L6480 Stepper Motor Power Supplies Question

Brad Jensen · ‎2017-02-20

Posted on February 21, 2017 at 07:11

Hello,

I am looking at using the L6480 in the externally supplied modes, where my system will be supplying:

VS = +24V DC

VSREG/VCC = +12V

VCCREG/VREG = +3.3V

The +24V supplied to VS by the system is removed (floating) during an emergency stop condition.

Can the L6480 handle this situation where VS and thus VBOOT is removed, but the other rails are still supplied with the +12V and +3.3V respectively?

On page 34 of the data sheet there is a note that states 'VSREG must be always less than VBOOT in order to avoid related ESD protection diode turnon'

When VS is removed due to an emergency stop condition, VSREG will be +12V greater than VBOOT (floating), causing the internal protection diode turnon.

It goes on to say 'The device can be protected from this event by adding an external low drop diode between the VSREG and VS pins' however there is no detail of direction, rating nor if this will cause undesired operation with such a large difference in voltage.

This is where I am confused about the diode. Regarding direction, the diode would have the anode connected to VSREG and the cathode to VS.

During the emergency stop condition this would pass the +12V, less the forward voltage drop of the diode, to the VS pins and charge pump, and try to continue to power the motor from the +12V supply via the diode.

With the diode in the reversed orientation, VS less the diode forward voltage would be applied to the VSREG +12V which violates the 7.5-15V limit of VSREG.

So can I SAFELY use the L6480 without diodes when VS is removed?

Or the L6480 cannot be used in a system where VS is removed?

Thanks,

Brad

#l6480-power-supply

Enrico Poli · ‎2017-02-28

Posted on February 28, 2017 at 14:35

The protection diode must be put with the anode on VSREG and the cathode on VS. You can also use the EVAL6480H-DISC schematic as reference.

In this way when the 24 V supply is removed the VS pin is kept high by the 12 V line. As soon as the 24 V supply is restored, the VS pin returns at 24 V.

TAKE CARE! You must absolutely avoid having the power stage operative when the main supply is removed, or you'll supply the power stage using the 12 V through the diode! Result: a burned diode.

About the reverse voltage rating of this diode, it just need higher than VS supply, so a 30 V diode is enough.

Brad Jensen · ‎2017-02-28

Posted on March 01, 2017 at 00:34

Thanks .

Poli.Enrico

That was my concern, that if the 24V supply is removed AND the power stage is active, then the power stage will try to continue to operate from the 12V rail via the diode, resulting in diode failure.

Unfortunately I cannot guarantee the power stage to be inactive when the 24V is removed (via emergency stop condition).

I was thinking about separating the VS supplies from the FET bridge and the L64

Could I supply the L6480 VS with constant 24V, but have the protected supply to the FET bridge?

So when the 24V is removed, it is only removed from the FET bridges, not VS on the L64

Is there an issue with the L6480 attempting to drive the gates of the FETs with no supply to the FETs?

Enrico Poli · ‎2017-03-02

Posted on March 02, 2017 at 13:01

Hi

Jensen.Brad

‌

It is not safe to decouple the power stage supply from the VS pin of the IC; there is the risk to violate the absolute ratings of the OUT pins (-5 V to VBOOT).

Do you think it is possible to force a reset of the device just before removing the supply? In this way all the MOSFETs will be turned off and there will be no possibility to overstress the protection diode.