LMR70503: Inverting DC/DC Converter

Hi Seijiro:

Never mind, just for confirm...The Vout is too large for LMR70503, I will suggest you use a Buck converter. 

Here is an AN about that. https://www.ti.com/lit/an/snva022e/snva022e.pdf

Seijiro,

To get a non-isolated negative output voltage from a positive input voltage you would have to configure Inverting buck-boost topology.

See appnote: https://www.ti.com/lit/an/slyt286/slyt286.pdf

For 5V to -30V the SW voltage would be VIN-VOUT = 5-(-30)= 35V, so you would need at least a 40V device for this.

The duty cycle is D = VOUT/(VOUT-VIN), (-30)/(-35) = 0.86.

Also, the inductor current IL_avg = IOUT/(1-D).

At 5V the SW current would be 4.5A and at 3.3V it works out to ~5A, so you need at least a 5A device.

I think the LM76005 in inverting buck boost would be the best converter for this, LM76005 does not have inverting-buck boost WEBENCH model yet, but it should work.

I suggest you start a related thread specific to operating LM76005 in inverting buck-boost, you can link this thread for reference.

Thanks,

-Orlando