Selecting parts can be a PITA. Comparators have a LOT of very detailed spe cs that all need to be checked. I have been using an LM393A that is alread y in use on the board for some new circuits. I just noticed the common mod e range is only up to Vcc-2. With a recent change to the MCU, the power su pply for the comparators and MCU was changed to 3.3 volts. So that would m ake the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
I could have all the power pins changed to 5V which is still on the board f or some sensors, but then both voltages have to be routed to the comparator s which makes the routing harder. Also the power consumption is around 1 m A which is more than it should be in this design. It will be easier to jus t toss the LM393A and find another part that is lower power, works on 3.3 v olts Vcc and works with a large common mode range. Finally, it has to be i n stock at various distributors so 10,000 can be ordered without delay and a low price is always good.
I'm sick of digging through digikey looking for parts. Any suggestions? W hat's your favorite comparator for general work?
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Rick C.
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specs that all need to be checked. I have been using an LM393A that is al ready in use on the board for some new circuits. I just noticed the common mode range is only up to Vcc-2. With a recent change to the MCU, the powe r supply for the comparators and MCU was changed to 3.3 volts. So that wou ld make the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
rd for some sensors, but then both voltages have to be routed to the compar ators which makes the routing harder. Also the power consumption is around 1 mA which is more than it should be in this design. It will be easier to just toss the LM393A and find another part that is lower power, works on 3 .3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without delay and a low price is always good.
? What's your favorite comparator for general work?
The input common mode range pf the LM393 is 0 to Vcc-2. That's my problem. On a 3.3 volt supply it only leaves 1.3 volts of working range.
No need for speed, most parameters are relaxed. I'd like to keep the Iq to less than 150-200 uA. We have 15 volts in the design, so I'd like to have a part that can be used directly on that voltage if needed. These compara tors are mostly detecting thresholds to sound alarms or the one I'm using n ow shuts off power to the motor. High precision is not really required.
I found the TLV1702A which meets all the criteria. But it's a bit pricey a t $0.80 in quantity. The LM393 are two for a quarter.
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ed specs that all need to be checked. I have been using an LM393A that is already in use on the board for some new circuits. I just noticed the comm on mode range is only up to Vcc-2. With a recent change to the MCU, the po wer supply for the comparators and MCU was changed to 3.3 volts. So that w ould make the common mode range 0 to 1.3V. Also, the device is not rated f or Vcc below 5V.
oard for some sensors, but then both voltages have to be routed to the comp arators which makes the routing harder. Also the power consumption is arou nd 1 mA which is more than it should be in this design. It will be easier to just toss the LM393A and find another part that is lower power, works on 3.3 volts Vcc and works with a large common mode range. Finally, it has t o be in stock at various distributors so 10,000 can be ordered without dela y and a low price is always good.
ns? What's your favorite comparator for general work?
m. On a 3.3 volt supply it only leaves 1.3 volts of working range.
to less than 150-200 uA. We have 15 volts in the design, so I'd like to ha ve a part that can be used directly on that voltage if needed. These compa rators are mostly detecting thresholds to sound alarms or the one I'm using now shuts off power to the motor. High precision is not really required.
at $0.80 in quantity. The LM393 are two for a quarter.
I also found the LMC6772 which is about $1.
There just don't seem to be a lot of choices. I'm surprised.
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Rick C.
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pecs that all need to be checked. I have been using an LM393A that is alre ady in use on the board for some new circuits. I just noticed the common m ode range is only up to Vcc-2. With a recent change to the MCU, the power supply for the comparators and MCU was changed to 3.3 volts. So that would make the common mode range 0 to 1.3V. Also, the device is not rated for V cc below 5V.
for some sensors, but then both voltages have to be routed to the comparat ors which makes the routing harder. Also the power consumption is around 1 mA which is more than it should be in this design. It will be easier to j ust toss the LM393A and find another part that is lower power, works on 3.3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without delay an d a low price is always good.
What's your favorite comparator for general work?
