boost regulator with LDO mode when Vin > Vout ?

[Bo-Ming Tong]

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

 

[Ken Smith]

In article <1111714731.791868.113660@l41g2000cwc.googlegroups.com>,
Bo-Ming Tong <cyberstudio@shaw.ca> wrote:

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.
[....]
this thin, as these chips requires 100uF capacitors for filtering. Cost
is not a huge concern but I am severely constrained in space.

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

 

[James T. White]

"Bo-Ming Tong" <cyberstudio@shaw.ca> wrote in message
news:1111714731.791868.113660@l41g2000cwc.googlegroups.com...

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.

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

 

[Joerg]

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

http://www.analogconsultants.com