PWM output short to ground/battery protection - Page 1
Forum section: electronics / projects

author: dmg
author_rank: Regular Contributor
timestamp: July 14, 2016, 05:39:18 pm
content: Hi,I need to output four low frequency PWM signal (50 Hz to 1 kHz) with an ON voltage (Von) selectable between +5V and +15V (i.e. switching between ground and Von). It needs to be able to both source and sink current with low output impedance (up to 50mA or so without droping more than 0.5V) and it needs to withstand a short to ground or to battery, where the battery can have a voltage above Von (typically Vbattery = 24V, but can go up to 32V). Von supply is externally available and its voltage level is decided and set elsewhere, and the inputs need to work at 3.3V CMOS levels. (i.e. input is 3.3V CMOS PWM, output should be 0-Von PWM protected against shorts to 0-32V no matter what Von is)I need to minimize the part count and component area because I'm constrianed in area, so I thought about using an integrated solution that has it already implemented but I'm not having too much luck, I've looked and the only candidates I found for this are too expensive and overkill (see L6374 for example).Do you know how to search for something like this? I'm not sure in which IC category would it fall, as it's a line driver/buffer with elements that are usually found in load drivers. Any discrete compact implementation?Thanks,David

author: Scrts
author_rank: Frequent Contributor
timestamp: July 14, 2016, 05:46:48 pm
content: High-Side switch should do:http://www.st.com/content/st_com/en/products/automotive-analog-power-ics/high-and-low-side-drivers-switches/high-side-switches.html?querycriteria=productId=SC1037I was using 2ch switch from ST: VND5050AJ-E. Not sure if it mentions in datasheet, but it supports PWM at 1kHz without issues.

author: dmg
author_rank: Regular Contributor
timestamp: July 15, 2016, 09:59:16 am
content: Quote from: Scrts on July 14, 2016, 05:46:48 pmThanks but I need something that can both source and sink, not just souce current.

author: ajb
author_rank: Super Contributor
timestamp: July 15, 2016, 11:55:07 am
content: What do you mean by 'withstand'? Could you use a polyswitch, or does it need to go into and out of protection modes immediately?Having to tolerate a short to above the positive switch voltage makes it a bit tricky if it's the latter. Can't use a simple FET on the high side because the body diode will conduct from Vbat to Von. You could do a FET+blocking diode, but that may violate your <0.5V drop requirement. A pair of anti-series FETs would work, but that probably eliminates any integrated load driver solutions that would otherwise meet your needs.Since you only need ~50mA, maybe an overload-protected op amp? I don't know if there are any tolerate >Vdd on the output, so you might need to power it from Vbat if you go that route.

author: dmg
author_rank: Regular Contributor
timestamp: July 15, 2016, 05:48:08 pm
content: Quote from: ajb on July 15, 2016, 11:55:07 amIn the end I've chosen to use an overload protected opamp acting as a non-inverter amplifier and feed the opamp with Vbattery. The 50mA requirement turned out to be peak, normally it won't output more than 5mA tops. I've added an output series resistor with the opamp inside the feedback loop so that I have extra protection against shorts and a significant part of the power dissipation happens in the resistor (a pulse-proof one with higher power rating than usual). Calculations say it should work.