Using Darlington Tranistors as BJT - Page 1
Forum section: electronics / projects

author: Oleenick
author_rank: Contributor
timestamp: October 08, 2019, 04:34:07 am
content: 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.pdfThe 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?

author: Ian.M
author_rank: Super Contributor
timestamp: October 08, 2019, 06:12:18 am
content: You could get rid of the whole mess of BJTs by replacing the 74HC595 chips withTPIC6C595power 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: TPIC6C596to '595. The link and description were correct. ATPIC6C596has 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.

author: Oleenick
author_rank: Contributor
timestamp: October 08, 2019, 11:49:10 am
content: Quote from: Ian.M on October 08, 2019, 06:12:18 amThats 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/YPIjzMTAre these large ground copper areas on top and bottom neccesary? Is this predominantly for current or thermal reasons?I appreciate your help.

author: oPossum
author_rank: Super Contributor
timestamp: October 08, 2019, 12:48:26 pm
content: 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.

author: Oleenick
author_rank: Contributor
timestamp: October 09, 2019, 11:05:53 am
content: Quote from: oPossum on October 08, 2019, 12:48:26 pmVery 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!

author: thinkfat
author_rank: Unknown
timestamp: October 09, 2019, 11:55:10 am
content: I guess you'll be needing high-side drivers, too, I've used TBD62783 for LED matrix applications with good success.Another suggestion to further reduce BOM items, use a TLC5916 as the low-side sink driver. It's similar to the TPIC6A595, but it works as a constant current sink with a single resistor to set the current.

author: Oleenick
author_rank: Contributor
timestamp: October 14, 2019, 08:43:00 am
content: Quote from: thinkfat on October 09, 2019, 11:55:10 amUnless I'm mistaken I don't need high side drivers as I am only connecting and disconnecting the GND of the LEDs using the TPIC6A595 or possibly the TLC5916.The TLC5916 looks very interesting and since I have 64 connections to switch of each colour, I'll be able to use 8 drivers per colour with current settings to match said colour.I'm always astonished to find these magical application specific ICs, is there somewhere like the TI website that makes it easier to find them or is it down to experience?

author: thinkfat
author_rank: Unknown
timestamp: October 14, 2019, 09:27:30 am
content: Well, unfortunately I don't have a recipe here for finding these, the landscape is just too big. It basically boils down to finding the right "enchantment" to search for at mouser or digikey and then using the parametric search to home in.If you want to further reduce BOM count, I think there's also a 16 channel version of the TLC5916. I leave it to you for hunting it down, now that you know the manufacturer and the family, it shouldn't be that hard.Regarding the high-side drivers, I thought you were doing an LED matrix, and those are usually arranged in rows of which only one is enabled at a time.

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 14, 2019, 04:02:48 pm
content: Quote from: Oleenick on October 14, 2019, 08:43:00 amI have spent many late nights just leafing through some Aliexpress dealer's collection of surplus ICs and searching the web for part numbers and documentation to see if the IC is interesting to me. Sometimes it isn't very interesting at all, just another switching converter or low-rent MCS-51 microcontroller. Sometimes it is just what I need at that moment.And sometimes it's a chip with some very interesting capabilities that I want to file away for later. By this method, several months ago I found the Macroblock MBI5050, a 16-column PWM LED driver with 8 rows of memory. If you can find the "confidential" app note, find a source suited to your quantity needs, and meet its slightly convoluted timing/clocking requirements, it might be just perfect for you.

author: Oleenick
author_rank: Contributor
timestamp: October 17, 2019, 09:22:27 pm
content: Quote from: thinkfat on October 14, 2019, 09:27:30 amQuote from: jhpadjustable on October 14, 2019, 04:02:48 pmThank you all for your help. I'm well on my way to deciding that hardware to use now.For the high side drivers I'll be using 8 IRF9Z34N P-MOSFETs, for the 8 horizontal rows that I will be quickly cycling and then turning on individual LEDs in verticle columns.This will draw MAX currents of 6A so I believe MOSFETs are the way to go, unless you advise otherwise @thinkfat?