USB supplied circuit problem

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Hi there,

I have a problem here. I am writing the source code for a USB supplied product. The USB ic I use is the FT232R that comunicates with the pc using the latest Virtual Com Port driver from FTDI.

Before suppling the circuit from the USB port, I used an external 5Vdc power supply. The function was this:

1. function for 80ms - I consumption = 160mAdc
2. stop function for 70ms - I consumption < 10mAdc
3. step 1. again

After this, I suuplied the circuit from the USB port and I realised that when it functions, the supply voltage drops from 5Vdc to 4.7Vdc. A stable supply voltage is very critical for the application and I can't decrease the current consumtion (160mAdc) nor the function time (80ms), nor even the stop function time (70ms).

It is clear that the USB port cannot give enough current. Going at the Device Manager/Universal Serial Bus controllers and looking at the corresponding USB Root Hub/Power(tab), I see this list:

    Description Power Required
    USB Serial Converter 90mA
    JTAGICE mkII 500mA

That seems to me that this USB port could give up to 500mA if a there was a mkII connected.

Note: The point is that the product must function connected at any PC USB port.

Is there any good idea of how to pass this problem?

Thanks.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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A buck/boost converter.

The USB spec specifies the voltage on Vbus can be between 4.75V and 5.25V. For lower power devices the minimum is 4.40V

http://www.beyondlogic.org/usbnutshell/usb2.htm

A USB device is not allowed to draw more then 100mA until it has been enumerated and has advertized its power needs.

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Ah,

Thanks for this nice information.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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And, a host is not required to supply more than 500mA to a USB device after is has been identified, so your device should not require more than this amount to be used in all/widest variety of PC devices.