Break-before-make behaviour in analog switches - Page 1
Forum section: electronics / projects

author: pmbrunelle
author_rank: Regular Contributor
timestamp: July 11, 2017, 03:45:44 pm
content: Does anyone have experience using analog switches?I was planning on using the 1G3157-series switch:https://www.digikey.ca/product-detail/en/texas-instruments/1P1G3157QDCKRQ1/296-15610-1-ND/571008I would like to usetwoof them together in a DPDT configuration. My objective is to do a polarity-reversal setup, kind of like this:Essentially, it's the switch setup used to swap two wires back-and-forth.My concern is that, if both switches don't have the same reaction time, then I will be briefly shorting two leads together. The select pins will be shorted together to ensure that both switches change state together.A CMOS push-pull output will be driving the select pins at about 1 MHz.Opinions?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 11, 2017, 04:12:10 pm
content: My REAL concern would be, to employ a 7 to 15 ohm switch (from the datasheet) to drive a motor.If the motor stalls, or if the load has too much inertia, the current drawn by the motor will be many times the rated full-load current.I've seen motors which draw 60 mA no load, 200 mA full load, and well over 1 amp stalled.Unless you are driving a micro-motor with little inertia, you should be using a Mosfet full bridge instead.

author: Fungus
author_rank: Super Contributor
timestamp: July 11, 2017, 04:26:14 pm
content: The maximum switch time is 5.7ns, resistance is about 6 Ohms when fully on so at lest 12 Ohms under short circuit (circuit path goes through the chip twice).Quote from: pmbrunelle on July 11, 2017, 03:45:44 pmIn the worst possible case they're shorted for less then 0.5% of the time. You can work out a power dissipation from that.In reality it will never be a full short circuit if the inputs are tied together. When the fastest switch of each pair is fully open the other one will be in an intermediate state which is close to fully closed and will only last a nanosecond or so.I suspect you'll be fine. The danger of the "short circuit" will be swamped by the inductances in the motor, etc.nb. I'm assuming the motor is tiny. These switches are not suitable for driving anything more than a micro-motor.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: July 11, 2017, 04:36:48 pm
content: The motor picture (found on the internet, not mine) was for illustrative purposes only, to show the wiring of the switch.Source resistance: around 1 k?Load resistance: around 100 k?In reality, I would like to demodulate an AM signal, using the carrier (squared-off) of said AM signal to control the switches.The possible consequence of the shorting here is a degradation of the signal linearity, I wasn't too concerned about heating.The more I think about it, what I've done is really not kosher, and I should expect shoot-through.The next iteration of the circuit should use 4x SPST switches, and have a "fast turn-off/slow turn-on" scheme on the clock edges to guarantee windows of high impedance.I think...

author: Marco
author_rank: Super Contributor
timestamp: July 11, 2017, 05:38:32 pm
content: It's only a MHz, add some series resistance and forget about the shorts.

author: Fungus
author_rank: Super Contributor
timestamp: July 11, 2017, 06:12:58 pm
content: Quote from: pmbrunelle on July 11, 2017, 04:36:48 pmIn that case just put in some resistors on the source and stop worrying.