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L6470 / L6472 VREG line question

Jacek Pieczaba
Associate II

Hello,

I am designing the system with both L6470PD and L6472PD stepper drivers. Since my system is operating on 3.3V (MCU is STM32F103 ) I have tight the VReg with VDD line as suggested. The problem which I am facing is following. I supply the drivers from 48V while the MCU via DC/DC converter from 24V to 3.3V. If I am turning of the 24V only the logic is still supplied via VREG embedded in Stepper driver ( since 3.3V line is tight with Vreg ). This condition is not good for my application since a) ST not recommended to supply anything from this VREG b) I wanted to be able to turn of the logic completely when I turn on the 24V.

Please advice since it is not clear for me, can I maybe place the Schottky diode between VDD and VReg so there will be no possible supply the system from Vreg?

Or maybe I can leave open the Vreg and not tight this to VDD

Any suggestion is more than great.

Thank you very much

Jacek

13 REPLIES 13
BRYAN SPEED
Senior

Is it possible you can draw and post a block diagram of the power sources and how you have them connected or wish to have connected ? Not 100% following the setup. You can also submit this as a support request via support home via web request on st.com

Jacek Pieczaba
Associate II

Hello Bryan,

Thanks a lot for quick replay.

Here is the block diagram..

0690X000008iOGMQA2.jpg

Basically I am planning to use 48V to supply the power stage of the H-bridge in L6470 to drive the motors with defined torque. +24V is my main power supply for the rest of electronic system. I am rectify from 24 to 3.3V to supply MCU, Following to spec from L6470 I tight the VREG and VDD to 3.3V line. Now there is a scenario when I unplug the +24V input the MCU is still alive since is sourced from internal VREG from L6470 which is not good in my case ( i need switch off logic completely ). Can I place the Schottky diode in the way that I will be not able to source from VREG the logic , and if yes what I am risking there?

Thank you very much

Jacek

Hi,
Well, I am not an expert on this part, but it appears the :
The Vreg and VDD are indeed supplied externally.. I had to clear that up in my mind.
If you turn off 24volts, it seems all the 3.3 lines will be 0 volts, both the motor driver and MCU.
So I don’t see any need for steering diodes. Sorry, I have not worked with the part before, just reading datasheet, and trying to put myself in your situation.
Best Regards,
Bryan
Jacek Pieczaba
Associate II

Hi,

Thanks a lot for replay. Unfortunately is working in this way if I turn off 24V I will have 3.0V on the VDD rail ( not 0V ) from the internal regulator embedded in the L6470PD ( Internal VREG rectify from 48V to 3.0V ) , so my MCU will still respond. So my idea around Schottky diode was to not allow to feed anything from VREG.

Thanks a lot

Jacek Pieczaba

IS this measured ? I did not see the Vreg being powered from the connections from main power ☺
I will look again
Ah yes I see it now…
Jacek Pieczaba
Associate II

Yes it is measured...

Yes, of course, I did not read enough..! has been a rough morning.
So.. most elegant and simple perhaps, a “switch�? in line with L6470 Vreg, that the truth table is 24v = 1, switch on. 24v = 0 , switch off. So a series FET - so neither 24v supply output or mcu can see l6470 vreg voltage when 24v is off
Does that makes sense to you ?
But of other concern, is maybe the SPI lines could be “active�? but with no mcu power. I have no clue if that’s ok or not. ;)