Been using TLV1805 recently. $0.85 on Digikey which might be too high for what you're looking for.
specs that all need to be checked. I have been using an LM393A that is al ready in use on the board for some new circuits. I just noticed the common mode range is only up to Vcc-2. With a recent change to the MCU, the powe r supply for the comparators and MCU was changed to 3.3 volts. So that wou ld make the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
rd for some sensors, but then both voltages have to be routed to the compar ators which makes the routing harder. Also the power consumption is around 1 mA which is more than it should be in this design. It will be easier to just toss the LM393A and find another part that is lower power, works on 3 .3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without delay and a low price is always good.
? What's your favorite comparator for general work?
r what you're looking for.
Thanks for the suggestion. That one is not open drain which is required si nce most of them are being used as window comparators with the wire ORed ou tputs. It's also a single which is not a show stopper, but makes it more e xpensive.
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Rick C.
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ed specs that all need to be checked. I have been using an LM393A that is already in use on the board for some new circuits. I just noticed the comm on mode range is only up to Vcc-2. With a recent change to the MCU, the po wer supply for the comparators and MCU was changed to 3.3 volts. So that w ould make the common mode range 0 to 1.3V. Also, the device is not rated f or Vcc below 5V.
oard for some sensors, but then both voltages have to be routed to the comp arators which makes the routing harder. Also the power consumption is arou nd 1 mA which is more than it should be in this design. It will be easier to just toss the LM393A and find another part that is lower power, works on 3.3 volts Vcc and works with a large common mode range. Finally, it has t o be in stock at various distributors so 10,000 can be ordered without dela y and a low price is always good.
ns? What's your favorite comparator for general work?
for what you're looking for.
since most of them are being used as window comparators with the wire ORed outputs. It's also a single which is not a show stopper, but makes it more expensive.
OK, I searched on Digikey for open-collector duals, sorted by price. One o f the first without "393" or "2903" in its part number is BU7253. maybe it will work.
iled specs that all need to be checked. I have been using an LM393A that i s already in use on the board for some new circuits. I just noticed the co mmon mode range is only up to Vcc-2. With a recent change to the MCU, the power supply for the comparators and MCU was changed to 3.3 volts. So that would make the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
board for some sensors, but then both voltages have to be routed to the co mparators which makes the routing harder. Also the power consumption is ar ound 1 mA which is more than it should be in this design. It will be easie r to just toss the LM393A and find another part that is lower power, works on 3.3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without de lay and a low price is always good.
ions? What's your favorite comparator for general work?
h for what you're looking for.
d since most of them are being used as window comparators with the wire ORe d outputs. It's also a single which is not a show stopper, but makes it mo re expensive.
of the first without "393" or "2903" in its part number is BU7253. maybe it will work.
U7253F-E2CT-ND/5720201
Thanks for trying. Digikey was the first place I looked after making the f irst post. So I was able to exclude parts that use too much power, don't h ave the range of Vcc and don't have the right output type. Combine with th e preferred package (and only selecting reels so each part doesn't show up five times) drops the list to 41 devices, mostly flavors of the 393. The L M393A has higher idle current so is excluded, but there are plenty of other variations on the theme that show up but still have the common mode range limitations.
The BU7253 is a good part, but with 15 volts on the board I want to use a p art that can be used with that as Vcc/Vdd. So the BU7253 is out. Otherwis e a good fit.
Thanks again. I suppose if Digikey and Mouser don't have anything better I know what my options are. I just thought I'd ask if there was something I 'm missing.
I'm so glad Digikey and Mouser have their searching capabilities. It reall y makes a big difference in doing this work. Digikey is really good about cleaning up problems in their data base. I'd say I report issues to them t wo or three times a year and they are always quick to respond and sound app reciative. Just last week I found reed sensors that had the wire terminati on descriptions messed up. In one or two days they got back to me that the y fixed the one I reported and all the others in that product line. They d o a great job.
But not everything you want to select on is in their database. The common mode range is one of the things they don't make searchable. Fortunately no t many comparators have that issue.
The LM393A is a great chip in many ways. Even when powered from 5V you can yank the inputs and output to 36V without damage. That's nice. Just don' t expect it to function properly while both inputs are above 3 volts.
