Over voltage / over current sensing ic - Page 1
Forum section: electronics / projects

author: halloween360
author_rank: Contributor
timestamp: December 12, 2015, 04:48:41 pm
content: I am currently working on a design where I need to protect an output from current over 2 amps. Basically loads that are too large or hard shorts. Im struggling with how I want to do this.The load can be considered resistive as it will be LEDS. I would like to find an IC to do this for me that would drop a flag or latch when this scenario is encountered and cant be reset without a power cycle to the circuit. I am thinking that I would like to place a small resistance in series with the load and monitor the voltage that develops across the resistor. So if I were to place lets say a .05 ohm resistor in series when the current reaches 2 amps or more the voltage would be .05 x 2 or 100mV. What ever the solution is, it needs to be fast acting and very low cost as there will be multiple of these outputs in the same project. Basically creating a circuit breaker that is electronic instead of mechanical to protect the output.Any thoughts would be appreciated.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 12, 2015, 04:51:36 pm
content: There are many..... but I use Linear LT1910.Sent from my SAMSUNG-SM-G890A using Tapatalk

author: rx8pilot
author_rank: Super Contributor
timestamp: December 12, 2015, 05:10:09 pm
content: I spoke too soon. The LT1910 will only cover your over current needs. I also use in qty the LT4356-3 for electronic fusing and overvoltage protection. Very simple devices to ue but make sure you understand what a Kelvin Connection is when you lay it out on a PCB. You will probably use a very small resistor for current sense - around 20-30mOhm (.03 Ohms).The LT4356-3 is the latching version of the family. It has an open collector output that can drive a status LED or an IO pin on a micro-controller. It also has an enable pin that can be used with a tiny switch or MCU to turn the load on/off. These are external MOSFET controllers but you only need a few extra parts and they are very flexible.

author: halloween360
author_rank: Contributor
timestamp: December 12, 2015, 05:21:47 pm
content: Thanks for the quick response! The LT1910 looks like it would be very easy to use. I may have to get a few to play around with. At a quick glance I see that it is a fairly pricy IC. At least on DigiKey...

author: rx8pilot
author_rank: Super Contributor
timestamp: December 12, 2015, 05:40:03 pm
content: All the Linear stuff is kinda pricey. If you are looking for a 1 off it may not be bad. For a low cost solution, Linear is generally not a player. The good news is that most of their devices work very well. Is this a hobby project or a commercial projects?Sent from my SAMSUNG-SM-G890A using Tapatalk

author: halloween360
author_rank: Contributor
timestamp: December 12, 2015, 05:43:53 pm
content: Kind of a mix haha more of a hobbyist project atm. Just looking down the road if i am to make multiples.

author: langwadt
author_rank: Super Contributor
timestamp: December 12, 2015, 05:49:30 pm
content: Quote from: halloween360 on December 12, 2015, 04:48:41 pmsomething like an fpf2701, for higher currents auir3315

author: halloween360
author_rank: Contributor
timestamp: January 02, 2016, 11:29:10 pm
content: Im getting a little deeper into this project than I originally thought I would. I have run into a small issue with some of the led lights I am controlling. They seem to be on the capacitive side. This causes an obvious issue with a chip that is as fast as the LT1910 without some additional circuitry, as the inrush current exceeds the minimum trip current of 2amps that i have set. I have been able to over come this issue using a suggested RC combination in the data sheet but am struggling with exactly how the extra circuitry works.In the data sheethttp://cds.linear.com/docs/en/datasheet/1910fc.pdfOn page 8 there is a current limit delay network. Obviously this is adding a tripping delay and I do understand that. The part that I dont quite get is why is the circuit calling for the 1N4148 diode? What is its purpose? The delay works without it.Any insight would be greatly appreciated!

author: kuromaku
author_rank: Newbie
timestamp: January 03, 2016, 03:44:51 am
content: Quote from: halloween360 on January 02, 2016, 11:29:10 pmProbably so the MOSFET doesn't spend a bunch of time in the linear region as it is turning off at high Id and burn a bunch of power for SOA reasons.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 03, 2016, 07:58:40 am
content: I have used the LT1910 extensively in my designs. The intention of the diode is to delay the current trip but only to a point. I don't use it because my circuits don't need it. I do like it and it is very reliable for eFuse and control of current. I wish it had an current amplifier output. The reset timer is very handy. Gate drive is weak so you have to pay attention to big capacitive loads that can violate the SOA of your MOSFET. I have most of mine tweaked to deal with pretty heavy transients, the delay control gives you great control over how you deal with over-current without much fuss.There is no OV sensing - only current. The price is fairly high for what it does, but it is also fairly unique device.

author: Tim F
author_rank: Regular Contributor
timestamp: January 03, 2016, 08:50:48 am
content: An opamp and a pair of BJTs will be cheaper than the LT part:The opamp runs as a differential amplifier and the gain is configured such that the opamp output exceeds ~0.7V when the desired load current limit is reached. Just make sure that you observe the common-mode input voltage range of the opamp.This turns on Q1, which then turns on Q2, latching Q1 on such that the output remains low even if the load current then drops and the opamp output goes low.The capacitors in the BJT latching circuit affect how fast it latches. Primarily R8 and C1. The larger the current overload, the faster the latching speed.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 04, 2016, 05:15:13 pm
content: The integration of the LT part is worth the $1.50 or so difference in my case. A current sense amplifier and all the other features would be a solid effort to replicate with discreets and would take a lot of pcb space.Sent from my SAMSUNG-SM-G890A using Tapatalk

author: nctnico
author_rank: Super Contributor
timestamp: January 04, 2016, 05:21:04 pm
content: If accuracy isn't that important it is worth to consider measuring the voltage drop over a length of PCB trace. Expect the tolerance to be around +/-20 though due to the temperature dependant conductivity of copper.