Questions about TTL and converter TTL 5V to 24V (1Mhz) - Page 1
Forum section: electronics / beginners

author: FlowzZz
author_rank: Newbie
timestamp: January 11, 2016, 11:16:48 pm
content: Hi everyone,I would like to convert a TTL 5V signal to 24V at 1Mhz.For that I read the TTL spec but I don't understand how to interpret TTL spec.IOL = 16 mAIOH = 400 µAIIL = 1.6 mAIIH = 40 µADoes someone could explain me?I would prefer that both circuit (5V/24V) was isolated, eventuallly protected against polarity inversion and limited to 5V on the 5V side. Could you advise me about these protection ?I have read about optocoupler to do a part of the job (high frequency) and found two interesting references :- ACPL-064L-060E (1.6mA but not sure that is it possible to take 24V on the output side)- ACPL-054L-000EAre these references ok? Do you know references that allow to get 1Mhz with 1.6mA Led and which can deal with 24V on the phototransistor side ?With these references, is it necessay to have a transistor stage on the input side (5V TTL) or is the optocoupler sufficient to respect TTL spec.?What about the output side? Is the output current limited in the case where the optocoupler is not followed by a transistor stage ?Thank you in advance and sorry if I am not really clear,

author: Paul Price
author_rank: Super Contributor
timestamp: January 12, 2016, 01:04:25 am
content: Your part "references" are made for <6.5Volts max and wrong for this application.Fairchild Semi H11A11 series of optocouplers might be the ticket for this ride using an active low level of a TTL gate with a 220-ohm resistor in series.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 12, 2016, 03:38:10 am
content: For what purposes?What is required to protect against, on the 5V/TTL side?What is required to drive, on the 24V side?What risetime are you expecting?Is 1MHz the clock frequency, bit rate, maximum pulse repetition frequency...? Is it a continuous signal, or variable, with 1MHz being a typical or guaranteed maximum?How much delay is tolerable? Phase shift? Skew? Do you need multiple channels with matched delay (e.g., gate driver)?Tim

author: FlowzZz
author_rank: Newbie
timestamp: January 13, 2016, 11:19:28 pm
content: Thank you for your answers.If I am not mistaken ACPL-054L-000E could have until Vo=24V on the output side.I don't understand the CTR, does it mean that the maximum output current is 130% of 3mA (if 3mA drive the diode) ? So if the system in the output side need more than 3mA, I need to use a transistor or a gate on this side?The purpose is to synchronize a hardware system which take 0/24V signal in input.I am expecting about or below 1us for the risetime and falltime. I think that 1Mhz can be viewed like a clock frequency. In input, it is a 0/5V TTL signal in input (continuous signal). A delay of 3us is tolerable. One channel is sufficient.On the 5V side, I want to protect the optocoupler if a 24V signal is connected by mistake for example.Thank you for your help,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2016, 07:24:12 am
content: Uh, well, 1us is a full cycle of 1MHz, so you're saying you don't need any signal on the 24V side..?What kind of hardware system is this? Perhaps you mean ms rather than MHz?Please answer all these questions in as excruciating detail as you can muster!Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2016, 11:20:20 am
content: A logic opto-isolator+74LS07 high voltage buffer with pull-up resistors.http://www.vishay.com/docs/83685/sfh6731.pdfhttp://www.ti.com/lit/ds/symlink/sn74ls07.pdf

author: FlowzZz
author_rank: Newbie
timestamp: January 16, 2016, 04:22:20 pm
content: Of course, I want a signal on the 24V signal, so yes bellow 500ns (for risetime / falltime) should be better. The current to drive is at least 10mA at the output. I don't know about skew or phase shift.Thank you for your suggestion Hero999. I did not mention that the current to drive (at the output) was 10mA, so is it ok to drive 10mA with that solution?What I don't understand with the TTL and the phototransistor (logic opto-isolator) is that according to the TTL spec Ioh is 400uA. If I connect a phototransistor on the TTL output (with for example If=1.6mA), the diode of the phototransistor should not be active with that low current. So I can't directly connect a phototransistor to the TTL output, right?

