ST7 VddF and i/o protection against external Vcc (Gng , 3V or 5V)

[pascal239955]

Posted on June 02, 2005 at 03:30

ST7 VddF and i/o protection against external Vcc (Gng , 3V or 5V)

[pascal239955]

Posted on June 01, 2005 at 12:51

Hello,

we have done an application with ST72651 able to Read/Write serial flash memory embedded on the board , on a socket and driven by USB. The power supply of Serial Flash is VddF output. Others signals are ST7 i/o with Vddf voltage. All this work very well.

Updating the board, we want to provide to the customer a external connector , linked to this signals vddf & i/os to help him to connect board to his application and prog / read serial flash directly on his application; so vddf will be connected to Serial Flash Vcc AND ALSO on external pin of external connector ; same for others i/o. So the customer will be able to read/program serial flash on the embedded socked of this board OR on his application.

The question is : are VddF output and i/o protected ? If the customer link the Vddf output (3V for exemple) to HIS Vcc power supply (3.5 or 5V or ground). Is there any protection or the ST7 will died ? or only VddF decrease ?

Thanks for your reply,

Best regards.

[alok2]

Posted on June 02, 2005 at 03:30

Pls note that Vddf current sourcing capability is 50mA. if the external serial flash consumes less that this ( i am sure it will), it is OK for external connections.

Pls do not short the Vddf with another Vcc ( coming from different supply). This is not a recommended configuration. If you want to use external Vcc, keep the internal regualtor of 7265 in OFF condition and short the extrnal supply to Vddf. So all IOs shall be powered frm single supply.

Another configuration possible is that there are 2 supplies ( external supply = Vddf) but there is no physical connection between these two supplies. The IOs can then be connected for data transfer. Supply voltage shall not be different.

Vddf is protected only upto Vddf Rail. Pls do not inject current into Vddf IO with Vin > Vddf. It is not a recommended configuration.