Comparator with Wide Common Mode Range...

[boB]

On Sat, 18 Jul 2020 08:49:05 -0700 (PDT), Ricketty C
<gnuarm.deletethisbit@gmail.com> wrote:

Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?

There is an Op-Amp from TI I think that has a common mode input range
to 75 volts. Maybe that could be used, or adding a comparator after
the amp.

 

[George Herold]

On Monday, July 20, 2020 at 3:27:47 PM UTC-4, boB wrote:

On Sat, 18 Jul 2020 08:49:05 -0700 (PDT), Ricketty C
gnuarm.deletethisbit@gmail.com> wrote:

Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?


There is an Op-Amp from TI I think that has a common mode input range
to 75 volts. Maybe that could be used, or adding a comparator after
the amp.

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.)

George H.

 

[Piotr Wyderski]

George Herold wrote:

One could gain up the input some and put a voltage divider before the
comparator.

Only if there is some current to spare.

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.

Best regards, Piotr

 

[Piotr Wyderski]

George Herold wrote:

One could gain up the input some and put a voltage divider before the
comparator.

Only if there is some current to spare.

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.

Best regards, Piotr

 

[Piotr Wyderski]

George Herold wrote:

One could gain up the input some and put a voltage divider before the
comparator.

Only if there is some current to spare.

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.

Best regards, Piotr

 

[piglet]

On 18/07/2020 6:10 pm, Ricketty C wrote:

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 comparators 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.

I found the TLV1702A which meets all the criteria. But it\'s a bit pricey at $0.80 in quantity. The LM393 are two for a quarter.

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?

piglet

 

[Ricketty C]

On Monday, July 20, 2020 at 4:51:16 PM UTC-4, George Herold wrote:

On Monday, July 20, 2020 at 3:27:47 PM UTC-4, boB wrote:
On Sat, 18 Jul 2020 08:49:05 -0700 (PDT), Ricketty C
gnuarm.deletethisbit@gmail.com> wrote:

Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?


There is an Op-Amp from TI I think that has a common mode input range
to 75 volts. Maybe that could be used, or adding a comparator after
the amp.

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.)

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 are 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 threshold 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.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Monday, July 20, 2020 at 4:51:16 PM UTC-4, George Herold wrote:

On Monday, July 20, 2020 at 3:27:47 PM UTC-4, boB wrote:
On Sat, 18 Jul 2020 08:49:05 -0700 (PDT), Ricketty C
gnuarm.deletethisbit@gmail.com> wrote:

Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?


There is an Op-Amp from TI I think that has a common mode input range
to 75 volts. Maybe that could be used, or adding a comparator after
the amp.

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.)

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 are 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 threshold 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.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Monday, July 20, 2020 at 4:51:16 PM UTC-4, George Herold wrote:

On Monday, July 20, 2020 at 3:27:47 PM UTC-4, boB wrote:
On Sat, 18 Jul 2020 08:49:05 -0700 (PDT), Ricketty C
gnuarm.deletethisbit@gmail.com> wrote:

Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?


There is an Op-Amp from TI I think that has a common mode input range
to 75 volts. Maybe that could be used, or adding a comparator after
the amp.

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.)

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 are 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 threshold 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.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Tuesday, July 21, 2020 at 4:47:41 PM UTC-4, piglet wrote:

On 18/07/2020 6:10 pm, Ricketty C wrote:
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 comparators 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..

I found the TLV1702A which meets all the criteria. But it\'s a bit pricey at $0.80 in quantity. The LM393 are two for a quarter.


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?

piglet

System issues. The comparators are used for alarm thresholds and need to run 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 behind the power switch. But the LM393 is not so low power using a half mA per unit or 4 mA total compared to self discharge of the battery of 83 uA at 20°C. Even the comparator I picked is 10\'s of uA so I will want to only run the ones I must when in low power mode. Heck, it will be easy to keep the MCU power consumption in the low uA range and the comparators will be the nail that sticks up.

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209

 

[Phil Hobbs]

On 2020-07-18 19:54, Ricketty C wrote:

On Saturday, July 18, 2020 at 7:00:32 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 3:50:07 PM UTC-7, Ricketty C wrote:
On Saturday, July 18, 2020 at 4:15:35 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 8:49:10 AM UTC-7, Ricketty C wrote:
Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

Been using TLV1805 recently. $0.85 on Digikey which might be too high for what you\'re looking for.

Thanks for the suggestion. That one is not open drain which is required 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 of the first without \"393\" or \"2903\" in its part number is BU7253. maybe it will work.
https://www.digikey.com/product-detail/en/rohm-semiconductor/BU7253F-E2/BU7253F-E2CT-ND/5720201

Thanks for trying. Digikey was the first place I looked after making the 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 with 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. The LM393A 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 part that can be used with that as Vcc/Vdd. So the BU7253 is out. Otherwise 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 really 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 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 termination 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. They do 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 not 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.

