On 4/28/14, 11:33 AM, "Britton Kerin" <[email protected]> wrote:
>On Mon, Apr 28, 2014 at 3:34 AM, Timbo <[email protected]> wrote: >> Either there is a bad connection somewhere between the barrel jack and >>the >> header pins (unlikely), or one of the power supplies is causing the >>problem. >> I would check the resistance between the jack contacts and the >>corresponding >> header pins, and then try powering via the headers with the known-good >> supply. If that's ok, then (with the load connected) look for sag and >> ripple on the other supply. > >Well, its the supply. In some hideous way that I can't figure out how >to measure. > >It works fine going straight from a wall wart to the header. > >The on-board 12V to 5V supply *looks* fine, it shows up a totally >clean 5V on the scope, no noise that I can measure. There's 220 uF of >output filter. But somehow 500 kHz triangular fuzz is showing up on >the bone IO pin -- exactly the switching frequency of the step-down >switcher. > >Its a design from TI switcher-pro. It also agrees almost completely >with the reference design in Fig. 11 of the attached spec sheet. I >feel sorta stupid to be using a design I don't completely understand, >but I'd like to at least be able to measure where the problem >originates. The important part is the switch on time (power on ramp time). Does it ramp linearly and are there any glitches in the ramp. What is the ramp time? The 500KHz noise sounds like a PCB layout issue. Also, what is the ESR of the output cap? Don¹t use electrolytic or tantalum capacitors as they have higher ESR. I recommend you use ceramic capacitors, even if you have to reduce the size of the capacitor. Try adding a smaller ceramic capacitor in parallel. Regards, John > >Any hints? > >Thanks, >Britton > >-- >For more options, visit http://beagleboard.org/discuss >--- >You received this message because you are subscribed to the Google Groups >"BeagleBoard" group. >To unsubscribe from this group and stop receiving emails from it, send an >email to [email protected]. >For more options, visit https://groups.google.com/d/optout. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
