exceed allowed input voltage of NAND Gate   - Page 1
Forum section: electronics / projects

author: max@rp-engineering.de
author_rank: Newbie
timestamp: April 26, 2022, 08:14:59 am
content: Hello,i need a low power NAND for a 1-4,2V Application. Sadly there is no compatible device on the market.So i ask myself if it will work to use a SN74AUP1G00 which allows 0,9 to 3,6V, but has absolut maximum ratings of 4,6V on the input.I dont have any expericene with going into the max area of such components.What do you think would this be a solution? Or should i forget it.ThanksMax

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 26, 2022, 08:22:05 am
content: That's a disgustingly wide supply range, what is it?Tim

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 26, 2022, 09:38:20 am
content: Quote from: max@rp-engineering.de on April 26, 2022, 08:14:59 amYou can supply more than the supply voltage by limiting the current through the protection diode. The datasheet for the microcircuit must have allowable currents. Those the series resistor will limit the current and you can work safely.

author: Zero999
author_rank: Super Contributor
timestamp: April 26, 2022, 10:25:47 am
content: If it doesn't need to be fast and an open collector output, with a pull-up is acceptable, use a couple of BJTs.

author: max@rp-engineering.de
author_rank: Newbie
timestamp: April 27, 2022, 02:46:45 pm
content: Thanks for your fast answers.the cirquit should be able to power via AA batery (1-1,5V) but also Lithium Batteries up to 4,2V. thats why the range is that strange.I used some ultra low voltage mosfets to build the logic, this works fine, speed is no problem here.Thank you guys

author: Zero999
author_rank: Super Contributor
timestamp: April 27, 2022, 06:42:24 pm
content: BJTs will work down to 1V and a little below.

author: magic
author_rank: Super Contributor
timestamp: April 27, 2022, 08:22:03 pm
content: Quote from: MegaVolt on April 26, 2022, 09:38:20 amWhile CMOS ICs are said to typically contain zeners that limit VCC overvoltage in the event of ESD, it isn't common for them to be rated for operation under continuous breakdown of said zener.The rating which typically is provided for 74xx-style gates is continuous current through I/O pin protection diodes. These are ordinary diodes that simply redirect the current to appropriate supply rail.

author: hans
author_rank: Super Contributor
timestamp: April 27, 2022, 08:50:29 pm
content: Perhaps something to investigate: how much current can the supply rail "sink". Normally these overload protection circuits try to dump the extra current onto a live supply rail, assuming that the active load on that rail far exceeds the input overload current. It's basically assuming that the power rail is 2 quadrant to a limited degree.Say an input is 'protected' with a 1k series resistor, and a diode dumps excess input current into VCC. Then at 1V overload, the supply rail will need to sink approximately (1-0,7)/1k= 300uA. Normally the voltage regulator will supply 300uA less to keep the rail stable. An inconsiderable amount of current typically, but for a battery powered device (with non-rechargable battery) that's taking uA's it might pull the rail up high. Some voltage regulators (or batteries) may not like it's output being pulled higher,..(or if the device is unpowered, you could actually power the device via these diodes - or put it in an undefined state causing issues)Obviously you can increase the resistor to 100k or something large, but also need to watch out what the input leakage/current is for the gate itself, and that it stays operational at the lower end of the supply range.Alternatively, if the inputs are normally at a specific state, you could add some inverters made with discrete FETs and a pull-up/down, before the NAND gate, that only conduct a small bit of current whenever an input is active. Then you only need to find a suitable FET that has a low threshold voltage, and can sustain a reasonable gate voltage.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 27, 2022, 09:42:34 pm
content: I've used gates in the NLSV series, such as the NLSV1T244 in one project. 0.9V to 4.5V.Caveat: it's tough to hand-solder (if you need to), but it's doable.