regulator output protection (in micro circuit)

[Electro nS]

hello i am designing a small circuit containing a micro controller and switching regulator + usb programmer .

when the USB is connected to the computer 5v will be present in the circuit in addition to the 5v supplied by the regulator (switching LM2575) is this harmful ?? will they fight each other ???

another thing if there is NO input for the switching regulator , and the 5v is supplied only from the USB port (which is more common during programming and debugging ) will this be harmful for the regulator which means that there is 5v on its output and no voltage at the input ??

what type of protection should i use in each case

i have attached a block diagram to clarify


please help and thanks in advance

 

[FvM]

Usually the voltage sources are combined by schottky diodes or active MOSFET switches.

 

[dick_freebird]

You might look for a regulator that advertises reverse
polarity / reverse current protection, and set it to just
below what the USB will force. Then (so long as the
regulator doesn't have significant sinking ability) it
might turn out to be almost a freebie.

 

[FvM]

The other critical point is that you must never drive VBUS of an USB device connector.

 

[Electro nS]

The other critical point is that you must never drive VBUS of an USB device connector.

what do u mean ? i shouldnot give voltage to the usb port ?

- - - Updated - - -

Also just to make sure i got this right , if no input is present on a LM2575(or similar ) and 5v is present on the out put from other source , THIS IS OK ! and WILL NOT CAUSE DAMAGE to regulator ??

 

[FvM]

Also just to make sure i got this right , if no input is present on a LM2575(or similar ) and 5v is present on the out put from other source , THIS IS OK ! and WILL NOT CAUSE DAMAGE to regulator ??

There might be a risk of damage by inrush currents if larger capacitors are connected on the input side. I would at least connect a reverse diode as suggested for linear regulators.