Low Dropout 5 - 24 V 2A Switcher?

Thanks, John!  These look like they'll do it.

Regards,

Gerrit

Hmm, the devil's in the details as usual.  The TPS54340 has this line buried in the section about low-dropout operation: "To reduce the losses of the small low side MOSFET at high output voltages, it is disabled at 24 V output and re-enabled when the output reaches 21.5 V".  This sounds like the low-dropout behavior doesn't persist for >= 24 V input, and wouldn't you know my application requires low-dropout 24 V input.

The TPS54335 doesn't actually discuss low-dropout operation like the '54340 does.  The only hint I can find is a minimum high-side switch off duty of 0%.  I don't see anything describing or spec'ing low-dropout operation specifically, or what the dropout might actually be.

Is there more about low-dropout operation with the '54335, or can you suggest another switcher that doesn't have the limitation the '54340 does?

Many thanks,

Gerrit

What is your output voltage? 

My input voltage can be from 5 to 24 V, and I will program the output voltage from as close to zero to as close to the input voltage as I can get.  I want the output to be less than 1 V from the input rails if possible.

If you are always wanting Vout to be as close to Vin as possible, then maybe an LDO controller would be your best option.  For dc/dc converters, operating over both a wide input and wide output voltage range provides other challenges besides drop out, specifically inductor selection and loop compensation.  You can read this to understand about the limitations using non-synchronous solutions:

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slva547&fileType=pdf

For TPS54335, the low side FET can always turn on to refresh the BOOT cap.

Yes, an LDO controller is what I want.  The original problem posted here is that I can't seem to search for the LDO types, and I am looking for recommendations.  The link you sent mentions a couple other LDO controllers, which I will study.  Are there any others I should know about?

My loop can be overdamped, so I hope not to run into compensation problems.  

It's good to know that the TPS54335 will continue to refresh the boot cap even up to 24 V input.  Can I assume that its low-dropout behavior will be the same as the TPS54340 in general?  There is a section in the '340 data sheet devoted to low-dropout operation, but not in the '335 data.

Thanks,

Gerrit

TPS54335 is a synchronous converter.  It does not operate in the same manner as TPS54340 and others in it's family.  It is a relatively new device, and I have not done too much investigating into low drop out situations yet.

For LDO controllers, here is a link to the landing page.  It has a parametric search function to help you choose the best part for your application:

http://www.ti.com/lsds/ti/power-management/ldo-controller-external-fet-products.page?paramCriteria=no

I saw that category under "Linear Regulators" and didn't think it applied to my application.  I'll look into them further.

Thanks,

Gerrit

Nope, that's not what I need.  LDO controllers are to build linear regulators.  I need a low-dropout switcher.

The TPS54240 mentioned in the SLVA547A app note you sent might do it.  At least it doesn't have the warning about low-dropout not working for >= 24 V in.  I'll check into it further and get back to this thread with what I find.

Thanks,

Gerrit

If you are operating with Vout very near Vin, then the advantages of switchers over LDOs diminish.  A "switcher" operating at 100% duy cycle is essentially an LDO. For 24 Vin put and say 23.5 V output, the efficiecny would b3 over 97%.  On teh low end, for 4.5 V input and 4 V output, efficiency would still be 89%.  Depending on your choice of pass transistors, you may be able to get even higher outputs.  I guess I do not fully understand your application.  It seems an LDO controller would be the best solution.

This thread is getting a little unwieldy, and maybe I didn't make it clear.  I will be programming the output voltage during use to be 0 - Vin, or as close to those rails as I can get.  So sometimes it will be like an LDO but sometimes the output will be almost zero.

Latest update: I went back to WebBench and began entering trial design parameters like 5 V in / 4.5 V out, 12 V in / 11.8 V out, etc, and the TPS54332 kept popping up as a candidate device.  Checking the data sheet, there are actual graphs of min and max Vout vs Vin!!  Why aren't these graphs a common feature of switcher data sheets?  It's been like pulling hen's teeth to get this information, and I didn't learn about this part on this forum either.

I will pursue this IC further, and I've ordered an eval board for it.  I hope it will do the trick.

There is some variability in the datasheet contents.  If we get a lot of questions on certain aspects that aren't covered we try to add them in to new device datasheets and add them in when older datasheets are revised (although we tend not to revise datasheets unless absolutely necessary).  If you are going to operate at light loads, make sure to pay attention to the SLVA547 document I referenced earlier in this thread.  It is applicable to TPS54332 as well.

John's suggestions are good integrated FET switchers.    Another alternative  that operates at a true 100% duty cycle in dropout is the LM25085A PFET controller.  A PFET does not require a bootstrap capacitor for gate drive so there is no need to turn of the buck switch occaisionally to refresh to capacitor voltage.

Thanks to both John and David for your helpful suggestions.  The LM25085 looks very interesting, because it's more important to get close to the + rail than ground in this application.  WebBench also supports it, so I will chase this one next.