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Help with power issues

L

linnix

Jan 1, 1970
0
I am runing a sensor (1.5mA),
op amp lm/lmv/lpv358 (1/0.2/0.1mA)
and µC (0.5mA) on two button cells

The batteries range from 4V to 6V.
The sensor needs at least 4V.
The µC can have at most 3.3V.
The op amp is a sigma-delta A2D from sensor to µC.

So, I am thinking about a 4V zener to the sensor,
minus diode drop to the µC.

Should I power the op amp before or after the diode?
Also, 4V zener is hard to get.
Should I use 5V plus two more diodes?

I have to commit to PCB in a few days,
no time for more prototype/testing.
 
L

linnix

Jan 1, 1970
0
Please clarify that last sentence.

Op amp analog input from sensor and digital output to µC. I am just
worrying about peak output voltage hitting the op amp or the µC.
Zener regulators are terrible. When the battery is 4 volts and the
sensor needs 4 volts, the required series resistor is zero.

At 4V, the batteries are dead anyway. I should have said operating
range 4V somthing to 6V.
So when
the battery is 6 volts, everything fries. The math is terrible.

Use a low-dropout linear regulator to make the 4 volts; a diode from
there to the uP is OK, or use a second LDO.

But a 4V LDO is expensive, compared to the others.

4V LDO: 0.30
Zenar: 0.05
Diode: 0.01
If you really need to get this right, in a couple of days, post a
complete schematic that people can comment on.

OK, will try that.
 
L

linnix

Jan 1, 1970
0
You left out the series resistor. And its problem isn't the cost; it's
the value.

OK, OK. I will have both on the PCB and decide later.
How much will it cost to do it again?

A thousand jumpers by yours truly.
 
E

Eeyore

Jan 1, 1970
0
linnix said:
I am runing a sensor (1.5mA),
op amp lm/lmv/lpv358 (1/0.2/0.1mA)
and µC (0.5mA) on two button cells

The batteries range from 4V to 6V.
The sensor needs at least 4V.
The µC can have at most 3.3V.
The op amp is a sigma-delta A2D from sensor to µC.

So, I am thinking about a 4V zener to the sensor,
minus diode drop to the µC.

Should I power the op amp before or after the diode?
Also, 4V zener is hard to get.
Should I use 5V plus two more diodes?

I have to commit to PCB in a few days,
no time for more prototype/testing.

Suggest using micropower voltage regulators. Zeners are horrible.

Graham
 
L

linnix

Jan 1, 1970
0
Suggest using micropower voltage regulators. Zeners are horrible.

Graham

Yes, but standby power is less than 100µA, with everything else off
except the µC. Active power is only for several minutes once in
several days or weeks. Standby power needs to be several months on
two CR2032.
 
L

linnix

Jan 1, 1970
0
Yes, but standby power is less than 100µA, with everything else off
except the µC. Active power is only for several minutes once in
several days or weeks. Standby power needs to be several months on
two CR2032.

Here is the simplified schematic:

http://linnix.com/SigmaDeltaEpilson.gif

Actually, we have considered a custom made 4V LDO with 3.5µA standby
current. But we are trying to avoid the $3000 minimum order for the
moment, until necessary, we will just stuff it with zeners or even 6
regular diodes.
 
L

linnix

Jan 1, 1970
0
Calculate the value of the resistor that feeds the zener.

John

Yes, perhaps I need one (10K) for the µC and another (1K) for the
rest. Two zener is still cheaper than the LDO. I know LDO is the
right solution, but we need something to bridge us there.
 
L

linnix

Jan 1, 1970
0
Yes, perhaps I need one (10K) for the µC and another (1K) for the
rest. Two zener is still cheaper than the LDO. I know LDO is the
right solution, but we need something to bridge us there.

never mind. 10K won't run the µC
 
J

James Arthur

Jan 1, 1970
0
linnix said:
Here is the simplified schematic:

http://linnix.com/SigmaDeltaEpilson.gif

Actually, we have considered a custom made 4V LDO with 3.5µA standby
current. But we are trying to avoid the $3000 minimum order for the
moment, until necessary, we will just stuff it with zeners or even 6
regular diodes.

