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Evaluation module TPS40304

Rob vreden
Intellectual 310 points
Other Parts Discussed in Thread: TPS40304, TPS40304EVM-353

Hello, I am testing with the evaluation module TPS40304 revision A. It was delivered with an 1.2Volt output Volt and 20 Amps setting.

What I changed is the FB resitor from 10K to 1k and get about 6,6Volts. That works ok.

I also want to reduce the output current to about 8 Amps. I tried different resistors ( R8 was 4.02k) but no luck sofar. I have tried 3K and 2.35k. Amps output still far over 10 amps. my meter stops at 10.

I have used webench for my new design, but it uses different fets. so I cant check on the values for the EVB. 

I bought the EVB for testing and get some feeling about the current limiting mode.

How can I set the output current to limit at say 8 Amps? 

can you help?

kind regards,


Rob van Vreden

  • 0
    TI__Guru**** 191445 points

    Looks like there is more than one EVM for the TPS40304.  Can you tell me which one you have?

  • 0 in reply to JohnTucker
    Intellectual 310 points

    Hello John,

    it says on the box: TPS40304EVM-353. is that sufficient info for you?

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Guru**** 191445 points

    yes.  Let me look into it.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    I think your first problem is inductor value.  The inductor value is related to Vout and Iout.  For an 8 A current limit, that implies a maximum nominal load current of about 6.5 A.  You may want a 2.2 uH inductor.  From there I think you can calculate a new current limit at 8 A and R = 1.29 kohm

  • 0 in reply to JohnTucker
    Intellectual 310 points

    Hello John,

    Ah, ok. I see. There is at the moment( according the schematic) an 0,38uH coil mounted. I will order an different coil from Farnell and go from there. . resistors is not an problem.

    what about the max Voltage in? it is for this module specified for 14 Volt max. but the Chip can handle 20 Volts max. Thing is I want to test with 18 Volts in. 

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Guru**** 191445 points

    If you are going to change the operating conditions, i suggest you go thru the entire datasheet design procedure.  You will likely have to change the inductor value (I calculated the value based on Vin = 12 V).  It will be instructive to go thru the entire procedure to see how the various components are inter-related.

  • 0 in reply to JohnTucker
    Intellectual 310 points

    Hello John,

    I have my design ready and modeled in Webench. that one is 18Volts in and 8 Amps limited. but before to go to production, I wanted to test and see how it behaves. That is the reason I bought the EVB.

    I will go over the documents and see what else needs to be altered. seen from the values, not that much.

    I will start with the coil. 

    Thanks sofar.

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Guru**** 191445 points

    If it were me, I would still go thru the exercise of working thru the datasheet calculations.  I think it will be instructive, plus it would be interesting to see what differences arise between the manual calculations and the Webench results.

  • 0 in reply to JohnTucker
    Intellectual 310 points

    Hello John,

    Soldered in the 2,2uH inductor and also a 1k resistor. It seems to work; current tripp is about 8 amps. thing I am now facing is that it tripps during startup. I cannot get it starting up unless I take away the resistor load. It also wont startup at higher resistance where it takes about 1 amp if running. 

    I also tried a different powersupply( 30V20Amps linear) but no difference. I will continue to search for an solution.

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Guru**** 191445 points

    During start up additional current is required to charge the output capacitors.  You may have to adjust your current limit accordingly.  Typically, current limit function is more useful for "short circuit protection" rather than to limit the output current to a certain value.  That is why the current limit is usually set quite a bit higher than the maximum load current.

  • 0 in reply to JohnTucker
    Intellectual 310 points

    Hello John,

    I understand the tripping point a bit better now. what I see with the current configuration and coil of 2,2uH is that tripping does not occus at 10 Volt in. repeatingly ok. output Voltage is set at 8,6 Volts now.

    At this moment I use an 3rd FET to switch the load to gnd; this works not very wel in this topography. I get a very voltage sike of 60 Volts on the Drain-source of the 3rd FET. 

    It worked ok in an other step up scenario very well; with a different component. thats why I tried it here.

    Maybe I should look for another device than this one. What I need is a current limit and Voltage limit( adjustable Voltage) of say 8 Volts 8 Amps. And a need to PWM the output so the output current can be adjusted. Like a pwm for a high power led module. I was not able to find an part that fitted for this. MAybe you can advice on some parts TI sells.

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Guru**** 191445 points

    i would recommend that you post your complete requirements in a new thread.  i will close this one out as "answered"