Question for Dual input XOR Power Switch and Battery

Apr 18, 2007 3 Replies
C

Hi,



I have a question: In my design I have 2 power input: USB (5V) and Serial+Power Jack (9V). I see in differents computar that the voltage in the USB port not is 5.0V exactly. I want to know which is the best way for implement a automatic xor and power switch regulate for ouput



5V 500mA. The idea is that is the user connect the USB the hardware have power and communication for the USB PORT and if connect the Serial
+Jack the hardware have power for this last. And is connect the two together the priority is the USB.

SERIAL+JACK------|====================| | XOR SWITCH REGULATE |---------OUPUT 5V



500mA USB---------------------|====================|

The other point is that I use a SuperCap for battery operation, is the user disconnect the USB and/or the SERIAL+JACK the hardware can finish the operation. How to put the SuperCap in the this design.



Which is the best design for this problem???????



Thank you very much!! for help me.



Best Regards,



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T

The first thing to bear in mind is that the USB will only release 100mA until you specifically instruct it to allow 500mA.

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C

I not understant that you say. Please check this link

formatting link
that say:

  1. How much power does a system in S3 need to supply to USB?

A: 500mA per USB port. See section 7.2.3 for the details of device behavior during suspend and resume. The Device Working Group is working on power management extensions to the common class definition that should alleviate most of the S3 power supply issues with USB. Contact snipped-for-privacy@usb.org for more info.

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T

The USB port certainly _can_ supply 500mA (assuming it really meets the specs) but only after a negotiation with the host to request the extra current. With no handshaking and just using the port as a voltage source then the port will only permit 100mA to be drawn.

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