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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
video of the problem (this link will not be valid permanently) no load is connected during this test. sometimes i am pressing 1 button, sometimes i am pressing 2 buttons, after about 0:48 i am holding 1 button and then alternating another button while it is down (buttons are active low) when the LEDs are dim, its using a lot of current, the UCC chip heats considerably and rapidly, and make a mess on the TV reception. switching of one input seems to be able to have an effect on other inputs, even if they are grounded
any idea what i've done wrong, and how i can remedy it preferably without starting over on a new board?
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
the ucc is probibly oscilating maby add some bypass caps to the power pins of your IC. prefribly tantalums and electrolytics at the power for the totem
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
wow 1000uF on the supply rails stopped the 'dim led' problem! now i just have the inter interference flickering problem. tantalums for close to the chip decoupling? are you sure? is that instead of or aswell as the ones ive got?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
The Irf 9540 is a P channel device; I would guess it should be wired source to +vcc. Or maybe it’s just an error in the way you’ve drawn it.
The Tv interference is likely to be oscillation as already pointed out.
Are the inputs left floating when the problem occurs? You could try a pull up (or down) resistor too. You didn’t say what was driving them but if from logic level then that shouldn’t be required.
It is usually good practice to use series gate resistors, typically a few, to a few tens of ohms, perhaps this will help with the oscillation problems.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I was going to suggest bypass caps too. I made a little board with 3 555 circuits on it, as I would change the frequency on one, the other frequencies would also change. Tantalums directly over the ucc rails should fix that, I had to try different values IIRC.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
You are correct, i have drawn the mosfets incorrectly, although they have been wires correctly. No inputs are floating, they are switches for now, it will be a micro-controller later. I have added 11 ohm series resistors from the UCC, don't really notice much difference though. What value tantalums are suggested?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Perhaps Also try bypass caps across the + and – rails of each totem pole right where the mosfets are, as any cross conduction will cause spikes on the supply rails just as on the UCC drivers. If using tantalums 10uf is a convenient value.
Are you sure they will work with 3.3v in the inputs too? I’ve only ever used 5v, but I’m not saying they won’t, perhaps somebody else can comment.
Edit.
Another Trick is to use individual gate resistors (which is what I was originally thinking) but with a diode in parallel across each one. Anode to gate on the n channel fet, and cathode to gate on the P fet. The trick is to get each mosfet to switch off faster than it’s partner turns on.
Assuming this is the sole cause of you oscillations of course. As others have said, good grounding layout is just as important.
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trouble with a 4x totem pole board i made
