TPS5430: IBB for 12V in = +5V/-5V out @ 0.7A

Hi Frank,

Thank you for the response. I read through the document you provided (including slva317b and SLVA721A). However, I have trouble understanding the current calculation part. 

On the snva856a there is an example that uses LMR33640, however, it is rated at 3A, if 1A is my requirement, what would be the changes I should do? (or do we have a converter that can convert + voltage to a - voltage directly?)

I hope you can explain further. 

Regards,

Marvin

Hello

In your case you would first calculate the inductor current assuming an efficiency of 0.85, from eq. 4.  That

gives about 1A.  Then assuming 30% ripple you get 0.3A of ripple.  Using eq. 6 you get a peak of bout 1.2A, or so.

So, you need a device with at least a current limit of 1.5A to be safe.  

You can use the LMR33620 and the same circuit shown in the app note for the LMR33640 and change the inductor.  Or another device with 

a 1.5A or 2A current rating.  The inductor size is calculated with eq. 8.

A good way to test is to get one of the buck EVMs for the device you choose and configure as an IBB as shown

in the app note.

Thanks

Hi Frank,

Thank you so much for the help. We have now progressed with the schematic and tried simulating using Pspice. However, I encountered an error. the pin 4 (PG) is having error if left floating. I checked the datasheet for the device and I found that of Pin 4 is not used. I can left it floating but the simulation model does not allow that. Can you help? Here is the file.

LMR33640_TI.zip

Also, can you check if the schematic and values are correct? Thank you kindly.

Regards,

Marvin

Hello

Your calculations are correct.

You can not hook up the simulation model as an IBB the way you have it.  The simulation model has

internal ground connections that are global.

You will need to hook up the model as shown in figure 8 of the application note SNVA856.

You would hook up the input supply as shown in the figure.

To avoid a floating node on PG, you can connect a resistor from the PG node to the GND pin of the model.

Thanks

Hi Frank,

I updated the schematic and the issue with the PG has been resolved. I checked the input supply against figure 8 but I am unable to find any difference? Would you kindly specify? 

I run the simulation using the updated schematic and got this:

Thank you. 

Regards,

Marvin

Hello

You need to start with a normal buck schematic with all the grounds in the normal place.

Get the buck to work.

Then just take the input supply and connect its negative terminal to the normal buck output.

Thanks

Hi Frank,

I did what you instructed but I still cannot get this to work. 

There must be something I have done wrong in the schematic or simulation setup. Can you show me how its done? 

5.zip

Thank you.

Regards,

Marvin

Hello

Let me re-assign this post to the engineer assigned to E2E for this month.

He will be able to help you further.

Thanks