Solution for wide input power (2.7V to 42V in, 11V/120mA out)..?

[Tomle]

I'm in need of a power supply solution that should fit the following requirements, but are a bit at loss about topology/solution to choose.

1. Input case 1: 2.7V to 4.2V (3 cell NiMH)
2. Input case 2: 15V to 42V
3. Efficiency >= 75-80% in both cases. High efficiency is very important
4. Output: 11V / 120mA

I'm struggling to find suitable switch regulators both supporting the high/low voltage and the efficiency, and what topology could be used. Buck/boost, SEPIC...

Could actually the best solution be to have two separate power supplies each optimized for one of the two input voltage cases...?

 

[Orson Cart]

for the first case you need to find cmos gate ic's that have threshold voltages in the 1.5 volt region - you can then build discrete solutions to provide the 11V o/p, for the second case there are plenty of buck solutions to implement a solution, 2 power supplies to provide the two seperate inputs...feeding a common o/p.

 

[cks3976]

You may try LM2735 (**broken link removed**) for Case 1 and LM2840 (**broken link removed**) for case 2

Hope that helps

 

[mtwieg]

I found two possible chips after a bit of searching: the LM3578 and LM78S40 (the latter one only goes up to Vin of 40 though). I'd recommend a buckboost converter, so long as you're okay with the inverted output voltage. Less components than a SEPIC will make higher efficiency possible. For this purpose, you shouldn't use the internal BJTs in these chips, but rather have them drive an external FET for the converter. Selection of that FET, along with your switching frequency, will be key to getting the best possible efficiency.

Whether you can get your desired efficiency over your entire input range is uncertain. At low input voltage, high input currents will and higher switching losses will hurt you, while at higher input voltages the quiescent current of the controller will become an issue. Having separate power supplies for each case would work, but there's the added complexity and cost that comes with that....