ST Micro app note AN4241 shows ways of connecting logic power on L6470's to external 3.3V & 5V supplies. For external 3.3V it shows VREG & VDD are connected. Is this correct? It seems like the internal L6470 3.3V would content with the external.

[EMcCa.1]

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[Cristiana SCARAMEL]

In the L6470 the VREG​ pin is the output of internal 3 V voltage regulator (starting from the motor power supply) and provide an external logic supply that can be used to supply the VDD pin in order to make the device digital output range 3.3 V compatible.

The VDD pin voltage sets the logic output pin voltage range.

VDD is not internally connected to VREG, an external connection is always needed.

The AN4241 show the possible alternative supply scenarios:

• Fully self supplied device: VDD supplied by VREG (SDO 3.3 V compliant)
• VREG self supplied and VDD externally supplied at 5 V (SDO 5 V compliant)
• VREG and VDD externally supplied at 3.3 V (SDO 3.3 V compliant)
• VREG externally supplied at 3.3 V and VDD externally supplied at 5 V (SDO 5 V compliant)

Applying an external 3.3 V regulated to the VREG pin in order to supply all the internal logic you can avoid power dissipation of the internal 3 V voltage regulator.