Switching Regulator minimum output voltage question

[asifjahmed]

Hello! This is my first post in this forum!

I am designing a custom board and am currently focused on the power supply. I am trying to use a Rohm BD9G101G (http://www.mouser.com/pdfdocs/bd9g101ge-2.PDF) due to its relatively low cost.

Here are my design specs:

Input voltage:             6.6 - 33.6 V
Output voltage:           3.3 V
Output current (RMS): 250 mA

In the data sheet, it says that the minimum output voltage is based on the minimum on pulse, which is around 100nsec. According to my calculations, this means that I cannot achieve 3.3 V if the input voltage is > 22 V. However, in one of their application notes (http://rohmfs.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/bd9g101g_brd_appli-e.pdf), they have a reference circuit with 3.3 V out and 24 - 40 V in. Can someone please help me understand this better?

Apologies in advance if I didn't provide all the necessary info. Any help is much appreciated!

Best,
-asifjahmed

[T3sl4co1l]

Lower duty cycle is possible due to pulse skipping operation.  Ripple will be higher and stability will be poor, however.

If you need a large ratio of Vin/Vout, consider using a tapped inductor or a transformer, and a controller or topology suitable for that method (i.e., forward or flyback converter, rather than buck/boost).

Tim

[rsee]

If you are into power supplies and want a little more theory, there is a multiple part course on coursera by the guys at University of Colorado, Boulder.  I can't vouch for that particular course series, but I did go through some of the lectures when there was only one offering.  Erickson did a great job explaining concepts I had been asking myself for awhile.  Also, Erickson's book is on sale from the publisher for nearly half off.  I picked up my copy two days ago and I'm super excited to dig in!

[asifjahmed]

Thanks Tim.

My target voltage range is 6.6 - 33.6 V. So I don't necessarily need a large Vin/Vout ratio in all cases. Also, one requirement is low cost.

Is using a switching regulator in pulse-skipping mode a bad idea?

[jwm_]

How efficient do you need to be? You can drop the input to 20v max with a linear regulator and do the rest with your switcher. Will need a heart sink, but nothing absurd.