Buck Converter minimum input voltage

[maurosmartins]

Hello all,

I have a big question and I'm not being able to find an answer.

If I have a DCDC buck converter and I need a output voltage of 5V how can I check the minimum input voltage that I must supply to the DCDC converter? On a linear regulator this would be a parameter called Voltage drop, but on a DCDC converter I don't have such parameter.

I need to supply power to a raspberry pi from a 7.2V NiMh battery that can go as low as 6V. Besides the raspberry I have some more electronics and I estimate the total current draw to be between 2 and 3Amps peak.


Best regards, Mauro.

[mariush]

I would assume they would output 5v from as low as 5-5.1v but it depends on what inductor you use, the frequency the converter runs at, the duty cycle.. and other things.

If you can't determine from datasheets you could build a simple constant current load and test the dc-dc converter... here's an example of such simple load : http://www.sleepyrobot.com/wp-content/uploads/2012/01/currentload-rev2.pdf

Alternatively, pick a dc-dc converter chip that can also be simulated or has design software made for it...
texas instruments has  software "Switchers Made Simple" for LM2xxx dc-dc converter chips and others and they have some other online tools here: http://www.ti.com/lsds/ti/analog/webench/overview.page
linear technology has ltspice and lots of their chips available to be simulated if you want to ..

[Kremmen]

Does it have to be a buck converter? With minimal extra complexity you could use the SEPIC topology by adding just 1 cap and 1 inductor. Several switcher chips can run the SEPIC configuration as well as buck. Then your input voltage can be below, above or same as output voltage.

[justinjja]

They are all different, What DCDC Conterter are you using?

Maximum duty cycle, Diode voltage drop, transistor voltage drop, inductor voltage drop, output current...
all can play a role in dropout voltage.

[CarlG]

The long answer:
Minimum duty cycly is one parameter you can check. May not be in all datasheets though. Theoretical minimum is 5/6 = 83,3%, but you'll probably need around 90% duty cycle depending on efficiency.

Some buck converters can run at 100% duty cycle, then you'll be safe. Otherwise you'll probably need a synchronous buck to get high enough efficiency.
 
The short answer:
Use e.g. LT, AD, TI power design tools to see what fits your design.