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Saturating a MOSFET with a constant current sink driver
Oleenick:
Hello EEVBlog forum,
I'm designing an LED cube and for its LED drivers I'm using TLC5927 16 Channel constant current sink drivers (https://datasheet.lcsc.com/szlcsc/1804162351_Texas-Instruments-TLC5927IDBQR_C181418.pdf).
I was wondering if I could use the constant current sink outputs of the TLC5927 to drive a P-channel MOSFET (for the ~7amp max layer current).
As per the diagrams bellow I just want to use some outputs from the TLC5927 to drive the logic level of the gate of the MOSFET.
Intuitively it seems that it would work fine in my head, but I'm not experienced enough to know if there will be any problems saturating a MOSFET with a constant current sink driver.
Thanks in advance!
Wimberleytech:
I think you want to exchange the source and drain connections.
moffy:
If you are looking at 7A current, then you would tend to use a switchmode constant current source to limit power dissipation in the switching device.
Oleenick:
--- Quote from: moffy on December 31, 2019, 04:28:50 am ---If you are looking at 7A current, then you would tend to use a switchmode constant current source to limit power dissipation in the switching device.
--- End quote ---
That makes sense, but in the design I'm already using these constant current sink drivers for 512 LEDs (30mA sink per channel).
I don't have much experience yet, so would you say it's worth it to implement switchmode constant current source ICs for the 8 anode control point (the ~7A points)? This is just for a one off project.
--- Quote from: Wimberleytech on December 30, 2019, 07:20:49 pm ---I think you want to exchange the source and drain connections.
--- End quote ---
At the moment the top pin the the drain and the bottom is the source on the MOSFET in that diagram. When it is saturated I intend for the LVL0 net to be placed at a +5V potential. I think I see what you mean, that the drain pin should be where the current is "drawn" from through source. But then that diode on the diagram shows that current flows from drain to source, or am I wrong assuming that? Could you elaborate on this to help me out of my confusion.
For reference I based it on this design from Kevin Darrah with the same component. Maybe the symbols have the D and S in different places?
Thanks.
mikerj:
--- Quote from: Oleenick on December 31, 2019, 08:58:26 am ---
--- Quote from: Wimberleytech on December 30, 2019, 07:20:49 pm ---I think you want to exchange the source and drain connections.
--- End quote ---
At the moment the top pin the the drain and the bottom is the source on the MOSFET in that diagram. When it is saturated I intend for the LVL0 net to be placed at a +5V potential. I think I see what you mean, that the drain pin should be where the current is "drawn" from through source. But then that diode on the diagram shows that current flows from drain to source, or am I wrong assuming that? Could you elaborate on this to help me out of my confusion.
For reference I based it on this design from Kevin Darrah with the same component. Maybe the symbols have the D and S in different places?
(Attachment Link)
Thanks.
--- End quote ---
The direction of the body diode on your MOSFET should give you a big clue something is not correct :) You have a P channel device, so the gate must be at a lower potential then the source to turn it on. This means you need the source pin connected to the supply rail, and the gate pin then gets pulled toward 0v to switch the transistor on.
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