1. Home
  2. Electronics Design
  3. boost regulator with LDO mode when Vin > Vout ?

boost regulator with LDO mode when Vin > Vout ?

Hi all,

I am trying to design a power supply for a 5V/50mA circuit, which is powered by either a non-rechargeable lithium battery through a TMOS switch (measures 5.7V), or from another switch-mode power supply giving around 4.2-4.8V. I don't have any control over this power source.

As you can see the desired output voltage is sometimes higher and sometimes lower than the input voltage. I looked at MAX1672 and TPS61027 because the boost regulator + linear LDO approach seems to make a lot of sense in this application. However, I have only 2mm of height (plus thickness of PCB) for this and I couldn't find capacitors this thin, as these chips requires 100uF capacitors for filtering. Cost is not a huge concern but I am severely constrained in space.

What other chips should I look at ? And pardon me I am a digital designer who knows nothing more than the very basics of voltage regulation. Thanks in advance.

Bo-Ming Tong

Reply to
Bo-Ming Tong
Loading thread data ...

TDK makes 100uF ceramic caps that are only 2.3mm thick. If that is still too thick, they also make 47uF ceramic caps that are only 2mm thick so you could parallel two of them.

--
James T. White
Reply to
James T. White

[....]

I think that Linear has a chip that becomes a capacitive voltage doubler when needed and has a low drop out circuit built in.

--
--
kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Hello Bo-Ming,

Look at the SEPIC architecture. These are boost converters with an extra cap and an extra inductor. That makes them capable of delivering voltages above and below Vin.

Look at the SEPIC part of the National LM3478 data sheet. Check the data, maybe you could even use this chip. It is around a Dollar and works very nicely. The frequency is high enough so you shouldn't need large capacitors.

Regards, Joerg

formatting link

Reply to
Joerg

Thank you for all the very useful responses !!!

Sorry for asking you the basics but what is the difference between ceremic and tantalum ? Also my output is 5V what should be the rating for the filter capacitor ? (From what I have heard for tantalum it needs to be rated at 3 times the voltage across it but not sure about ceramic.)

Reply to
Bo-Ming Tong

Initially, I did not choose a capacitive voltage doubler because of the very low efficiency. But as I searched more, it turned out to be the solution best suited to my application.

Don't laugh but I "discovered" that there are plenty of 1.5x charge pumps around. Given my voltage range of 4.2V - 5.7V, the output would be 6.3V - 8.55V, and I could use an LDO after that, with 60-80% overall efficiency. While not as efficient as an inductor-based boost converter, this solves my problem of finding very low-profile capacitors.

Most of these 1.5x charge pumps are for driving LEDs, which is fine because I will have an LDO at the output anyway, but how do I turn off the current regulation of for example a Maxim MAX1575 ? Also, what would you suggest for a simple 1.5x charge pump with very small footprint ? But not so small that I couldn't solder the prototype though.

Thanks in advance !

Reply to
Bo-Ming Tong

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.