TTL level converter - 1.5V to 5V - Page 1
Forum section: electronics / beginners

author: Cj1
author_rank: Contributor
timestamp: June 24, 2018, 01:55:05 am
content: https://www.sparkfun.com/products/12009I have 2 1.5V signals that i want to convert to 5V signals (VGA H/V sync). Would this board work for that purpose? I was going to use this circuit:https://imgur.com/a/DJPT0Lwbut would rather not build it if i don't have to.

author: sleemanj
author_rank: Super Contributor
timestamp: June 24, 2018, 02:05:04 am
content: Unlikely to function, 1.5v is too low with regard the FET gate threshold.

author: exe
author_rank: Unknown
timestamp: June 24, 2018, 09:12:39 am
content: It might need swapping fets with those that work from 1.5V. I'd ask them what fets are used (hopefully they know). Or may be they can even check this for you... Although, there should be some headroom because parameters may drift with temperature and age.

author: DuPe
author_rank: Regular Contributor
timestamp: June 24, 2018, 09:22:53 am
content: hi folks,i dont see any fet's in the imgur circuit, so why it should not work in principle?only issue might be, that first transistor might go into saturation so speed can be too low for H-sync signals

author: sleemanj
author_rank: Super Contributor
timestamp: June 24, 2018, 09:31:10 am
content: Quote from: DuPe on June 24, 2018, 09:22:53 amThe poster is asking about the sparkfun board, which uses fets to accomplish the shift. The specs are on the sparkfun page.

author: exe
author_rank: Unknown
timestamp: June 24, 2018, 09:44:25 am
content: Quote from: sleemanj on June 24, 2018, 09:31:10 amOh, I missed that. They use BSS138 fets with threshold voltage 0.8-1.5V (with 1mA drain current => ~3kOhm Rds at 3V Vds in the worst case ). So, may work, but...

author: rstofer
author_rank: Super Contributor
timestamp: June 24, 2018, 04:49:31 pm
content: There are many level shifter ICs. The first I stumbled across today is the Maxim 14611.https://datasheets.maximintegrated.com/en/ds/MAX14611.pdfI didn't check timing but level wise it should work.Of course, there are many others. TI makes a some...

author: Cj1
author_rank: Contributor
timestamp: June 26, 2018, 02:48:10 am
content: I guess i will order the Maxim 14611, looks like it might work? Unless there is a better one...

author: rstofer
author_rank: Super Contributor
timestamp: June 26, 2018, 05:20:09 pm
content: Quote from: Cj1 on June 26, 2018, 02:48:10 amIs 20 Mbps sufficient? I don't know your application and VGA encompasses a lot of video rates but it would seem that even the slowest might be more than the 14611 can handle.https://www.maximintegrated.com/en/products/interface/level-translators/MAX14611.html/tb_tab1TI makes a series of translators but with a quick search all I found was 1.8->3.3V and 3.3V->5V. I didn't see a device that went from 1.5V->5V in one step. Then again, I didn't do a lot of searching.1.5V seems like an odd voltage. 1.8V is pretty common.You need to do some research before picking a device. Some of the TI devices will go to 50 Mbps so that is a help. I didn't look for any other parts at Maxim either.The Maxim part may be fine as you are only using it for the sync pulses and they are pretty slow. I doubt that it is fast enough for the pixels.

author: Cj1
author_rank: Contributor
timestamp: June 27, 2018, 04:22:13 am
content: Here is all the information I have. First is a diagram with an explanation and second is the user running it through his scope, and showing success. So i know for sure the original circuit works at least:https://imgur.com/a/4nDwoTH(1.52 VPP - 5.20 VPP)This would be for resolution of 640x480 @ 60hz, no other resolutions would be used.

author: FrankBuss
author_rank: Unknown
timestamp: June 27, 2018, 06:54:12 am
content: Quote from: rstofer on June 26, 2018, 05:20:09 pmDigikey is very useful for searching. I found this:http://www.ti.com/lit/ds/symlink/lsf0102.pdf2 channel, by some analog magic bidirectional without a direction pin, 100 MHz.

author: rstofer
author_rank: Super Contributor
timestamp: June 27, 2018, 04:53:41 pm
content: Quote from: FrankBuss on June 27, 2018, 06:54:12 amOutstanding search! I didn't stumble across that device yesterday. I was looking at uni-directional and I probably should have opened up the search.100 MHz ought to cover VGA.

author: Cj1
author_rank: Contributor
timestamp: June 28, 2018, 05:35:18 am
content: Hm, so looking at the diagram for the LSF0102, it would be wired like:A1:H 1.5V inputA2:V 1.5V inputB1:H 5V outputB2:V 5V outputEN:+5VGnd:GroundIt looks like the Vref is for some kind of switching? Im assuming the level converter is always on due to +5V and doesn't need to have switching applied? Or did i read that wrong?

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2018, 07:52:52 am
content: Quote from: Cj1 on June 28, 2018, 05:35:18 amVref_A and Vref_B are the power supply voltages. Please read the data sheet in full. I know data sheets are often difficult to read, but we're here to help.http://www.ti.com/lit/ds/symlink/lsf0102.pdf

author: rstofer
author_rank: Super Contributor
timestamp: June 28, 2018, 02:05:13 pm
content: When you look at Figure 6, you need to recognize that they are coupling logic at 1.2V, 2.5V and 3.3V. GPIO3 is 1.2V, GPIO 4 is 2.5V and GPIO1,2 are 3.3V It's quite a circuit.VrefA needs to be the lowest level logic voltage, or 1.5V in your case.VrefB needs to be the highest level logic voltage or 5.0V in your case, don't forget the pull-up resistorVen = 5VAny device that has a 21 page datasheet is going to take some reading.It doesn't seem that the device wants to deal with very much capacitance 30 pf for 100 MHz (50 pf for 40 MHz). I have no idea how much capacitive loading the monitor, cable and PCB are going to provide. I wonder if this circuit is going to require some kind of cable driver. What is driving the other signals?