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TPS54620 not stable with different load conditions.

Nick McMilan
Prodigy 70 points
Other Parts Discussed in Thread: TPS54620

I am using a TPS54620 buck converter with 12V input to drive 6A LEDs

I have 6 conditions I want to make stable with one set of compensation components.

Vout 2V and 5V

Iout 0A, 2A and 6A

I am using the TPS54620 EVM reference design schematic with Switcher Pro compensation recommendations. I am not able to get all combinations to be stable with one set of compensation values. Specially with 6A and 0A loads.

Please recommend correct compensation values to be stable at the 6 conditions.

Thanks

Nick

  • 0
    TI__Guru**** 191445 points

    If you are driving LEDs you are probably trying to drive constant current thru the LED string.  That ill probably require different compensation for each application.  You  may be able teo design a low performance , low crossover design that wil work for both, but I would not count on it,  Can you share your schematic and design equations?

  • 0 in reply to JohnTucker
    Prodigy 70 points

    Hi John,

    Thanks for your quick reply.

    From Vout I have the 6A LED then resistor and FET to GND (for 2A), I have a second resistor and FET to GND (for 6A). When both FET's are off is the no load condition.

    I have 2 different LED's, thats why I have  Vout of 2V and 5V.

    I am using the TPS54620EVM, the schematic is on page 17 here;

    http://focus.ti.com/lit/ug/slvu281/slvu281.pdf

    Thanks

    Nick

     

  • 0 in reply to Nick McMilan
    TI__Guru**** 191445 points

    If you just have an led in series with a resistor in series with a FET like such:

    http://e2e.ti.com/cfs-file.ashx/__key/CommunityServer-Discussions-Components-Files/196/3173.620_5F00_LED.eps

     

    then it will just be a simple load.  In general you need different compensation for the two different output voltages though.  AAlso you have to change teh resistor divider to set 2 V or 5V so it should not be an issue to change a couple other component values.  You can use on line switcherpro to design teh appropriate circuits for 5 V and 2 V output

  • 0 in reply to JohnTucker
    Prodigy 70 points

    Hi John,

    The requirements of my design does not allow me to change components.

    Can you please recommend one set of compensation values for my 6 conditions?

    Vout 2V and 5V, at Iout of 0A, 2A and 6A.

    When using Switcher Pro, it recommends 6 different set of compensation.  When I try 2v or 5V outputs and 6A using Switcher Pro, when testing at 0A (and sometimes 2A) the unit is not stable (6A is OK). A 6A design should be stable from no load to max load.

    Thanks again,

    Nick

     

  • 0 in reply to Nick McMilan
    TI__Guru**** 191445 points

    Before I try any design calculations, what leads you to beleive your current design is not stable?  Do you have any loop response plots I can look at?  Also, what other care abouts do you have?  Any restriction on output filter components?  One way to over come this is to use an output capacitor with some ESR.  let me know..,

  • 0 in reply to JohnTucker
    Prodigy 70 points

    Hi John,

    By not stable I mean the output will oscillate at ~250mV with no load and is stable at Iout of 6A (25mV ripple).

    I do not have a bode plot, no restrictions on output capacitance, besides load transient response should not be to slow.

    Design parameters; Vin=12V, Vout=2V and 5V, Iout=0A, 2A and 6A.

    Thanks again for all your help.

    Nick

  • 0 in reply to Nick McMilan
    Prodigy 70 points

    Hi John,

    Looking to hear back from you, you probably got busy with some other project. Please help when you have time.

    Thanks again,

    Nick

     

  • 0 in reply to Nick McMilan
    TI__Guru**** 191445 points

    I would really like to see a waveform, just to be sure I am not missing anything, like OVP.  Do you have one?

     

     

  • 0 in reply to JohnTucker
    Prodigy 70 points

    Hi John,

    I do not have OVP waveform.

    Thanks

    Nick