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Indeed, try to select a low I_DSS power MOSFET (down to the nanoamp level), and you will quickly learn what a royal PITA is.
I am unable to define what general work would be in my case. Every application needs something unique. I have just deployed the LT1716 -- looks pricey, but it has turned out to be the most cost-effective solution in a low volume design.
A buck and change is definitely pricey compared to $0.12 for a dual LM393. We were running them on the 5 volt rail, but we changed to a 3.3V processo r and moved them to the 3.3V rail. They are comparing the same analog sig nals that go to the alarms. If given enough thought I'm sure we could make the cheap part work but it's a LOT of I's to cross and T's to dot to make sure all the voltages work out.
I'm not actually the guy driving the schematic entry, but he seems to be a lot busier than I am and misses lots of details. He's the guy who didn't r emove the thermal breaks on the the thermal pads of the 5V regulator and mo tor controller. Many other details missed in the first two sets of boards.
How many of the LT1716 is your board using? This design has eight comparat ors, so the cost adds up fast.
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It depends. In volume it indeed is; small scale specialistic runs call for using "how many minutes of your work a buck is" as a metric. Engineers are an expensive resource. Hard to find the optimum.
Exactly one, just for this very specific task. LM393 elsewhere.
specs that all need to be checked. I have been using an LM393A that is alr eady in use on the board for some new circuits. I just noticed the common mode range is only up to Vcc-2. With a recent change to the MCU, the power supply for the comparators and MCU was changed to 3.3 volts. So that woul d make the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
d for some sensors, but then both voltages have to be routed to the compara tors which makes the routing harder. Also the power consumption is around
1 mA which is more than it should be in this design. It will be easier to just toss the LM393A and find another part that is lower power, works on 3.
3 volts Vcc and works with a large common mode range. Finally, it has to b e in stock at various distributors so 10,000 can be ordered without delay a nd a low price is always good.
What's your favorite comparator for general work?
One could gain up the input some and put a voltage divider before the comparator. (A cheapish R-R in opamp, mentioned above, sounds like a better solution.)
d specs that all need to be checked. I have been using an LM393A that is a lready in use on the board for some new circuits. I just noticed the commo n mode range is only up to Vcc-2. With a recent change to the MCU, the pow er supply for the comparators and MCU was changed to 3.3 volts. So that wo uld make the common mode range 0 to 1.3V. Also, the device is not rated fo r Vcc below 5V.
ard for some sensors, but then both voltages have to be routed to the compa rators which makes the routing harder. Also the power consumption is aroun d 1 mA which is more than it should be in this design. It will be easier t o just toss the LM393A and find another part that is lower power, works on
3.3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without delay and a low price is always good.
s? What's your favorite comparator for general work?
Better than what??? Replacing a comparator with an op amp and comparator? The problem isn't that the input isn't high enough, it's that the inputs a re too high. Also the comparators are being used as window comparators so one threshold will need to be high and one low. At some point the low thre shold just gets to be too low. Mostly I don't want to have to re-engineer eight circuits. The quad package device is only about $0.25 per comparator , so it's cheap enough.
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If you want to sense voltage only (meaning: only the crazily low input leakage current of the comparator is allowed to flow), things get interesting if the voltage is beyond the rails.
So why do you have to power the LM393 from 3.3V? It has open collector outputs so you can power the LM393 from 12 or 15V and keep the high common mode input range and pullup the outputs to logic 3.3V?
lem. On a 3.3 volt supply it only leaves 1.3 volts of working range.
q to less than 150-200 uA. We have 15 volts in the design, so I'd like to have a part that can be used directly on that voltage if needed. These com parators are mostly detecting thresholds to sound alarms or the one I'm usi ng now shuts off power to the motor. High precision is not really required .
ey at $0.80 in quantity. The LM393 are two for a quarter.
System issues. The comparators are used for alarm thresholds and need to r un when the MCU is running to detect battery voltage and input power. The
5 volt power can be shut down as well as the 12-15 volt power.
