = {5 µA/V x (VIN - VOUT) + 25µA} * Ton/Cramp.

    You can also calculate Ton = Vout/(Vin*Fsw).

    Let me know if you have questions.  

    -Orlando

  • 0
    Prodigy 20 points

    Thanks Orlando,

    let me put numbers. (5µA/V x (28V-24V) + 25µA) * (24V/(28V*108.132kHz) / 2.2nF = 162mV = V_RAMP

    Now going to exp. (15)         R_RAMP = (7.8V - 0.162V) / (24V x 5µA - 25µA)  = 80.4k ohm

    Webbench came up with something close. What VIN do i use? min or max?

    Something else I really do not grasp is the effect of varying Fsw periode.

    My design runs on 28V VIN, 24V VOUT ist seen, load is 2A appox. R_RT is set to 59k. So Fsw should be 108.132kHz   Tsw = 9.25µs.

    Looking at mosfet "on" edges I would expect to habe them at 9.25µs each. (C_RES set to 0 ohm) However I do observe 9µs, 12µs and 13µs distance in time. Current waveform is all but regular.

    What might be the reason to this? - Is it a broken controler? 

    Thanks for u help

    Andreas

  • 0 in reply to Andreas Peterswerth
    TI__Mastermind 22301 points

    Andreas,

    Please use VIN-nominal for slope compensation or use WEBENCH recommended value.

    The frequency should be fixed and not vary. It's hard to tell without looking at the design and the waveforms.

    If you are still experiencing issues feel free to post the information stated above.

    -Orlando