Using Isolated Gate Driver as General Purpose 3.3 to 12V Isolated Level Shifter - Page 1
Forum section: electronics / beginners

author: rheb1026
author_rank: Regular Contributor
timestamp: May 09, 2017, 04:54:47 pm
content: I'm wanting to isolate my digital and analog sections from my power section which includes a few solenoids. I was initially going to use opto isolators or digital isolators to send control signals across the barrier, but was curious if I could use an isolated gate driver as a general purpose level shifter instead? I'd imagine I would have to use a current limiting resistor of some sort between the gate driver and the receiving ICs. My control signals would be 3.3V from a MCU and the highest frequency would be ~35 kHz.I'd like to just use 12V in this power section and skip having to make a 5V power rail only for the digital isolators. Is this feasible?

author: Zero999
author_rank: Super Contributor
timestamp: May 09, 2017, 10:31:44 pm
content: Quote from: rheb1026 on May 09, 2017, 04:54:47 pmIt's possible but a bit perhaps a bit overkill.What output current do you need and how many channels?

author: retrolefty
author_rank: Super Contributor
timestamp: May 09, 2017, 11:06:30 pm
content: QuoteNo, Ohm's law says the load resistance of the receiving ICs will determine the max current draw, not the driver's max current output capacity. If your driving LEDs then we can talk about the need for current control/limiting.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2017, 07:04:43 am
content: You mean like those bootstrap gate drivers? Those need a supply on the "flying" side, and it can only "fly" "above" logic ground. Never more than, say, -6V below, and they're not functional while below.This is fine for the intended switching application, because the switching node is always coming back down to ground periodically, where the flying supply can be recharged with a diode: a bootstrap supply. Also, with logic ground = power ground, the flying side never goes below ground more than a little bit (due to transient overshoot).An isolated gate driver IC is just another opto, or logic isolator or whatever, but with extra guts and expense. (And yeah, if the output drive capacity suits your loads, that can be very handy.)As for digital isolators, those are usually 3.3V, what's wrong with that..? Okay so the output side isn't, but logic interface is very easy...Solenoids: are they not better to drive with an open-collector (or drain) switch? You can get arrays to do that (like the ancient ULN2003 and such), and you can save on isolator channels by using a deserializer (usually a 74HC595 shift register, or any number of SPI IO expanders).Tim

author: rheb1026
author_rank: Regular Contributor
timestamp: May 10, 2017, 02:01:05 pm
content: Thanks for the responses. I was thinking about something like TI UCC21520 or similarI should have been a little clearer in my initial post. I do need to drive one low side MOSFET (to turn on and off the solenoid) and need another control signal for the over current IC latch reset. Ideally I'd like a 3.3V to 12V digital isolator, but couldn't find any. By no means do I need a dedicated gate driver in my circuit, they just seemed like the only thing I could find that would do 3.3 to 12V isolated signals without having to go to a discrete opto-isolator.Is there another alternative?I'm trying to save board space and not have to put another 12 to 5V regulator on the board solely for the standard digital isolators

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2017, 05:45:14 pm
content: Quote from: rheb1026 on May 10, 2017, 02:01:05 pmA 5V regulator needn't take up that much space. You can get the LM78L05 or similar in a SOT-89 package which doesn't take up much space. A digital isolatior such as the Si8620BB is available in a SOIC-8 package.What would take up more space is if you needed 12V out, which would require a level shifting IC such as the CD4504

author: rheb1026
author_rank: Regular Contributor
timestamp: May 12, 2017, 03:08:08 am
content: No I suppose it doesn't have to, but dropping 12V to 5V is a fairly significant heat generator even at low current levels - especially with the small package regulators. eg: (12V - 5V) * 100 mA = 0.7WIf I can run the entire isolated section from 12V I can save myself the board space, component cost, and the heat generated by the conversion. All the other components in the isolated section can safely operate with 12V inputs which lead me to looking for a 3.3V to 12V isolator

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 12, 2017, 10:36:38 am
content: Quote from: rheb1026 on May 10, 2017, 02:01:05 pmCheck out the IsoPower series from Analog Devices:http://www.analog.com/en/products/interface-isolation/isolation/isopower.htmlThey have integrated isolated dc-dc converters for powering the output side of the isolators. Very handy for what you are trying to do.

author: rheb1026
author_rank: Regular Contributor
timestamp: May 15, 2017, 01:38:02 pm
content: Thank you both for the information! I will definitely look into the IsoPower ICs