Wrong way usb connexion to MCP73831 lipo charger

Question

I have a problem with my mcp73831 lipo charger; i have a connection with two neodym magnet connected to usb in one side and my circuit with the MCP73.. in the other side.

There is no problem when i respect the connexion polarity (5v from usb -> to 5v mcp73831 and Gnd from usb -> to Gnd mcp73831).

But, when i change the polarity, the mcp73831 burned... and i was wondering why this IC is not protected from wrong polarity connection ..

So, i want to add a diode with low drop voltage connected to 5v from mcp73831, in forward-biased mode.

I add two circuits to better understand the problem.

Without diode:

And with diode :

Am I allowed to put a diode between the MCP73, would the voltage drop of the diode be too much?

Answer 1

Yes, this kind of protection against a badly designed connector is fairly usual. The only thing would be to place the diode BEFORE C3 capacitor, to make sure the input source (capacitive) impedance is within recommended value, to avoid potential instability of the linear regulator.

Answer 2

Ali Chen has address the diode aspect. I will address the connector aspect:

You should consider modifying your magnet connector to either -

• be keyed so it will only go in the correct orientation

or

• add a third magnet in line with the other two and use centre positive.

^{simulate this circuit – Schematic created using CircuitLab}

*Figure 1. With a three-contact arrangement the power lead can be connected in either orientation without damage. The mechanical design would have to ensure that the contacts could not be offset by one pitch to reverse connect.

I seems to me that you may have missed a trick with the magnet orientation. If you put them opposite polarities you won't be able to stick the connector on the wrong way.

         plug                case
 + -------S=N                S=N----- +
 - -------N=S                N=S----- - (GND)

It won't prevent someone forcing the contacts together but it won't allow them to hold.