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TLVM13660EVM: Switching frequency does not adjust to the jumper settings

Part Number: TLVM13660EVM
Other Parts Discussed in Thread: TLVM13660

I am using the TLVM13660 EVM with the jumpers set for a 5.0V output and 800KHz switching frequency.  I am getting good output voltage and efficiency with low ripple, but the ripple frequency seems to be defaulting to about 250KHz or so.  I have tried the other FSW settings and still the ripple freq is only about 250KHz.  I confirmed the resistor for the 800KHz jumper setting is 16.5KOhm.  Please advise.

  • Hi Steve,

    Default Fsw for a 5V output is 1MHz, but 800kHz should be fine as well. Here is the applicable section of the schematic. Can you also try the 1MHz setting - note you can measure the Fsw at the SW pin (pin 2) or even easier at resistor R3 that is connected to CBOOT (pin 3). Also check the output caps as if they are somehow cracked, the low Cout can cause instability.

    Regards,

    Tim

  • Hi Tim,

    thanks for your reply.  I checked the SW pin and I do see 800KHz square wave, though it's very low level, on order of 30-40mV.  Is that correct?  I'm measuring the ripple output at the eval board SW+ to SW- pin and the ripple is pleasingly very low.  It's so low that it's hard to tell the actual frequency.  I did move the RT resistor (R17) closer to the chip (had it soldered to a via next to chip) and that seemed to have reduced the ripple quite a bit. Could it be that with the original placement of RT resistor far away that was affecting the switching frequency?  Even in that case, the ripple was still low, though not quite as low as after I moved the RT resistor, and I'm getting great efficiency (94%).

    Steve

  • Hello Steve, 

    Tim will probably take a look at this tomorrow and get back to you. 

    Regards, 

    Denislav

  • Hello Steve,

    Can you please take a scope shot of the Switch node, also please take a scope shot of the 250kHz you are seeing?

    Thanks.

    David.

  • Steve, 

    The SW voltage should go from 0 to VIN at the programmed switching frequency -- please check your probing technique as the pin is difficult to access. There is only one SW pin (no SW+ and SW- pins). I doubt there is any issue here, especially if the efficiency is as expected. If you suspect an issue, send on the SW voltage and output ripple waveforms.

    Regards,

    Tim

  • Hi Tim,

    I just doublechecked and confirm the SW node switches from 0 to VIN0 9at the correct frequency.  I must have been looking at a different pin previously.  I noticed that node is very sensitive as I see spikes on the ripple output when I probe that pin. Otherwise, the ripple output is very low now, almost to a point that it's hard to decipher the amplitude/frequency which is a good thing.

  • Hi Steve,

    Seems like it's operating correctly. Be careful probing SW to avoid shorting it to adjacent pins.

    Regards,

    Tim