Protecting regulator from alternate power supply

Question

I'm designing a module built around a Teensy LC MCU which can either run off of Eurorack power (+12/gnd/-12) or USB (mainly during development). The Teensy wants 5V and regulates down to 3.3V onboard, so I want to use a regulator to drop the 12V to 5V. In my first prototype this is a 7805:

D3 is protecting against reversing the Eurorack power connector (pins 1-2 carry -12V, which I don't need in this module). I gather that you don't want to put voltage on the output side of a 7805 with the input unconnected, so D2 is intended to protect it when the 5V is supplied from USB and the Eurorack is disconnected.

The module should be drawing under 60mA, so I'm not too concerned about temperature, but on general principle I'm considering using a switching regulator in place of the 7805, for sake of argument let's say an LM2574.

My questions are:

• Is D2 a suitable way of protecting the 7805 from output-side voltage? Does the drop across the diode leave a small reverse voltage through the 7805, and is that okay?
• Is a similar diode arrangement suitable for protecting an LM2574? What part of the datasheet tells me if it needs protection?

Answer 1

Diode D2 protects the regulator from reverse current spoiling the LDO.

The diode can be either schottky or normal diodes.

1. The schottky will be costlier but conducts sooner when the output voltage is higher than the input voltage (during shutdown or when input voltage is not present, but the output capacitors still have charge stored)

2. The topology of the regulator determines whether the safety protection is needed or not

3. Refer to the datasheet of the regulator for both topology and recommendations example below)