boost regulator with LDO mode when Vin > Vout ?

From: Bo-Ming Tong (cyberstudio_at_shaw.ca)
Date: 03/25/05 

Date: 24 Mar 2005 17:38:51 -0800

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

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