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2016, 10:54:31 pm
content: Is that sourcing, sinking 10mA or both? The 74LS07 can sink 40mA but it can't source any current so a pull-up resistor is required to enable it to go high.You mean connect a phototransistor to a TTL input? Yes, that would work but phototransistors are too slow for what you want to do.The SFH6731 optocoupler datasheet, I linked to in my previous post, can drive a TTL input with no problem and will operate at 1MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2016, 04:19:55 am
content: This is why it's so important to know what the load consists of.But OP doesn't know, or isn't dropping any clues to help...Tim

author: FlowzZz
author_rank: Newbie
timestamp: January 17, 2016, 07:31:31 pm
content: Sorry I try to be as clear as possible. I attached a schema to be clearer.Quote from: Hero999 on January 16, 2016, 10:54:31 pmCurrently it is a sinking load, so I need to source at least 10mA. But I would like to be able to connect sourcing load as well (if it is possible) in the future. I read that the second element attached could be useful to be abe both to sink and to source a load.Quote from: Hero999 on January 16, 2016, 10:54:31 pmNo I mean connect the TTL output of an external device to the optocoupler part of the converter [(1) in the schema]. As TTL output (Ioh = 400ua < If = 10mA) I don't understand how it could work, but I certainly made a mistake.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2016, 08:52:35 am
content: That Darlington opto-coupler won't be fast enough to switch at 1MHz.The output of the 74LS07 with a pull-up will be able to drive an opto-coupler but the voltage will drop, depending on the load. If you used a 1k2 pull-up when you could just connect the opto-coupler's IR LED directly between the output and 0V and it will work, but the output voltage will fall to that of the forward voltage of the LED.

author: tggzzz
author_rank: Super Contributor
timestamp: January 18, 2016, 09:22:04 am
content: The OP is falling into the traditional newbie trap of telling us their proposedsolution. They would get a more helpful response if they characterised theirproblem. That would also have the advantage that we might spot problems that might arise even if we "solve" their proposed solution. For example, having an optocoupler tends to indicate electrical isolation is required - and there are other ways of achieving electrical isolation, and other nasty issues that can arise.A good start would be to completely characterise what they are driving, i.e. the range of loads and operating environment.In the OP's diagram, I don't see the relevance of the 24V section; it could be omitted if the optoisolater diode is connected from a totem-pole TTL output to +5V rather than 0V.

author: FlowzZz
author_rank: Newbie
timestamp: January 21, 2016, 11:10:46 pm
content: Ok, let's say that it could be a sinking or sourcing load between 0 and 24mA. Is there anything else I should mention to better characterize my load ?What do you mean by operating environment?[quote author=tggzzz link=topic=61178.msg844930#msg844930 date=1453108924In the OP's diagram, I don't see the relevance of the 24V section; it could be omitted if the optoisolater diode is connected from a totem-pole TTL output to +5V rather than 0V.I am a little bit lost.Thank you,

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2016, 11:17:00 pm
content: Quote from: FlowzZz on January 21, 2016, 11:10:46 pmWe need to see the bigger picture.What are you trying to achieve? What is it for? What does the device you're designing actually do?EDIT:The SFH6731 opto-coupler with the UC1708 to level shift the signal to 24V be able to what you're talking about but it's probably way overkill.http://www.ti.com/lit/ds/slus171c/slus171c.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2016, 03:51:02 pm
content: Quote from: FlowzZz on January 21, 2016, 11:10:46 pmYes! What *is* it?If you don't know what's physically inside the box, then what is the box? Something off-the-shelf? Does it have a datasheet?Can you take a picture of the thing!? Open it up and take it apart?Tim

author: tggzzz
author_rank: Super Contributor
timestamp: January 22, 2016, 05:34:30 pm
content: Quote from: FlowzZz on January 21, 2016, 11:10:46 pmYes. Tell uswhatproblem you are trying to solve; don't tell ushowyou are trying to solve it.Tell us what the load actuallyis, not just a few of itsmanycharacteristics.