Usb port power supply issues/ experiences

[longpole001]

Hi guys , was looking to use the power supplied from a USB PORT connector.


although usb spec says that its should never get to more than above 5.2v , i would like to ensure it never does , has someone a good known cct to ensure it never gets below 4.9v or above 5.1v ,

most LDO regulators have a volt drop of 0.5v that i have found

the cct needs 5V for lcd and other parts


regards

sheldon

[pedja089]

If you really want to do that you need buck/boost regulator, eg LTC3440 (http://cds.linear.com/docs/en/datasheet/3440fc.pdf).
Another solution is to use boost regulator to boost 5V to 6V or so, followed by LDO to get clean 5V.
But in most cases that is unnecessary, most LCD will work down to 4.5V, and up to 5.5V. Some pic work up to 6V.
So you only need to sort out ESD on USB connector, there is no need for OV/UV protection.
If you really want OV protection, you can use zener diode 5.2V 5W.

[longpole001]

yes ESD was on my mind found this as possible solution

http://www.nxp.com/documents/data_sheet/IP4220CZ6.pdf

[pedja089]

I think that will do a job.

[longpole001]

A need to find a cct that allows the usb power from the pc to the device to allow fore charge via the usb interface , but also i need the device to power other external items via the same usb interface when not on trickle charge

has any one a cct that is suitable


cheers

Sheldon

[pedja089]

You need charger and step up with enable pin. But you will need step up with load disconnect during shutdown.
If there is no power on usb port, enable step up and check if slave device is connected. If no, turn it off. Repeat that every 1-2sec.
Or you can monitor current sensor to usb current, and if there is no current, then no device is attached.
If you have power on usb, just enable charger, and that is all you need.
Other option is to bust battery voltage to 4,6V. If that is enough for slave device. When Vusb is higher than 4,5V then disable step up, and enable charger.

[longpole001]

not done a current monitor using a pic have you a reference to that

[pedja089]

Just google it...
You can use resistor and differential amp. Or hall current sensor, current monitor digital sensor...
Advantage of hall sensor is that doesn't have voltage drop across it.
Just pick what ever fit you budget, it would be enough to get detection of slave device.
http://www.ti.com/lsds/ti/amplifiers-linear/current-sensing-products.page
www.google.rs/search?q=current+monitor
etc...

[longpole001]

cheers pedja089