Logic Gate Clamping - Page 1
Forum section: electronics / beginners

author: Tnixsw
author_rank: Contributor
timestamp: February 21, 2023, 08:36:45 am
content: Hi all,Anyone know if logic gates or logic family that can handle Vin > VCC and lower than GND via a resistance and rely on the input protection diodes to clamp the voltage.Tony

author: shapirus
author_rank: Super Contributor
timestamp: February 21, 2023, 09:56:50 am
content: Quote from: Tnixsw on February 21, 2023, 08:36:45 amlike this?

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2023, 10:58:30 am
content: Quote from: Tnixsw on February 21, 2023, 08:36:45 amThe answer is any logic gate with clamping diodes, so long as the resistor limits the current to within the maximum rating. Note that the current through the clamping diodes, also flows through the supply rails, so be sure the supply voltage isn't pulled up or down by the current. A zener diode with a 10% higher voltage than the supply can prevent the power supply voltage going to high.Also note that maximum voltages is often also specified on the data sheet. These indicate what the input can tolerate without a current limiting resistor. It's typically 0.3V outside of each power supply rail, to ensure the protection diodes never conduct.

author: Circlotron
author_rank: Super Contributor
timestamp: February 21, 2023, 11:21:55 am
content: So the gate won't be damaged if the input current through the clamp diode is not exceeded, but can proper operation always be guaranteed if the clamp diode is conducting? Might the diode current find it's way into the substrate and cause unexpected results?

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2023, 10:45:26 pm
content: Quote from: Circlotron on February 21, 2023, 11:21:55 amThe IC shouldn't go into latch-up, as long as the clamping diodes current isn't too high.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 22, 2023, 11:39:46 am
content: Indeed.As long as you stay within the specified clamping current it will work.Some 30 years ago Microchip made an application note for a light dimmer for one of their uC's and it uses a few resistors in series directly connected to the mains voltage as voltage crossover detection.But as time progresses, dies are shrunk and electronics get more fragile. The DC clamping current may be as low as 2mA, and for a lot of IC's it is not specified at all in their datasheets.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 23, 2023, 03:46:31 pm
content: If the driving signal has a fixed voltage swing then all you need is a simple voltage divider.Using the internal diodes and an external resistor to clamp the input is very a popular solution.In many applications you can get away with it.It may not always be the way to go if you have timing parameters to meet.The resistor and the intrinsic gate input capacitance will slow down the rise and fall time at the logic input.The resistor value will need to be low enough to minimise slew effects and high enough to protect the clamp.The over voltage clamp current will cause the diodes and the ic to heat up.During reverse recovery period the clamp diodes will still conduct causing unexpected artifacts at the gate input.The internal diodes are not designed to be in repetitive or continual conduction.They're intended to operate during transient over voltage events.

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2023, 06:12:23 pm
content: Quote from: Terry Bites on February 23, 2023, 03:46:31 pmdepends, plenty of ICs that specify operating conditions (not abs max) as a max voltage / max current and you just have to satisfy one of them

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2023, 06:15:56 pm
content: Quote from: Doctorandus_P on February 22, 2023, 11:39:46 amor they need 5V tolerance at <5V supply, so they can't have a diode to supply, but need a zener clampfor example STM32, and they are usually specified for +0mA