LM46002: Inverting Buck Topology - some questions

Graham North29

Intellectual 385 points

Part Number: LM46002

Design:

24V In

-17V Out

0.5A max

At what load should I expect the part to drop into PFM operation? How is this affected by the switching frequency set by RT?

What amount of output capacitance should I have for -17V Out, and what type of capacitor?

Thanks,

over 3 years ago

0 Marshall_Beck over 3 years ago

TI__Mastermind 29480 points

Hello Graham,

PFM mode is determined by peak inductor current.

If peak inductor current is below ipeak min, it is in PFM mode.

Alternatively, if you want to operate in constant frequency normally and "predicatably" move into a sleep mode by gpio pin, we have 60V device with mode pin.

Switching frequency will reduce ripple. So reduced ripple means ipeak min will most likely be triggered earlier for a given power condition.

Typical capacitance would be that expressed in datasheet. Inductor would be varied for desired ripple and transient performance.

0 Graham North29 over 3 years ago in reply to Marshall_Beck

Intellectual 385 points

Hi Marshall,

Thank you for your quick reply.

Is there a way for me to work out when the peak inductor current will go below ipeak min?

I'm unable to change the footprint now, is there a pin compatible part?

The data sheet only talks about +ve output buck designs. The Application note SNVA722B only gives a table (3) with three example output values.

Regards,

Graham

0 Marshall_Beck over 3 years ago in reply to Graham North29

TI__Mastermind 29480 points

Hello Graham,

I will need to check with designer what ipeak min is for the device. Typically, it is 20-30% of rated load current.

We don't have pin compatible part for this, as far as I am aware.

0 Graham North29 over 3 years ago in reply to Marshall_Beck

Intellectual 385 points

Hi Marshall,

Thanks for the reply. It would be good to get the answer from the designer.

Can you help with the recommended inductor and output capacitors for my application?

Thanks,

Graham

0 Marshall_Beck over 3 years ago in reply to Graham North29

TI__Mastermind 29480 points

Hello Graham,

13% of rated load current for the device (2A) would be ipeak min.

Webench would be good start for component selection. I can review the outputs.

0 Graham North29 over 3 years ago in reply to Marshall_Beck

Intellectual 385 points

Hi Marshall,

Thanks for the reply.

Webench won't do an inverting buck design for this part, so that's why I am asking here.

Many thanks,

Graham

0 Denislav Petkov over 3 years ago in reply to Graham North29

TI__Mastermind 18940 points

Hi Graham,

What inductor value do you have for this design and what is the RT resistor value?

It think both of these will have an effect on the entry point to PFM mode.

Cheers,

Denislav

0 Graham North29 over 3 years ago in reply to Denislav Petkov

Intellectual 385 points

Hi Denislav,

The inductor is 47uH.

RT is currently not fitted, but I'd really like to run it at 1MHz if possible.

Regards,

Graham

0 Marshall_Beck over 3 years ago in reply to Graham North29

TI__Mastermind 29480 points

Hello Graham,

Please reference equation 8 in attached doc for inductor selection for given switching frequency.

www.ti.com/.../snva856a.pdf

0 Graham North29 over 3 years ago in reply to Marshall_Beck

Intellectual 385 points

Hi Marshall,

The equation gives me 51.2uH, so I'm in the ball park with 47uH?

I sent you a friend request so I can get details of my other support question to you, did you see it?

Thanks,

0 Marshall_Beck over 3 years ago in reply to Graham North29

TI__Mastermind 29480 points

Hello Graham, 47uH would be a good inductor selection it seems.

I will accept your friend request

Power management

Power management forum

LM46002: Inverting Buck Topology - some questions