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Pls help me find extremely small 1:1 voltage-tracking regulator

B

Bo-Ming Tong

Jan 1, 1970
0
I am developing a product which requires interfacing my 5V circuit to
another 5V circuit whose power supply I have no control over, which
varies from around 4.75V to 5.25V in practice.

I wish to build my circuit's power supply to track the other circuit's
power supply instead of a fixed 5V. So, I am looking for a regulator
with an external voltage reference instead of an internal one. I need
only 40mA, and I have found ON Semiconductor CS8182 (SOIC-8) and
Infineon TLE4250G (SCT-595) for that purpose. However, they are not
stocked by suppliers such as Mouser and Digikey, and I have learned
from posts in this newgroup that I should never design-in
hard-to-source components. Is there any other tracking LDO you could
think of, which is extremely small and made by a more commonly
available manufacturer such as TI ?

I also found a lot of DDR termination tracking controllers but they are
all 1:2 and they are larger than my very constrained space would allow.

Thanks in advance !
 
S

Spehro Pefhany

Jan 1, 1970
0
I am developing a product which requires interfacing my 5V circuit to
another 5V circuit whose power supply I have no control over, which
varies from around 4.75V to 5.25V in practice.

I wish to build my circuit's power supply to track the other circuit's
power supply instead of a fixed 5V. So, I am looking for a regulator
with an external voltage reference instead of an internal one. I need
only 40mA, and I have found ON Semiconductor CS8182 (SOIC-8) and
Infineon TLE4250G (SCT-595) for that purpose. However, they are not
stocked by suppliers such as Mouser and Digikey, and I have learned
from posts in this newgroup that I should never design-in
hard-to-source components. Is there any other tracking LDO you could
think of, which is extremely small and made by a more commonly
available manufacturer such as TI ?

I also found a lot of DDR termination tracking controllers but they are
all 1:2 and they are larger than my very constrained space would allow.

Thanks in advance !

Maybe you could use an LM723 in SO-14 plus 2R and 1C ? ;-)



Best regards,
Spehro Pefhany
 
L

legg

Jan 1, 1970
0
I am developing a product which requires interfacing my 5V circuit to
another 5V circuit whose power supply I have no control over, which
varies from around 4.75V to 5.25V in practice.

I wish to build my circuit's power supply to track the other circuit's
power supply instead of a fixed 5V. So, I am looking for a regulator
with an external voltage reference instead of an internal one. I need
only 40mA, and I have found ON Semiconductor CS8182 (SOIC-8) and
Infineon TLE4250G (SCT-595) for that purpose. However, they are not
stocked by suppliers such as Mouser and Digikey, and I have learned
from posts in this newgroup that I should never design-in
hard-to-source components. Is there any other tracking LDO you could
think of, which is extremely small and made by a more commonly
available manufacturer such as TI ?

I also found a lot of DDR termination tracking controllers but they are
all 1:2 and they are larger than my very constrained space would allow.

A surplus error amplifier could be used to modify your presently-used
circuit, by driving the summing point through an impedance much larger
than that of the voltage divider that is already in place. This would
only 'trim' the voltage.

Perhaps this might not use much more real estate.

RL
 
W

Walter Harley

Jan 1, 1970
0
Bo-Ming Tong said:
I am developing a product which requires interfacing my 5V circuit to
another 5V circuit whose power supply I have no control over, which
varies from around 4.75V to 5.25V in practice.

I wish to build my circuit's power supply to track the other circuit's
power supply instead of a fixed 5V. So, I am looking for a regulator
with an external voltage reference instead of an internal one. I need
only 40mA, [...]


How about using a rail-to-rail opamp, in unity-gain buffer configuration?
I'm sure you can find plenty with that kind of current capability.
 
B

Bo-Ming Tong

Jan 1, 1970
0
Thanks. That's an excellent idea !

The only potential pitfall I see is that my power could be turned off
(hence turning off the opamp) but the reference voltage (the circuit I
am interfacing to) is still on. Then I will be applying a voltage much
higher than Vdd to the opamp.
 
S

Spehro Pefhany

Jan 1, 1970
0
Thanks. That's an excellent idea !

The only potential pitfall I see is that my power could be turned off
(hence turning off the opamp) but the reference voltage (the circuit I
am interfacing to) is still on. Then I will be applying a voltage much
higher than Vdd to the opamp.

