Freewheeling diode on regulator output

Question

A designer in our team has a habit of putting freewheeling diodes on the output of every voltage regulator. Buck converters, LDOs, you name it. The loads in our applications are mostly logic gates, FPGAs, DDR3 RAMs and similar. No motors or relays (inductive loads). The reason for this practice he says, is that if there is a sudden negative transient during power off, the freewheeling diode will catch this.

But. I have never seen this in any other application except for systems with inductive loads. Is he just wasting board space?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Assuming you are talking about a topology as can be seen below:

^{simulate this circuit – Schematic created using CircuitLab}

Then yes, there are valid concerns to at least consider doing this with (some) linear regulators (in some situations).

In the event that the input is forced low & especially if there is larger output capacitance than the local input capacitance, you end up in a case where the output voltage is higher than the input with a relatively high source of charge.

This is a valid concern in the automotive industry and you find regulators with such a diode on-die.

SHORTING THE REGULATOR INPUT

When using large capacitors at the output of these regulators, a protection diode connected input to output may be required if the input is shorted to ground. Without the protection diode, an input short will cause the input to rapidly approach ground potential, while the output remains near the initial VOUT because of the stored charge in the large output capacitor.

The capacitor will then discharge through a large internal input to output diode and parasitic transistors. If the energy released by the capacitor is large enough, this diode, low current metal and the regulator will be destroyed. The fast diode in Figure 21 will shunt most of the capacitors discharge current around the regulator. Generally no protection diode is required for values of output capacitance ≤ 10 μF.

http://www.ti.com/lit/ds/symlink/lm340-n.pdf