2389514_en-US

[#1]

33FS45xx - overvoltage capability of VAUX

I am using 33FS4500 and currently applying the VAUX supply as tracker to act as an external sensor supply. The use case includes short to battery level (32/40V) in error cases.

The 33FS* data sheet states max. 40V for all pins the of chip itself, but does not state the consequences for the external pnp transistor in case of an overvoltage and whether there is a reverse current protection between VAUX and the Buck.

- Is external sensor supply an intended use case of VAUX?

- do I need an extra protection for VAUX?
   and if - how does the circuit look like?
  a first idea might be a schottky diode between transistor and VAUX (and its feedback).

Re: 33FS45xx - overvoltage capability of VAUX

Dear HajoV,

using VAUX as an external/off-board sensor supply is an intended use case of the FS4500. VAUX can also operate in tracking mode to follow VCCA for ratiometric sensor applications. 

For a potential short-to-battery condition (32 V to 40 V), the FS4500 documentation specifies that the VAUX-related pins are rated up to 40 V absolute maximum and explicitly mentions that VAUX is suitable for off-board supplies. It is also recommended to use an output capacitor rated above 40 V.

We could not find any statement in the datasheet, Safety Manual, or FAQ that confirms internal reverse-current protection between VAUX and the Buck/VPRE regulator. Therefore, for robust automotive designs, we recommend considering external protection, such as a Schottky blocking diode and/or a TVS clamp, to prevent back-feeding and improve robustness against fault conditions and transients.

With Best Regards,

Jozef

Re: 33FS45xx - overvoltage capability of VAUX

Mhhh..

This information is not helping much and it is a bit disappointing as it only repeats the gaps in the data sheet.

Short to battery (and GND) is somehow the class 1 error condition, every mobile eletronics is expected to survive. If designing an SBC/sensor tracker supply , I would expect, that not only the chip but the required peripherals safely survive usual overvoltages.

External protection devices like (not so powerful) TVS diodes may act only above usual battery voltages.
So they do not protect the critical range this question is about  (8.. 32V).

Just to have a solution I am going to test a circuit with additional schottky diode in series to the pnp hoping that the linearity of the diode is sufficient to keep VAUX stable and test the circuit.
(I added some default load to enshure that the diode is always conducting.)

As I need << 100mA on VAUX I expect the diode drop to be tolerable.