Seiko Instruments has a number of uA-quiescent regulators.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=728-1023-1-ND

datasheet: http://datasheet.sii-ic.com/en/voltage_regulator/S812C_E.pdf

About $0.30 @ qty 3,000, as you said.

James Arthur
 
E

Eeyore

Jan 1, 1970
0
linnix said:
Here is the simplified schematic:

http://linnix.com/SigmaDeltaEpilson.gif

Actually, we have considered a custom made 4V LDO with 3.5µA standby
current. But we are trying to avoid the $3000 minimum order for the
moment, until necessary, we will just stuff it with zeners or even 6
regular diodes.

I though Vin was variable so you'd have to use them in shunt mode and that
*gobbles* current plus zeners aren't specced at currents that low. A
bandgap reference would be better.

Graham
 
L

linnix

Jan 1, 1970
0
J

James Arthur

Jan 1, 1970
0
linnix said:
Yes, I have looked at this. But the drop out is 120mV

Does that matter? The CR2032 is 98% utterly dead and gone at 2.5v,
or 5v for two in series. So you've got lots of headroom.

Extending the discharge cutoff to 2.0v/cell gets you 2% extra
battery lifespan, maybe.


The custom version is drop out of 70mV, but $0.30 @ 10,000.

Unfortunately, they are in different pinouts of 5 pins package.

The problem with your zener shunt regulator idea is that it
won't work.

Cheers,
James Arthur
 
L

linnix

Jan 1, 1970
0
Does that matter? The CR2032 is 98% utterly dead and gone at 2.5v,
or 5v for two in series. So you've got lots of headroom.

Extending the discharge cutoff to 2.0v/cell gets you 2% extra
battery lifespan, maybe.

Yes, either one will work. But the customer has already committed to
the other version, we don't want to change plan and layout again. The
PCB is already set for SC-70 package with the custom pinout.
Unfortunately, the check has been in the mail for weeks. I am just
going to jumper a 1k R, zener and a power switch (for standby), as
well as a $3000 ransom note to remove them.
 
L

linnix

Jan 1, 1970
0
Yes, either one will work. But the customer has already committed to
the other version, we don't want to change plan and layout again. The
PCB is already set for SC-70 package with the custom pinout.
Unfortunately, the check has been in the mail for weeks. I am just
going to jumper a 1k R, zener and a power switch (for standby), as
well as a $3000 ransom note to remove them.

But seriously, I would probably jumper the different pinouts until the
custom version is ready.

Back to the main issue. I am just wanting to make sure that the op
amp output won't go beyond Vcc2, since it is running at higher Vcc1.
In theory, the output should be 1.5V less than Vcc1.
 
E

Eeyore

Jan 1, 1970
0
James said:
The problem with your zener shunt regulator idea is that it
won't work.

You got it. They are the lousiest most power hungry 'regulators' in the world.
They have better uses.

Graham
 
E

Eeyore

Jan 1, 1970
0
linnix said:
Back to the main issue. I am just wanting to make sure that the op
amp output won't go beyond Vcc2, since it is running at higher Vcc1.
In theory, the output should be 1.5V less than Vcc1.

That's another story altogether. What are the devices ? Many are tolerant of such
issues now.

Graham
 
L

linnix

Jan 1, 1970
0
That's another story altogether. What are the devices ? Many are tolerant of such
issues now.

Graham

It's an LCD ASIC. Supply beyond 3.3V mess up the LCD, with all
segments lighting up. It was not a problem with the AVR, but a
problem with the ASIC. I guess the AVR clams down on the LCD drivers,
but not the ASIC.
 
L

linnix

Jan 1, 1970
0
You got it. They are the lousiest most power hungry 'regulators' in the world.
They have better uses.

Graham

We have also considered the TL431. 0.10 @ 10,000 is acceptable, but
the standby current could be higher.
 
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