Nucleo-32 regulator maximum input voltage

Question

I have the STM32's Nucleo-32(NUCLEO-F303K8) board with its data sheet here.

And according to the data sheet powering the board externally is possible up to 12V mentioned as follows:

But in the same document the schematic shows the following voltage regulator:

When I look into that regulator's datasheet here the electrical specs is given as:

Above the maximum input voltage is given as 15V.

My questions:

1. What could be the reason the Vin of the Nucleo-32 given as 7V-12V instead of 7V-15V?

2. In another Nucleo board's datasheet I saw the following:

In the above case there is a diode between the E5V and the regulator output. But in my board's datasheet there is no diode between E5V and the regulator:

What is the purpose of diode in the first one?

Answer 1

It's possible they are being (just slightly) conservative. It's ill-advised to operate right at the maximum input voltage. Also 12V is a reasonable maximum for most purposes, allowing a bit of tolerance, and it's a standard voltage for power supplies.

Certainly the Nucleo evaluation/development board will not be capable of safely operating from a vehicle 13.8V nominal supply because of all the protection required in such applications against transients.

It's also possible they are taking thermal limitations of the regulator into account. That's not a simple calculation and would require a lot of work to confirm (and it depends on the PCB construction and maximum Ta expected, as well as reliability safety factors).

---

The diode is a low-drop Schottky rectifier and is most likely to prevent E5V from back-feeding the regulator. That's a poorly specified region of operation for most regulators and the diode may or may not be required depending on factors such a limits on E5V current, input capacitor type and value, and so on. Leaving the diode out removes the associated voltage drop, so it is to be preferred if the regulator does not require it for reliability under all conditions.