Can you explain input capacitance? - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 02, 2012, 08:32:39 pm
content: I'm working with a 74HC4050 -http://www.ti.com/lit/ds/schs205i/schs205i.pdfI'm wondering about the inputs and how they might relate to mosfets. I'm seeing the spec for Input Capacitance and I'm wondering if this is the same as it is on a mosfet. If the input is High and then there is no longer signal there, will it stay high until the the capacitance bleeds off? Should pulldown resistor should be used on these inputs?

author: Psi
author_rank: Super Contributor
timestamp: November 02, 2012, 08:38:58 pm
content: Short answer:Everything has some capacitance and transistor/fet/logic gate inputs require practically no current, so always have something connected to an input, be it a pullup, pulldown or some other output.If it floats it will add uncertainty to your circuit.I'm sure someone will also post the long answer

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 02, 2012, 08:53:22 pm
content: OK, how about a rule of thumb for dissipating the capacitance with a pull down?

author: IanB
author_rank: Super Contributor
timestamp: November 02, 2012, 09:01:04 pm
content: Quote from: Szewczykm on November 02, 2012, 08:53:22 pmI guess it depends, but something like 10k would be my unqualified and unconfirmed suggestion as a starting point?

author: Psi
author_rank: Super Contributor
timestamp: November 02, 2012, 09:10:17 pm
content: Quote from: Szewczykm on November 02, 2012, 08:53:22 pmYou cant get rid of the capacitance, all you can do is take it into account when selecting parts and check if it will cause any issues in your circuit.If you want a rule of thumb for pullup/down resistor value anything from 1k to 50k is pretty normal.But even a 1Meg resistor will discharge a 10pf input in 10us or so

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 02, 2012, 09:28:57 pm
content: Is it fair to say that if you want to dissipate the capacitance as quickly as possible you need to balance having a lower value resistor with how much current will flow through the resistor when the input is high?

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 02, 2012, 10:50:47 pm
content: You don't dissipate capacitancebut you add or remove charge. The bigger the capacitance, the longer it takes to charge up/discharge down to a certain voltage level for a given current. If you want to switch faster, the current needs to increase. Once the capacitor is fully charged/discharged, no current will flow at all.Whether you need a pullup/down resistor depends on what is driving the input. If it's a push-pull output from another chip you need no resistor at all. How fast the cap charges depends on how much current the driving output can deliver or sink (not necessarily symmetrical). You obviously need a pull-up resistor if the driving output is a open-collector/drain type out that can only sink current, current will flow constantly while the output is low.You do not always maximum switching speed, unnecessarily high edge rates only cause EMI trouble. There is no point in 1ns rise/fall times for a 9600 bps serial port