You can put series resistance in there to limit the current. One thing
you should be careful of if you try this is that most op-amps will
happily oscillate away when connected directly in a unity-gain
configuration to typical power supply bypass capacitors, so you need
to decouple the op-amp output from any serious capacitive loading (and
make sure that your compensation doesn't allow it to overshoot too
much).


Best regards,
Spehro Pefhany
 
B

Bo-Ming Tong

Jan 1, 1970
0
Yes, thanks for the advice...

I am rethinking this a bit due to the advice of you and Walter. The
reason why I wanted my power supply to track the other power supply is
that I am afraid of the I/O pin protection clamp diode conducting (on
either side), blowing things up. But as I think about it, using a
high-current rail-to-rail opamp leads me back to exactly the same
original problem that I am trying to solve. I am just dealing with a
protection diode in a different place. And on top of that I need to
worry about output capacitance like you said.

As you can see I have about 0.3V voltage difference between the 2 power
supplies. I guess I can just add 10k resistors to each pin of the
interface. If the power supply differences turns out to be great enough
to trip the protection diodes (in the normal case this should not
happen), the protection resistors should be able to limit the current
and saving me from blowing things up.

Is there going to be any problem if I interface two CMOS logic systems
together with a constant Vcc difference of 0.3V ? I wish the design to
be both durable and reliable. I think the durable part has been
addressed by protection resistors, but I am not so sure about the
reliable part. I had the pleasure (sarcastic) to deal with an alarm in
my car where the control signals would get mixed up, such as the door
lock control crossing over to the siren, when the not-so-well-designed
zener diode protection circuit to the ULN2003 driver IC kicks in. And
because of this lesson I don't want my own product to act crazy under
any situation.

Thanks again to the advice.
 
S

Spehro Pefhany

Jan 1, 1970
0
Yes, thanks for the advice...

I am rethinking this a bit due to the advice of you and Walter. The
reason why I wanted my power supply to track the other power supply is
that I am afraid of the I/O pin protection clamp diode conducting (on
either side), blowing things up. But as I think about it, using a
high-current rail-to-rail opamp leads me back to exactly the same
original problem that I am trying to solve. I am just dealing with a
protection diode in a different place. And on top of that I need to
worry about output capacitance like you said.

As you can see I have about 0.3V voltage difference between the 2 power
supplies. I guess I can just add 10k resistors to each pin of the
interface. If the power supply differences turns out to be great enough
to trip the protection diodes (in the normal case this should not
happen), the protection resistors should be able to limit the current
and saving me from blowing things up.

Let me preface this by saying that I don't have a clear idea of what
you are trying to do. Is this correct?

i) You have system A with a power supply that's "about" 5V.

ii) You are designing system B that has logic level connections to
system A.

iii) You wish it to be reliable.

If that's correct, maybe you can just use level translation methods to
deal with any mismatch.

Simplest way is to put some series resistance in the signal lines. I
won't go into that, there are plusses (cost, mostly) and minuses (may
violate the abs max input voltage rating, may cause unforseen effects,
slows down signals etc.).

You could also use 74HC4049/50s (with dual Vcc connections- one Vcc
for inputs- tied to the internal Vcc, and one Vcc for outputs- tied to
the external Vcc) or other level translation chips to inexpensively do
this. That part will tolerate inputs as high as 16V and has a
reasonable input threshold for CMOS. There are also other specialized
level-translation devices:

http://focus.ti.com/pdfs/logic/dsprodclip.pdf

The 74LVC245 is one jellybean choice (accepts inputs as high as 5.5V)



Best regards,
Spehro Pefhany
 
W

William P. N. Smith

Jan 1, 1970
0
Bo-Ming Tong said:
Is there going to be any problem if I interface two CMOS logic systems
together with a constant Vcc difference of 0.3V ?

IIRC, even CMOS doesn't swing rail-to-rail, so you shouldn't have any
trouble. The only time you would have a problem would be when one
power supply goes away, but why not run your circuit off the Vcc from
the other circuit, CMOS is pretty low power...
 
J

Jim Adney

Jan 1, 1970
0
I wish to build my circuit's power supply to track the other circuit's
power supply instead of a fixed 5V.

How about an op-amp with a transistor added, emitter follower style?

-
 
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