Driver recommendation that has decent performance down to 2.5V... - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: November 09, 2019, 05:11:45 pm
content: I'm working on a project that will drive four LTP-305 5x7 LED displays and then two rows of some high efficient LED's. Basically it will be handled with 20 columns and 9 rows (7 rows LTP, 2 rows indicator). I've done this successfully with an atmega4809 microcontroller doing the multiplexing. The columns are driven directly from the port pins and the rows need to be switched somehow. I've done this with some 2N7002, but I can certainly see a difference between 1 column being lit and 20 columns being lit in terms of current consistency. That can be overcome by firmware adjustment to give the display a consistent brightness whether 1 or 20 without too much difficulty. Obviously it would be better if it were more consistent in the first place aka a constant current driver.So now is where it gets more complicated: I'd like to make it support a voltage range of 2.5V to 5V and I want to do it with through hole components only. The atmega4809 has a DIP version and just enough pins so that works, but looking at the current graph of the 2N7000 shows it isn't passing much current at lower voltages like 2.5V. I fully expect it to be dimmer at 2.5V, even to the point of barely working, but the goal is that it is almost functional down to 2.5V if that makes sense.Any ideas on what I should look at? Do I just need better row switching devices? I was looking at Digikey and wasn't making much sense of the selection menus to find a more suited FET. Again, it has to be a through hole solution. I've used an HT1632C before to do this, but it is SMD.

author: alank2
author_rank: Super Contributor
timestamp: November 27, 2019, 09:00:32 pm
content: Still thinking about this. The current does not need to be super at 2.5V, but still enough to make the display visible.In summary, I'm looking for something that will work for 2.5V - 5V. 5V current might be around 250mA at 10% duty cycle.Should I just try something like a 2N7000, BS170, or BS270 to see how it does?This will be something someone might touch with static electricity - should I shy away from a FAT and use a BJT instead? How will a BJT do with the 2.5V to 5V range. I guess the BJT route would require base drive resistors...

author: thm_w
author_rank: Super Contributor
timestamp: November 28, 2019, 01:02:00 am
content: 2N7000:https://www.onsemi.com/pub/Collateral/2N7000-D.PDF- 200mA 60V- Vgsth = 3V max2N7002:https://www.diodes.com/assets/Datasheets/ds11303.pdf- 210mA 60V- Vgsth = 2.5V maxSo the 2N7000 is going to be worse.Digikey lets you search by Vgsth, I see 22 parts in TO-92 package with rating of 1-2V.You can search through those to see if any meet your requirements.

author: alank2
author_rank: Super Contributor
timestamp: November 28, 2019, 02:28:44 am
content: The Vgsth also has a current with it like 2.5V @ 1mA and so on - what is the relevance of the current part of it? Can you compare two that are like:2.5V @ 1mA2.0V @ 250uA

author: thm_w
author_rank: Super Contributor
timestamp: November 29, 2019, 12:12:40 am
content: Quote from: alank2 on November 28, 2019, 02:28:44 amThats the point at which 1mA flows through the drain. So you can't directly compare the two numbers, but they are close enough.You can look at some of the graphs as well.

author: alank2
author_rank: Super Contributor
timestamp: November 29, 2019, 02:39:55 pm
content: So it would take more voltage than that for any sizeable amount of current to flow? Sounds like the chart/graph might be better to determine how much current for how much voltage...

author: thm_w
author_rank: Super Contributor
timestamp: November 29, 2019, 10:45:01 pm
content: Quote from: alank2 on November 29, 2019, 02:39:55 pmCorrect, figure 2, 250mA ~4.5V. Or 0.6A at 6V.TN5325N3 bit better at 0.6A 4.5V:http://ww1.microchip.com/downloads/en/DeviceDoc/20005709A.pdfSeems like no TO-92 is going to be ideal though.

author: alank2
author_rank: Super Contributor
timestamp: November 30, 2019, 12:30:12 am
content: Will a BJT be any better at what I am looking at? Does it depend on the base drive strength?

author: thm_w
author_rank: Super Contributor
timestamp: November 30, 2019, 01:36:32 am
content: It looks like it would be better at low voltages yeah, at the cost of efficiency and higher IO output current requirement.MPSA05:Vbe = 1.2VHfe = 1005mA base current at 2.5V = 260R base resistorBase current at 5V = 15mA

author: mariush
author_rank: Super Contributor
timestamp: November 30, 2019, 09:57:10 am
content: Would you be able to make a single exception for a SOT-23-5 / SOT-23-6, or maybe you'd consider using a SOT-23 to DIP8 adapter pcb?You could basically use a switching regulator to boost any voltage to 3.3v or 5v and then your hassles are over.Or have the microcontroller or whatever power everything directly if the voltage is higher than 3v (use a mosfet or something to power the rest), or enable a switching regulator to boost to 3.3v if the voltage is below 3v.You can get SOT-23-5 regulators likeAP301or SOT-23-6 regulators likeAP1603orMCP1640They don't have the same pinouts, but if you make your adapter boards you can easily make your own pinout (rearrange the pins)You can easily find SOT-23 to DIP pre-made boards, here's an example - 20pcs for 2.5$:https://www.ebay.com/itm/20pcs-6-pin-SOT23-TO-DIP-Adapter-PCB-Socket-Breakout-Convertor-Board/332579763865