Positive to Negative DC/DC

Daniel Mazzer

Other Parts Discussed in Thread: LM20323, LM2673, TPS54140, TPS54160, LM3102, LM22670, LMZ14203

Hi everyone,

I planning to use the LM20323 as a negative to positive DC/DC converter, but I don't know if it will work.

The input voltage is around +20V, the output must be -15V @300~600mA.

The Application Note 1157 address this use, but for LM2673.

It is possible to use the LM20323 to generate negative voltage, like the application note show?

Regards,

Mazzer.

over 12 years ago

0 Chris Glaser over 12 years ago

TI__Guru*** 142750 points

I suggest you read the ground rules post at the top of this forum and then edit your post's title to make it easier for the correct engineer to identify the part you are asking about.

If that IC won't work, there is the TPS54140 with this app note: http://www.ti.com/lit/an/slva317a/slva317a.pdf

0 Marc Davis-Marsh over 12 years ago

TI__Genius 10095 points

Hi Daniel,

You are very close to the voltage limit of the LM20323 with this design, how much tolerance is on your 20V input supply. In general you need to look for a buck regulator that has a operating voltage > Vin + |Vout| . Or in your case 35V + some margin.

You can take many Buck converters and configure them into a positive to negative converter.

The main thing to watch out for in using this topology is that all of your ground referenced signals are now referenced to the negative output voltage. This causes you to need level shifting circuits to interface to Enable, and other signals.

I have a more detailed response in another post.

https://e2e.ti.com/support/power_management/non-isolated_dcdc/int-non-isolated_dcdc/f/198/t/163138.aspx

There are more details on level shifting circuitry and how to configure buck regulators for inverting applications in this document.

http://www.ti.com/lit/an/snva425/snva425.pdf.

Regards,

0 Daniel Mazzer over 12 years ago in reply to Marc Davis-Marsh

Prodigy 20 points

Hi Marc,

Thank you for the answer, that is exactly what I'm looking for.

My input voltage has about +/-5% tolerance, so the LM20323 is really not a choice for this design. I will look for another part.

Regards,

Daniel

0 JohnTucker over 12 years ago in reply to Daniel Mazzer

TI__Guru**** 191445 points

As Chris suggests, TPS54140 or TPS54160 would be a good choice depending on how much voltage headroom you desire.

0 Marc Davis-Marsh over 12 years ago in reply to JohnTucker

TI__Genius 10095 points

If you would like to stay with a synchronous buck converter you can use the LM3102

0 Hector Arroyo over 12 years ago

TI__Prodigy 565 points

Daniel

As mentioned previously, ICs with a 42V input range would be more appropriate for your application than those with a 36V range.

Here are a couple newer app notes that describe the inverting architecture in detail and how to implement it using off-the-shelf buck regulators, including the needed level shifters if you need to use EN or other external signals with them.

http://www.ti.com/lit/an/snva363d/snva363d.pdf (AN-1888)

http://www.ti.com/lit/an/snva425/snva425.pdf (AN-2027)

The first app note shows the LM22670 (regular buck IC) as an inverter (LM22670 is similar to the LM2673 but newer, switching at 500kHz)

The second shows the implementation using an LMZ14203 integrated inductor synchronous buck module, for increased simplicity and ease of implementation

Hope this helps

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Positive to Negative DC/DC