3-state buffer with switched input rail - decoupling? - Page 1
Forum section: electronics / projects

author: elcaron
author_rank: Contributor
timestamp: September 02, 2017, 11:53:44 am
content: Yesterday, I spend quite some time to find a3-statebuffer that can beswitched with 3.3V(so I would need HCT, not HC series) AND can deliver atleast 24mA current @ 5V. (This is to make a small footprint solution of a switchable strong pullup, switching between HIGH and high-impedance.) Eventually I found the74LVC1T45.It has the peculiarity that it is explicitly advertised as "3-state output", but does not have an OE pin. Instead, the intended way seems to be to pull the input rail low, resulting in all pins to assume high-impedance state. This is specified in the second paragraph of the datasheet. The input rail does not need a lot of current, so it can essentially be used just like an OE pin - with one caveat: power rails usually require decoupling, that that would reduce the switching speed.I cannot quite get my head around this. Is it a dirty hack to use Vcc(a) essentially like OE, or is it intended behavior, as the datasheet suggests? Is there a way to know for sure if a decoupling capacitor can be omitted? (In the end, i do not need fast switching speeds, but I would like to know if this is a clean solution or not.)Any thoughts?

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2017, 12:12:14 pm
content: The 74LVC1T45 is a single bit level translating bus transceiver, that supports partial power-down operation. The DIR pin controls its direction A to B or B to A and it does NOT have independently accessible output enable pins. I would count it as a mild hack to control it by driving the power pin on the 3.3V logic side from a logic signal, as you wouldn't want much capacitance on the line so you'll need to keep the 3.3V input side wiring as short as possible due to the lack of decoupling.

author: elcaron
author_rank: Contributor
timestamp: September 02, 2017, 12:15:21 pm
content: But if it is a hack, why does it stress the 3-state so much? "Dual supply translating transceiver;3-state", "The 74LVC1T45; 74LVCH1T45 are single bit, dual supply transceivers with3-stateoutputs"?Since I do not need fast speeds, I can still add decoupling, but I don't know what to think of this in general.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2017, 12:26:29 pm
content: Its a bus transceiver - a coupled pair of tristate buffers. Its the lineal descendent of the 'classic' 74HC245 octal bus transceiver, but to get it down to a single gate 6 pin SMD package they had to omit /OE as they needed a second supply pin to support level translation. They emphasise the tristate outputs as it isolates them in partial power-down situations, a rare and useful function.As you are using the 3.3V side only as an input (DIR hardwired), its decoupling needs will be minimal. The output side however will need good decoupling.

author: elcaron
author_rank: Contributor
timestamp: September 02, 2017, 12:35:11 pm
content: That is what I expected. Eventually, I am almost not switching at all, so in my case, it is not so much of an issue.I could go with a larger package, but I did not find any that would fulfill all three requirements, tthat is HCT-family or similar, 3-state with OE, and >24mA output current.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2017, 07:25:42 pm
content: Quote from: elcaron on September 02, 2017, 12:35:11 pmACT logic fulfills those requirements while maintaining a TTL input threshold but the 74ACT125 is of course a 14 pin package with 4 gates and not in smaller packages.Through experience I have become leery of relying on the maximum output drive specification for continuous operation so would tend to use either a discrete level shifter or double up 24 milliamp gates to provide 24 milliamps.

author: Ian.M
author_rank: Super Contributor
timestamp: September 02, 2017, 07:44:56 pm
content: If you don't need to be able to drive the output low in the same package, a small signal complimentary MOSFET might be a better choice e.g.ROHM EM6M2. Use the N channel as a level shifter, with a pullup resistor to +5V on the P channel gate.

author: elcaron
author_rank: Contributor
timestamp: September 03, 2017, 06:44:29 am
content: @DavidThanks fur the advice. Since I need the circuit 2 times, it is actually not that bad. In contrast to most buffers, it can actually switch all ports independently to Z. With the quad package, I could pair up to ports. I guess I connect them after the pullup resistors, right? Do I see correctly, that this IC does not have ESD protection?@IanThat is what I started my search with. It is still an option, though. I was trying to save component count with the 2-4 resistors involved (current limit to the gate is probably not really necessary, especially given the fact that I do not switch fast).

author: colorado.rob
author_rank: Frequent Contributor
timestamp: September 03, 2017, 11:22:21 am
content: NC7SZ125 seems to be rated to 24mA, with an Absolute Max of 50mA.

author: Ian.M
author_rank: Super Contributor
timestamp: September 03, 2017, 11:55:41 am
content: Yes, but it wont do upwards level translation to 5V as its Vih_min threshold is 70% of Vcc, which is above 3.3V.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2017, 04:20:18 pm
content: Quote from: elcaron on September 03, 2017, 06:44:29 amJust connect the outputs and inputs in parallel and use one pull-up resistor. The saturated drain to source resistances are about 25 ohms with a positive temperature coefficient so current sharing is not a problem.QuoteThe Fairchild datasheet specifies maximum input and output current through the protection diodes so as far as I know, all 74ACT125 inputs and outputs are protected against ESD.