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Using Darlington Tranistors as BJT

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Oleenick:
Hello EEVBog Forum!

Some Background:
I'm working on a version of Kevin Darrah's 8x8x8 RGB LED Cube:

He completes his using protoboard but I want to make mine in a compact PCB form. In the interest of compactness I started looking into arrays of components in a single package.
There are a lot of NPN 2N3904 transistors used throughout the design which will take up most of the space on a PCB.

I did some reseach and found transistor array ICs such as the ULN2803A (https://datasheet.lcsc.com/szlcsc/STMicroelectronics-ULN2803A_C73936.pdf).
I hit a stumbling block when trying to decide if this would be a suitable replacement for the 2N3904 in Kevin Darrah's design: https://www.kevindarrah.com/download/8x8x8/Cube_sch_3_16_13.pdf

The design uses them to switch PNP MOSFETs which drive currents of up to ~10A (192 overdriven LED anodes) as well as switching smaller currents of up to ~40mA (192 individual LED cathodes), all switched by shift registers (74HC595).
Therefore, I was thinking of replacing them with these ULN2803A darlinton arrays.

My Questions:

I searched on Digikey for BJT arrays but only found darlington transistor arrays (https://www.digikey.com.au/products/en/discrete-semiconductor-products/transistors-bipolar-bjt-arrays/277). Am I missing something? Or does this suggest that darlington arrays are drop in replacements for BJT arrays?

* Are there any limitations when using darlington transistors/arrays in place of BJTs that I'm not aware of?
* Do darlington transistors react well to fast switching speeds (8MHz) similarly to BJTs?
* What are your thoughts on using a transistor array such as the ULN2803A for switching small currents? Overkill or a neccesary compacting measure?

Ian.M:
You could get rid of the whole mess of BJTs by replacing the 74HC595 chips with TPIC6C595 power shift registers.   They are like a combo of a 74HC595 and eight output transistors except non-inverting, good for 100mA at up to 30V.  You'd still need the P-MOSFETs and their gate pullup resistors.

Edit:  corrected typo: TPIC6C596 to '595. The link and description were correct.   A TPIC6C596 has subtly different behaviour: unlike a 74HC595 or  TPIC6C595, instead of only the shift register being cleared by /CLR, both the output and shift registers are cleared. 

Oleenick:

--- Quote from: Ian.M on October 08, 2019, 06:12:18 am ---You could get rid of the whole mess of BJTs by replacing the 74HC595 chips with TPIC6C596 power shift registers.   They are like a combo of a 74HC595 and eight output transistors except non-inverting, good for 100mA at up to 30V.  You'd still need the P-MOSFETs and their gate pullup resistors.

--- End quote ---

Thats fantastic, thanks for reccomending that chip. I'm guessing this way I would also not require pullup resistors for either the P-MOSFETs or the LED cathodes.

My only concern is the heat dissipation of these ICs. I'll only be using each output for a max of 40mA. Is there a total package current limit or would that be 100mA * 8 = 800mA continuous sink?
In my use case I'd like them to be densely packed but from the thermal design advice in that datasheet I'm unsure if thats a good idea. Should I be worried about that at 40mA per output?
I'm unsure if this is revealed in section 6.4 Thermal Information, I don't know how or if that data relates to this.

Here is a screenshot from the datasheets layout example: https://imgur.com/a/YPIjzMT
Are these large ground copper areas on top and bottom neccesary? Is this predominantly for current or thermal reasons?

I appreciate your help.

oPossum:
Figures 6 & 7 provide some general guidance for maximum current.

The TPIC6A595 and TPIC6B595 have lower on resistance so they will run cooler at the same current.

Oleenick:

--- Quote from: oPossum on October 08, 2019, 12:48:26 pm ---Figures 6 & 7 provide some general guidance for maximum current.

The TPIC6A595 and TPIC6B595 have lower on resistance so they will run cooler at the same current.


--- End quote ---

Very interesting! I just checked out the TPIC6A595 and 596. The only difference I see with the 596 is that when SRCLR is low, all registers are cleared whereas on the 595 only the inputs are cleared.
That means I'll probably stick with the 595 varient as thats what I have considered so far.

Is it a good choice to use the TPIC6A595 over the TPIC6B595 just because of its higher per drain current capability of 350mA against the TPICB595's 150mA?
To me that seems like a way to reduce temperatures by capitalising on the higher rated output current.

Thanks again!

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