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

[Bo-Ming Tong]

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 !

 

[Bo-Ming Tong]

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.

 

[Bo-Ming Tong]

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.