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LTC54540: Compensation for switched loads

Sarah Jansen
Prodigy 40 points
Other Parts Discussed in Thread: TPS54540

Hi folks,

for the supply of an H-bridge (for a galvanometer) I've chosen the TPS54540. Vin = 24V, Vout+15V Ioutmax= 3A. This works wonderful with a static load, even at 3A the switcher doesn't get too hot.

But with a switched load it doesn't look that nice. When the load (5R) is switched on, Uout is still 15V after a short drop to 13V. When the load is taken off, Uout is ~16V.

Here some data:

Cin = 2* 4.7µ ceramic

Cout = 3* 22µ ceramic

Fsw = 400KHz

L = 22µH

Rcomp = 10K

ccomp = 22n

Anyone has an idea?

:-) Sarah

  • 0
    Prodigy 40 points

    I've made some experiments. Recalculating Rcomp and Ccomp according to the data sheet made it worse.... I came to Rcomp = 33k, Ccomp = 10p and this was awful....

    Now I have Ccomp = 470p and Rcomp = 15k. The short break in is much better but still present and the voltage rise after the load is off is now ~500mV. Still not good enough.

    Calculating the parallel capacitor to Rcomp-Ccomp gives me something about 5p. Does this makes sense? My guts tell me, this will have no bigger effect on the voltage rise.

    Anyone an idea?

    :-) Sarah

  • 0 in reply to Sarah Jansen
    Intellectual 530 points

    Hello Sarah

    going by what I think you are saying,, (its been a long few days I have had very little sleep) and a quick look over the data sheet I would guess,,,,

    If you don't want the drop in output power then I would go with a higher output caps (C6 and C7) take them up to say 470uf to 1000uf the higher the uf the longer they take to drop,,, those are filter/storage caps they are their to give a steady DC output.

     

    If you want the drop then lower c6 and c7 or even eliminate c7 all together.

    I hope this helps.

    Jeff / FrostyTheDadMan

     

  • 0 in reply to Jeff Lampe
    Prodigy 40 points

    Hi Jeff,

    thanks for Your reply, I appreciate.

    This is quite a step in Cout. What would You suggest, additionally to the ceramics some tantalum or aluminium caps? Like 100µ in ceramics for the quick responses and low ESR and filling up the rest with cheaper aluminium?

    :-) Sarah

  • 0 in reply to Sarah Jansen
    Intellectual 530 points

    Thanks

    If you want a clean DC then go with Electrolysis high uf,,, if you want the pulse then go with ceramic's low pf  

     

    I have to go to work now but I will check in with you after work. ;0)

  • 0 in reply to Jeff Lampe
    Prodigy 40 points

    Thanks, I'll try this on monday. You go to work and i have weekend now :-)

    What I've found out, there's quite some noise on the feedback line under load. This seems to cause the rise in Vout. Stupid me, I haven't embedded this trace between GND layers. Next version will be better.

    Have a nice weekend,

    :-) Sarah