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| Floating source on P-Channel |
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| advark:
Hi guys, A quick simple question (hopefully) here. I need to drive 8 7-segment displays using a PIC. Obviously, I'm going with a multiplexing approach using a P-Channel MosFET on the common anode of each display and a N-Channel MosFET to drive each segments. I have not yet calculated the refresh rate but it will probably be pretty fast since it's all the PIC is gonna do along with another half dozen of status LEDs (probably gonna need to put a delay between each refresh). Now the question is: Since the P-Channel MosFET is driving the common anode, which drives the LED segments, which in turn "feed" each N-Channel MosFET, what's happening when I turn off the P-Channel? My understanding is that its source pin is now floating. Is it best have a pull-down resistor on anode (or the source pin) or I could just leave it floating (that I will need an explanation). Because the "refresh" rate may be quite high, I'm afraid it may cause ringing. Is it possible to cause ringing in this situation? I'm a bit paranoid with floating pins so I included a pull-down anyway but I'm very curious nature. The more I think about what can happen at the source pin, the more confused I seem to get ???... (professional enlightenment needed ;) ) Thanks |
| DannyTheGhost:
I see you have good understanding how N-channel MOSFETs work, but let me clarify something - on schematics, P-ch and N-ch only differs with where the arrow inside is pointed. So, what you have here is that your source is actually connected to supply rail all the time (through your potentiometer, of course), and you can get rid of that 'pull-down' resistors Edit: your schematics indicate that you have 5v power source, so it will be no worries of putting pull-up resistors on P-ch gates, and pull-down on N-ch gates. It will get rid of possible accidental MOSFET on-states (without it, if you power down your MCU you can easily open your MOSFETs just by waving your hand right above them) |
| edavid:
The P MOSFETs need to have their sources connected to the +5V supply, and drains connected to the display anodes. It doesn't matter if the drains (not sources) float. You don't need the pulldown resistors. You probably don't need the gate resistors either, with small MOSFETs. Your brightness control pot won't work. It's easiest to use PWM to control brightness. Since your average segment current is only 3mA, the displays won't be all that bright anyway. |
| rs20:
#1: To answer your question, yes, the drain of the MOSFET becomes floating/open-circuit (except for the body diode clamping to the supply). But since the LEDs aren't inductive loads, who cares? A floating node cannot provide current, current is required for the LEDs to light, ergo, making the node be floating will certainly turn the LEDs off. Instantly. The 10k resistors to ground just waste power. #2: The brightness adjustment pot is a really dodgy idea, as depending on how many segments are on in your current row (which could be anywhere from 0 to 8), the amount of current flowing through the pot will vary by a factor of 8. This means the drop across in varies by a factor of 8 as well. In summary, any significant resistance on that pot will have a minimal impact on the brightness of a single segment, but a severe impact on the brightness of an all-on display. It would be much preferable to just modify the duty cycle on the driving waveform instead, or have global blanking. #3: The right-hand half of your diagram is horribly upside-down. The supply should always be at the top, ground at the bottom, and data flowing left to right. You've got all of those things backwards, which makes it very hard to read. P-type MOSFETs should always have their source terminals at the top. Put another way, those body diodes inside the MOSFETs should always be pointing upwards (as, generally speaking, they should never be carrying current except for weird spikes etc). |
| advark:
Thank you all for the insight. I should've mention it but the all gates do have their pull-down/up resistors. They're on another sheet: 10K pull-ups for the P-Channels and 10K pull-down for the N-Channels. I chose 10K because, well, I have plenty of them... The PWM will worth investigating to replace the 1K pot (which value was randomly chosen, for concept purposes). I understand its concept but never implemented it. That'll be for the next lesson... ;) Again, thank you all for the help. |
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