Ruggedizing a VOM1271 - Page 1
Forum section: electronics / projects

author: marc426
author_rank: Newbie
timestamp: December 29, 2017, 05:14:14 pm
content: Hi Everyone,It's my first post here as I'm trying to find a welcoming home to discus all my electronic-related dwellingsI've already asked a couple a questions on ee stackexchange but I'm not sure the way questions go really fits my old-school forum habits... So here I am!Now onto the technical bits, we're doing a poor-man's 40V/600A DC load with big 1kW resistors that will very handily heat our workshop this winter (and hopefully we'll find a way to turn into a barbecue by summer).The resistors will be switched by a small low RdsOn MOSFET himself turned-on through a VOM1271 which has the advantage of not needing an isolated power supply...My concerns lie in the input of this gate driver : I'm to protect it from reverse voltage and overvoltage all the while making it able to cater with a voltage range going from 1,8-24V.So here's my design thinking1,8V Zener diode for overvoltage protection (able to take 24V as a digital signal)Resistor for current protection (100R ==> 50mA@5V)This one I'm really not sure about : a 0,1µF in order for the whole contraption not to turn-off between two "cycles" of the multiplexer. I know that'd definitely be a no-go in a fast-switching application but that's not the case there... I'm also guessing that the MUX chip will not change an output state between two update cycles but...Hoping I'm not gonna bother too many double stackexchange/eevblog readers and that some of you will share some of your expertise!Marc

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2017, 09:27:29 am
content: There is no need for an overvoltage zener. The input is just an LED, which will need to have its current limited to the appropriate level. Driving the LED at the maximum current of 50mA, is a bad idea. The recommended operating current is between 5mA and 30mA. You will need an active constant current source, as 1.8V to 24V is too wider range for the usual current limiting resistor. Adding a capacitor will delay the turn on and turn off of the circuit and may cause brief current spikes to be drawn from whatever is driving it.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2017, 10:21:23 am
content: Quote from: blueskull on December 30, 2017, 09:48:56 amUnfortunately it isn't specified to work below 2.7V. The original poster wants it to work down to 1.8V.I had a play in LTSpice and this is what I came up with. The current regulation hangs on down to about 1.7V, before collapsing completely. It doesn't have any reverse polarity protection, beyond about 5V. No doubt there's a better way to do this.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2017, 08:45:02 pm
content: It turns out there is a better way. The LM10 could be used. It's a 200mV voltage reference + a buffer op-amp and general purpose op-amp. It will work from 1.2V to 40V.http://www.ti.com/lit/ds/symlink/lm10.pdfThere's still the question of reverse polarity protection, which could be implemented with a Schottky diode in series with the LM10 or a polyfuse and reverse parallel diode. The IR LED should protect the output stage from reverse voltage, as the breakdown voltage will be greater than 24V, even if a much lower reverse maximum voltage is specified on the data sheet.

author: marc426
author_rank: Newbie
timestamp: December 31, 2017, 08:52:49 am
content: Hi Everyone,Thanks for the answers, it gets interesting!Especially on the LDO option which indeed doesn't fit the 1,8V-up requirement but I'm willing to start at 3V or so I can have everything in a small package...Now here's the interesting part, I had actually ditched the LDO option after reading this post here :https://electronics.stackexchange.com/questions/99640/use-voltage-regulator-instead-of-zener-to-protect-gpio/99645#99645What's your view on this?Also, would an LDO be compatible with a high speed input? How does it behave at high frequency / high speed rise?Regarding the LM10, I'm not completely sure how to use it...Oh and also, I need this on a number of inputs (20-100) so the cost is a big criteria in all this... I'd be quite happy with the 0,30€ LDO should it be adequate.

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2017, 09:29:59 am
content: Quote from: marc426 on December 31, 2017, 08:52:49 amThe answer is on the data sheet. I've just looked at the data sheet for the TPS70918 and it doesn't have reverse polarity protection. A Schottky diode can be added in series with the regulator, but it will increase the minimum operating voltage. A MOSFET can also be used for reverse polarity protection, but you need one specified to pass 20mA or so, with a gate voltage equal to the minimum supply voltage.https://www.infineon.com/dgdl/Reverse-Batery-Protection-Rev2.pdf?fileId=db3a304412b407950112b41887722615QuoteThat might be an issue. Again, refer to the data sheet. The start-up time is listed on page 3. It depends on the output voltage.http://datasheet.octopart.com/TPS70925QDBVRQ1-Texas-Instruments-datasheet-21773742.pdfQuoteWhat do you mean? Please refer to the schematic I posted previously and the link to the datasheet. Ask specific questions.QuoteWhat's your budget? The LM10 is quite pricey.