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LM5085: Double switching pulse

Norbert Nestler
Intellectual 300 points
Part Number: LM5085
Other Parts Discussed in Thread: LMR38020

Tool/software:

Hi to all,

i did this design (18V-60V DC to 14.5V 1.2A) with the webbench power designer. It works, but i have a double pulse while switching (short on, before on). I read about Ripple Network, but i assume, that webbench will take this automatically into account?

Can you please tell me, what i have to modify? Please don't write only "Ripple Network", if this is the issue. Please specify circuit an components, thank you. Cout is populated with TAJC106M035RNJ (10uF/35V)

Thanks.

kind regards

Norbert

  • 0
    TI__Mastermind 48471 points

    Hello Norbert 

    Would you please send the link of your WEBENCH design ?

    -EL

  • 0
    Intellectual 300 points

    Dear Eric,

    here ist the link: webench.ti.com/.../SDP.cgi

  • 0 in reply to EricLee
    Intellectual 300 points

    Dear Eric, the only component that is different to the design is Cout with 10uF (Tantal).

    Can the Cvcc cause this problem? I used a X7R 470nF Ceramic. I think in one application i have seen a polarized one?

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Norbert 

    The link doesn't work....COUT affects the switching stability. VCC capacitor doesn't matter 

    -EL

  • 0 in reply to EricLee
    Intellectual 300 points

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=AA5922D684B3DD66

    it works on my pc..

    Do you have an email?

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Norbert 

    The link works now. Would you please share the schematic and PCB layout too?

    -EL

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Norbert

    C7//R13 should be directly connected to the positive connection of the current sense resistor. It is possible that the "short-on" pulse is caused by a noise injection to thi spoint. 

     

    -EL

  • 0 in reply to EricLee
    Intellectual 300 points

    Dear Eric, in the first post i forgot to mention, that i had a 10R resistor populated on R14. Now i changed this in 0R and the double pulse moved in the middle of the signal. (as you can see below). I changed the circuit as you suggested, but there is no change in behaviour. You can also see an oszi-picture from the gate signal.

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Norbert

    I guess you made a wrong connection. You have to connect the other side of RADJ 

    Based on your experiment using 10R, I believe the issue is caused by a noise injection between ADJ and ISEN pin. 

    I haven't tried, but I guess it is valuable experiment to check if any improvement after populating 100pF between ADJ and ISEN. 

    -EL

     

  • 0 in reply to EricLee
    Intellectual 300 points

    Hello Eric,

    the connection is correct. The unpopulated pads in front are Vin. Now the wire goes directly to Vin and the other side of R13,C7 goes to Pin1 (Adj).

    I tried your suggestion with the 100pF - nothing changed. This is the oszi with populated 100pF:

    I did some more measurement:

    This is the output ripple:

    and this is the signal at the current input:

    Now i populated the ADJ-Resistor with 4.7K and left the 1n away, since my LED's are controlling their current by themselves. This means, i do not need the current control of the LM5085. (I used it only for the voltage range 18V to 60V). And now, at a voltage of 22V the spike is away and switching frequency is correct (160KHz), with 18V it is only 65Khz, which is also wrong...

    22V:

    18V:

    Any suggestions?

    Kind regards

    Norbert

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Eric

    Good to hear that you saw an improvement by changing the ADJ-Resistor to 4.7K. This test result also matches with the theory that the issue is caused by a noise injection between ADJ and ISEN pin. Please double check the jumper connection  

    I think the jumper connection should be like this. 

    -EL 

  • 0 in reply to EricLee
    Intellectual 300 points

    Hello Eric,

    my connection is correct. The wire connects R13/C7 directly to +C4 and R18. I think a much shorter connection is not possible. As i use an aluminium pcb, i cannot put a groundplane to the bottom side.

    The improvement is only at a voltage of 22V and up. From 18V to 22V, the failure is present, even with the 4K7 at ADJ. So, this circuit is not usable for me.

    I think i have to set up a new layout, but i will not use the LM5085 again. What do you think, will the LMR38020 do a better job for this application?

    Thanks for your help,

    kind regards

    Norbert

  • 0 in reply to Norbert Nestler
    TI__Mastermind 48471 points

    Hello Norbert 

    LMR38020 should be much more noise immune than the current sensing using Rs since this device senses current inside the device.

    The switching noise also can be reduced by using low-QRR diode. Please try to replace the D2 diode with lower Qrr diode, and then check if any improvement.  

    -EL