In theory we could power the comparators from the 12-15 volt power rail beh ind the power switch. But the LM393 is not so low power using a half mA pe r unit or 4 mA total compared to self discharge of the battery of 83 uA at
ly run the ones I must when in low power mode. Heck, it will be easy to ke ep the MCU power consumption in the low uA range and the comparators will b e the nail that sticks up.
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Rick C.
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There's the TLV704x (open drain) and TLV703x (totem pole). 5.5V, RRIO, picoamp bias current, 0.3 uA supply current, 3 us, 21 cents in reels for the single.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
iled specs that all need to be checked. I have been using an LM393A that i s already in use on the board for some new circuits. I just noticed the co mmon mode range is only up to Vcc-2. With a recent change to the MCU, the power supply for the comparators and MCU was changed to 3.3 volts. So that would make the common mode range 0 to 1.3V. Also, the device is not rated for Vcc below 5V.
board for some sensors, but then both voltages have to be routed to the co mparators which makes the routing harder. Also the power consumption is ar ound 1 mA which is more than it should be in this design. It will be easie r to just toss the LM393A and find another part that is lower power, works on 3.3 volts Vcc and works with a large common mode range. Finally, it has to be in stock at various distributors so 10,000 can be ordered without de lay and a low price is always good.
ions? What's your favorite comparator for general work?
gh for what you're looking for.
red since most of them are being used as window comparators with the wire O Red outputs. It's also a single which is not a show stopper, but makes it more expensive.
One of the first without "393" or "2903" in its part number is BU7253. may be it will work.
he first post. So I was able to exclude parts that use too much power, don 't have the range of Vcc and don't have the right output type. Combine wit h the preferred package (and only selecting reels so each part doesn't show up five times) drops the list to 41 devices, mostly flavors of the 393. T he LM393A has higher idle current so is excluded, but there are plenty of o ther variations on the theme that show up but still have the common mode ra nge limitations.
a part that can be used with that as Vcc/Vdd. So the BU7253 is out. Othe rwise a good fit.
er I know what my options are. I just thought I'd ask if there was somethi ng I'm missing.
eally makes a big difference in doing this work. Digikey is really good ab out cleaning up problems in their data base. I'd say I report issues to th em two or three times a year and they are always quick to respond and sound appreciative. Just last week I found reed sensors that had the wire termi nation descriptions messed up. In one or two days they got back to me that they fixed the one I reported and all the others in that product line. Th ey do a great job.
mon mode range is one of the things they don't make searchable. Fortunatel y not many comparators have that issue.
can yank the inputs and output to 36V without damage. That's nice. Just don't expect it to function properly while both inputs are above 3 volts.
Thanks, that looks like a great part. But there is one spec it is shy on c ompared to the others, but maybe it won't matter. The max voltage on the o pen drain output is only 7 volts instead of 15V or 36V for the other parts.
I have a circuit where the comparator output pulls down on the source of a
2N7002 through a 1k resistor with the gate at the 3.3 volt rail. The drain is pulled up to 15 volts through a 46k resistor. In theory this should ne ver pull up on the comparator output beyond about 1.3 volts for a number of reasons including that the FET turns off at that point. But there is stil l some conduction through the FET.
The current through the FET is going to be very slight, 10 uA max. It's no t uncommon for a pin like the comparator output to be rated for over voltag e as long as some max current is observed, but I don't see that in this cas e.
The 2N7002 is spec'd for drain leakage at Vgs = 0. For the comparator ou tput to rise beyond 7 volts Vgs would -4V. Is it save to say that would be impossible in this circuit? I could add a high value resistor to conduct any very small leakage current, say 1 Meg. That would stay below 7 volts u
The other concern is supply. This looks to be a new part and there isn't m uch inventory. Digikey quotes 3 weeks for 10,000 and another 3 weeks for t he remaining 30,000 needed to build 10,000 units. That's probably an ok ti me frame. It's not like anyone can build 10,000 units in a month even if a ll the parts are on hand.
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Rick C.
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Nice part. I looked at various TLV types too but the OP said that he wants one that can be used "directly" on 15V. RRI, low power, at least 3-15Vcc/dd *and* cheap. I wonder if such a beast exists.
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