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LM3485: LM3485 Design issue and inductor selection issue

Cickey Xu
Intellectual 686 points
Part Number: LM3485

Dear TI partner,

My customer use LM3485 for a buck application, basic case is input 24v, output is 12v/3A; the schematic and pcb as below

there are several question need to be consult.

1.why our datasheet typical application is different from our Webench power design schematic. if we adopt datasheet schematic, the circuit cant run normally and no output voltage, if  refer to webench schematic, it is can output voltage.can you tell me the reason?

2. when we designed the schematic according to webench schematic, as above schematic. only the inductor is welded in one direction,the circuit is normal, in contrary, if we change the inductor welding direction, the LM3485 is very hot. the inductor is SDR1307-100ML,we also try other brand inductor such as MSS1210-473ME. can you tell me why, as i know ,the inductor don't have welding direction.

we also catch the PGATE pin (pin 7) waveform in these two case ,as below ,the first picture is normal waveform, the second is abnormal waveform ,please refer to.

look forward to your reply.

best regards

Cickey Xu

  • 0
    TI__Expert 6205 points
    Hello Cickey,

    The datasheet example is calculated for a 3.3 V output converter while you are designing a 12 V converter. Other than the feedback divider, several components, including the inductor must change. Also, the datasheet has an error – the input and output capacitors are swapped in Table 2. The Webench values should be correct.

    Inductors will have similar performance no matter which way they are connected. They can be much noisier if pin one does not face the switch node, however. Most inductor companies start the winding from pin one allowing turns closer to pin two to shield the noise. Pin one should face the SW node. The two inductors that you mention have different values. Since Webench recommends 100 uH for your design, alternate inductors should have the same value.

    Please let me know if you have more questions.

    Regards, Robert
  • 0 in reply to Robert Blattner
    Intellectual 686 points
    dear Robert,
    thank for your quick and detailed reply;
    the first issue, i think mainly reason is parameter select issue, datasheet's schematic test current by external mosfet Rdson, if mosfet Rdson is not suitable ,include divider resister selection,maybe make ISENCE pin over the threshold and result chip shutdown.
    the second issue, maybe i give your a wrong P/N,Actually, my customer changed several brand inductor, certainly,they have the same inductance value. but phenomenon is same. i also dont understand why appear different waveform when we welded inductor by different direction.
  • 0 in reply to Cickey Xu
    TI__Expert 6205 points
    Hello Cickey,

    I missed this the first time that I looked: You are wrapping the FB line around the inductor. The FB line is probably acting like a secondary for the inductor and may be picking up 10 to 50 mV of noise from the inductor. Try probing the 12V side of C39 right at C39. Use AC coupling to make mV level signals visible. If you reverse the inductor, the signal seen should be inverted. Having a CFF, C39, in this circuit is aggravating this coupling by passing the full signal to FB rather than attenuating this signal through the FB divider. If there is a 30 mV ripple injected by the inductor, it will look to the IC as though there is a 300 mV ripple on output. Hysteretic converters are very sensitive to this kind of noise. Also, try placing a capacitor from 12 V to ground right next to C39 to reducing this ripple.

    Also, since different inductors are wound and shielded differently, each type of inductor will behave differently when looking at this type of coupling.

    Regards, Robert
  • 0 in reply to Robert Blattner
    Intellectual 686 points
    Dear Robert,
    Thank for your help, i will adjust the external component parameter to test.
    Thanks again.
    Best Regards.
    Cickey Xu