Comparator for supply range 16.8V - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 19, 2016, 07:16:13 am
content: 1. I have a battery powered application which is powered application from 4S li-ion cell. (12V - 16.8V). I need a comparator for this. Comprator output will drive a mosfet FDS6690A, which is basically in current path.2. Comparator has to be very low powered since it is battery powered application. I have checked TLV3701 from texas instrument. Datasheet says it for upto 16V, but absolute max can go upto 17V. Can I use it here?3. Another option is to use LT1716(upto 44V), but its current can upto 85uA at 12V. So I haven't consider it yet.4. Any other very low powered comparator for 16.8V supply & to drive FDS6690A.

author: Simon
author_rank: Unknown
timestamp: January 19, 2016, 07:40:02 am
content: Never work at or near an IC's max voltage rating. Use something like an LM393 that goes to 30V. Also remember that sense resistors and voltage references will have a current draw. the comparator IC is the last of your worries.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2016, 12:10:17 pm
content: The problem with the LM393 is its common mode range doesn't include positive rail and as this is a battery powered application, the power consumption might be too high.Fortunately the common mode range for the TLV3701, exceeds the positive supply rail by 5V, so you could put a diode or two in series with the +V pin and it will still be able to monitor +16.8V, as long as the additional drop in output voltage is acceptable.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 19, 2016, 04:26:30 pm
content: Hi Hero999, voltage to be compared are less than 3.3V at both channels, so that is not the problem.Only problem is voltage at V+ pin of comparator.It seems good idea to put a smd diode like this :http://www.digikey.com/product-detail/en/1N4148X-TP/1N4148XTPMSCT-ND/789335It has drop of only 0.2V at 10uA current. I am browisng through digikey to look at smd diodes to have larger drop at this small current

author: Simon
author_rank: Unknown
timestamp: January 19, 2016, 05:47:11 pm
content: Quote from: Vindhyachal.takniki on January 19, 2016, 04:26:30 pmWhat are you proposing to with the diode ?

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2016, 07:54:36 pm
content: The diode could be used to reduce the power supply voltage to the comparator.What about an LED?

author: Simon
author_rank: Unknown
timestamp: January 19, 2016, 08:01:26 pm
content: bad idea, the comparators maximum is 16V the OP is relying on the absolute maximum rating of 17V and is working with 16.8V so 16.6V is still too high. A couple of decent solicone diodes are needed and probably a load which makes the whole thing unusable as a low current battery monitor. Also for this to work a low current voltage reference is needed and suitable resistors need choosing so that the standby current is a minimum. The comparators low current draw is just 1/3 of the whole problem that needs a solution.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2016, 08:24:12 pm
content: That's why I suggested an LED. A blue one should drop more than 1V, even at such a tiny current and will act as an indicator as a bonus.An easier option is to power it of a voltage regulator with a low quiescent current which can double as a voltage reference.http://cds.linear.com/docs/en/datasheet/3014fd.pdfhttp://www.ti.com/lit/ds/symlink/lp2950-33.pdf

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: January 20, 2016, 04:20:00 pm
content: Attached is my circuit.1. MCU takes one input(J3, voltage), do some processing & then turn P1.7 high or low. When P1.7 is high, it gives high to comparator, which in turn on the FDS6690A, & turn off if p1.7 is low.2. Can I turn on/off mosfet FDS6690A directly by MSP430(3.3V)? Datasheet says VGSmax = 3V, when VDS=VGS & IDS = 250uA.In my case max Vds will be 16.8V & max current will be around 7A.If yes, then I can remove comparator.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2016, 04:51:56 pm
content: It depends on what voltage drop is acceptable?It's quite likely it'll be fine but if in doubt, add a level shifter to drive the MOSFET from the MCU.

author: dadler
author_rank: Unknown
timestamp: January 20, 2016, 05:33:51 pm
content: The threshold voltage has a max of 3V, seems too close for comfort for me. Is this a one off device?You have 16.8V available, I'd prefer to use that to drive the mosfet.How quickly do you need to turn the mosfet on? Are you PWMing it?Comparators aren't spectacular gate drivers... May want to use a actual gate driver, something small like a ZXGD3003E6 or MCP1416.Or build one out of a couple of discrete transistors.