Regulator problem MAX1672

[GSXR1181]

I have a MAX1672 Step-up/down regulator. It is preset to regulate at 3.3v. Now the issue is when Vin is dropped below 5.1v, Vout will increase to 3.45v. I'm really needing the regulator to regulate at 3.3v with a MIN Vin of 4v due to a sensitive Vref thats getting pulled off the 3.3v rail.

I figure it has to do something with the 1672 switching modes. Can someone please explain why I'm seeing this? So I ask these two questions.

1. Is there a way to keep the Vout from moving +0.15v

2. Is there a regulator that's like the 1672 in a QSOP package that would keep this from happening. I can live with just a step down regulator that will take a Vin of at least 8v or more.

 

[Roff]

The MAX1672 has an internal linear regulator. Since your input only drops to 4V, you don't need the step-up internal regulator that is also in the MAX1672. You just need a low-dropout linear regulator. What is your maximum load current?

 

[GSXR1181]

Not for sure, I will check the bear minumum tomorrow morning. I beleive the 1672 say 350ma max. I will be turning a small motor, so I will say at least 300ma. I've got to be careful not to have the voltage drop when the motor rotates. Again I will get some numbers tomorrow.

 

[GSXR1181]

Just checked. I draw about 250ma max. So I'd say 300ma is at least what I need. I'm having major trouble finding one that that is just a step down with PGI/PGO Low batt comparator.

What would anyone recommend?

 

[Roff]

The integrated low battery comparator does make the search more difficult.
Also, do you need more efficiency than you will get from a linear regulator (the MAX1672 has a linear regulator)? The MAX1672 will not handle 300mA when the supply is above 5 volts. Any linear regulator you use will dissipate at least ((8V-3.3V)*300mA) 1.4 watts. Perhaps you need a buck regulator. You can buy small comparators with built-in references if you can't find the low battery function built into your regulator.

 

[OutToLunch]

what are the min and max input voltages?

you need an integrated comparator? could you elaborate on that as well - what input is considered good versus bad?

 

[GSXR1181]

Sry if I may seem confusing. I'm a programer thats been pulled into troubleshooting hardware. I know just enough to get by.

Here is what I know I need.

Vin wil be 7.5 to 0 (5 AA batterys)

Vout to be 3.3v (need this to be stable or it throws the calibration of the inst. off)

I'm looking to have the insturment to kick off around 4v-4.5v. I don't mind having a seperate comparator because the PCB is going to getting layed out again anyway.

Imin has to be 300ma.

How would a MAX710 work? Or is there something better?

 

[Roff]

As I read the datasheet, MAX710 is a linear regulator when Vin>Vout, which is always your case. If this is true, it doesn't appear to be able to handle the power dissipation. It might also have the same problem as the MAX1672, since it operates in a similar fashion.
I think MAX887 will work. I also looked at MAX921, which is a comparator with built-in reference, which you could use as your low voltage indicator. There are certainly other parts that will perform this function.
Having said all this, I should warn you that I have almost NO hands-on experience with switching regulators. I'm just an old retired (mostly analog) engineer who appears to know slightly more about them than you do.

Are you sure that constant voltage will satisfy your needs? What do you hope to accomplish with constant voltage? You won't get constant RPM if the mechanical load varies. Temperature changes (ambient and self-heating) will also probably cause RPM to change.