Can you use FT232RL LDO reg to power RS485 driver?

T Chap

Would you guys conclude from the info below that the FT343RL would be OK powering a MAX3430 RS485 line driver?

The FT232RL states that 50mA can be drawn from it's LDO 3v3 out. The MAX3430 Supply current states 10 mA.

Thanks

Phil Pilgrim (PhiPi)

Although the MAX3430 driver provides foldback current limiting, in case of short circuits, the output current peak can be as high as 250mA before the current limiting kicks in. This will translate to a peak in I_dd that exceeds the FT232R's ratings. It's possible that a large-enough filter cap on 3v3 out could absorb a transient like this, but there may be other factors, such as soft-start considerations, that limit the size of such a cap.

-Phil

T Chap

So you are suggesting that in a very controlled setting where a short was unlikely, that the idea my work I take it. In the real world where others may induce a short accidentally, I would provide a dedicated regulator. Thanks for the heads up.

Phil Pilgrim (PhiPi)

Todd Chapman said...
In the real world where others may induce a short accidentally, I would provide a dedicated regulator.

And in which parallel universe were you planning to install this?

Seriously, it looks like it should work, barring any overloads. Besides, the alternative is a lot more complicated. I'm not sure where they get the 10mA, BTW. The driver provides at least 1.5V into a 54-ohm differential load. That's 27mA. As a consequence, I would recommend using AC termination at the far ends of your network, rather than DC. Each would consist of a 120-ohm resistor in series with a 10nF cap. That will be enough to suppress echoes without excessive DC current draw.

-Phil

T Chap

That AC trick is a good idea. I will test it and see how it goes. Thanks for the information.

I think I may have the real world thing backwards.