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

--
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

 

[Ricketty C]

On Wednesday, July 22, 2020 at 9:26:00 PM UTC-4, Phil Hobbs wrote:

On 2020-07-18 19:54, Ricketty C wrote:
On Saturday, July 18, 2020 at 7:00:32 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 3:50:07 PM UTC-7, Ricketty C wrote:
On Saturday, July 18, 2020 at 4:15:35 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 8:49:10 AM UTC-7, Ricketty C wrote:
Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

Been using TLV1805 recently. $0.85 on Digikey which might be too high for what you\'re looking for.

Thanks for the suggestion. That one is not open drain which is required 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 of the first without \"393\" or \"2903\" in its part number is BU7253. maybe it will work.
https://www.digikey.com/product-detail/en/rohm-semiconductor/BU7253F-E2/BU7253F-E2CT-ND/5720201

Thanks for trying. Digikey was the first place I looked after making the 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 with 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. The LM393A 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 part that can be used with that as Vcc/Vdd. So the BU7253 is out. Otherwise 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 really 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 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 termination 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. They do 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 not 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.


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.

Thanks, that looks like a great part. But there is one spec it is shy on compared to the others, but maybe it won\'t matter. The max voltage on the open 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 never 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 still some conduction through the FET.

The current through the FET is going to be very slight, 10 uA max. It\'s not uncommon for a pin like the comparator output to be rated for over voltage as long as some max current is observed, but I don\'t see that in this case.

The 2N7002 is spec\'d for drain leakage at Vgs = 0. For the comparator output 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 up to 7 uA of current. Since the 10 uA spec is at 150 °C and it\'s only 1 uA at 25 °C (all at Vgs = 0), I\'m thinking 7 uA would get me to 70 °C and Vgs = -3.7 volts without any trouble, no?

The other concern is supply. This looks to be a new part and there isn\'t much inventory. Digikey quotes 3 weeks for 10,000 and another 3 weeks for the remaining 30,000 needed to build 10,000 units. That\'s probably an ok time frame. It\'s not like anyone can build 10,000 units in a month even if all the parts are on hand.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Wednesday, July 22, 2020 at 9:26:00 PM UTC-4, Phil Hobbs wrote:

On 2020-07-18 19:54, Ricketty C wrote:
On Saturday, July 18, 2020 at 7:00:32 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 3:50:07 PM UTC-7, Ricketty C wrote:
On Saturday, July 18, 2020 at 4:15:35 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 8:49:10 AM UTC-7, Ricketty C wrote:
Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

Been using TLV1805 recently. $0.85 on Digikey which might be too high for what you\'re looking for.

Thanks for the suggestion. That one is not open drain which is required 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 of the first without \"393\" or \"2903\" in its part number is BU7253. maybe it will work.
https://www.digikey.com/product-detail/en/rohm-semiconductor/BU7253F-E2/BU7253F-E2CT-ND/5720201

Thanks for trying. Digikey was the first place I looked after making the 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 with 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. The LM393A 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 part that can be used with that as Vcc/Vdd. So the BU7253 is out. Otherwise 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 really 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 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 termination 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. They do 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 not 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.


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.

Thanks, that looks like a great part. But there is one spec it is shy on compared to the others, but maybe it won\'t matter. The max voltage on the open 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 never 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 still some conduction through the FET.

The current through the FET is going to be very slight, 10 uA max. It\'s not uncommon for a pin like the comparator output to be rated for over voltage as long as some max current is observed, but I don\'t see that in this case.

The 2N7002 is spec\'d for drain leakage at Vgs = 0. For the comparator output 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 up to 7 uA of current. Since the 10 uA spec is at 150 °C and it\'s only 1 uA at 25 °C (all at Vgs = 0), I\'m thinking 7 uA would get me to 70 °C and Vgs = -3.7 volts without any trouble, no?

The other concern is supply. This looks to be a new part and there isn\'t much inventory. Digikey quotes 3 weeks for 10,000 and another 3 weeks for the remaining 30,000 needed to build 10,000 units. That\'s probably an ok time frame. It\'s not like anyone can build 10,000 units in a month even if all the parts are on hand.

--

Rick C.

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[Ricketty C]

On Wednesday, July 22, 2020 at 9:26:00 PM UTC-4, Phil Hobbs wrote:

On 2020-07-18 19:54, Ricketty C wrote:
On Saturday, July 18, 2020 at 7:00:32 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 3:50:07 PM UTC-7, Ricketty C wrote:
On Saturday, July 18, 2020 at 4:15:35 PM UTC-4, sea moss wrote:
On Saturday, July 18, 2020 at 8:49:10 AM UTC-7, Ricketty C wrote:
Selecting parts can be a PITA. Comparators have a LOT of very detailed 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 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 would make 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 for some sensors, but then both voltages have to be routed to the comparators 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.

I\'m sick of digging through digikey looking for parts. Any suggestions? What\'s your favorite comparator for general work?

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Rick C.

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Been using TLV1805 recently. $0.85 on Digikey which might be too high for what you\'re looking for.

Thanks for the suggestion. That one is not open drain which is required 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 of the first without \"393\" or \"2903\" in its part number is BU7253. maybe it will work.
https://www.digikey.com/product-detail/en/rohm-semiconductor/BU7253F-E2/BU7253F-E2CT-ND/5720201

Thanks for trying. Digikey was the first place I looked after making the 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 with 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. The LM393A 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 part that can be used with that as Vcc/Vdd. So the BU7253 is out. Otherwise 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 really 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 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 termination 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. They do 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 not 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.


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.

Thanks, that looks like a great part. But there is one spec it is shy on compared to the others, but maybe it won\'t matter. The max voltage on the open 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 never 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 still some conduction through the FET.

The current through the FET is going to be very slight, 10 uA max. It\'s not uncommon for a pin like the comparator output to be rated for over voltage as long as some max current is observed, but I don\'t see that in this case.

The 2N7002 is spec\'d for drain leakage at Vgs = 0. For the comparator output 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 up to 7 uA of current. Since the 10 uA spec is at 150 °C and it\'s only 1 uA at 25 °C (all at Vgs = 0), I\'m thinking 7 uA would get me to 70 °C and Vgs = -3.7 volts without any trouble, no?

The other concern is supply. This looks to be a new part and there isn\'t much inventory. Digikey quotes 3 weeks for 10,000 and another 3 weeks for the remaining 30,000 needed to build 10,000 units. That\'s probably an ok time frame. It\'s not like anyone can build 10,000 units in a month even if all the parts are on hand.

--

Rick C.

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[Pimpom]

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

 

[Pimpom]

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

 

[Pimpom]

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

 

[Ricketty C]

On Thursday, July 23, 2020 at 2:55:34 AM UTC-4, Pimpom wrote:

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

I might be able to get around the need for 15 volts on the output. That one comparator is pulling down the source of an nFET with the gate at 3.3 volts and the drain at 15V. In theory the 15 volts won\'t make it through to the comparator. But the nFET has some leakage. With damage not happening until exceeding 7 volts that would make Vgs -3.7 volts which should turn the nFET off pretty hard. Just to be sure, I added a 1 Meg resistor to ground to drain away any leakage and it should be good.

The higher voltage parts both cost more and use more power supply. At least two of these devices are used for alarms which need to be operative during the power \"off\" mode which is really just a minimum power level. The battery has a self discharge current of around 80 uA, so these parts at <1 uA each can be left on or whatever as desired without thought.

I\'m going to recommend the TLV7042DGK. The dual part only seems to be available in one package. They theoretically have a quad part, but not available yet.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Thursday, July 23, 2020 at 2:55:34 AM UTC-4, Pimpom wrote:

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

I might be able to get around the need for 15 volts on the output. That one comparator is pulling down the source of an nFET with the gate at 3.3 volts and the drain at 15V. In theory the 15 volts won\'t make it through to the comparator. But the nFET has some leakage. With damage not happening until exceeding 7 volts that would make Vgs -3.7 volts which should turn the nFET off pretty hard. Just to be sure, I added a 1 Meg resistor to ground to drain away any leakage and it should be good.

The higher voltage parts both cost more and use more power supply. At least two of these devices are used for alarms which need to be operative during the power \"off\" mode which is really just a minimum power level. The battery has a self discharge current of around 80 uA, so these parts at <1 uA each can be left on or whatever as desired without thought.

I\'m going to recommend the TLV7042DGK. The dual part only seems to be available in one package. They theoretically have a quad part, but not available yet.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209

 

[Ricketty C]

On Thursday, July 23, 2020 at 2:55:34 AM UTC-4, Pimpom wrote:

On 7/23/2020 6:55 AM, Phil Hobbs wrote:

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.

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.

I might be able to get around the need for 15 volts on the output. That one comparator is pulling down the source of an nFET with the gate at 3.3 volts and the drain at 15V. In theory the 15 volts won\'t make it through to the comparator. But the nFET has some leakage. With damage not happening until exceeding 7 volts that would make Vgs -3.7 volts which should turn the nFET off pretty hard. Just to be sure, I added a 1 Meg resistor to ground to drain away any leakage and it should be good.

The higher voltage parts both cost more and use more power supply. At least two of these devices are used for alarms which need to be operative during the power \"off\" mode which is really just a minimum power level. The battery has a self discharge current of around 80 uA, so these parts at <1 uA each can be left on or whatever as desired without thought.

I\'m going to recommend the TLV7042DGK. The dual part only seems to be available in one package. They theoretically have a quad part, but not available yet.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209