Question about mixing battery and 5 V on O'Micro

[flavigny]

Hello,
I am drawing a long lasting application. Board will be powered by LiIon battery. Time by time, I need to send a SMS; for prototype, I choose  external Usb modem (Wavecom Mod2, less than 40 $. )
So at this time I need Usb activated.

Is it correct to put momentarily, 5V (produced by an external DC/DC step-up converter) on the PWR jack (closing by a GPIO pin a TTL $ 2. relay as http://www.ebay.com/itm/New-One-1-Channel-Isolated-5V-Relay-Module-Coupling-For-Arduino-PIC-AVR-DSP-ARM-/330784564911?pt=LH_DefaultDomain_0&hash=item4d044be6af )

and to remove it (openning the relay) when the GSM communication is complete..

Regards.
PO.

[ngcoders]

You do not need to use a Relay , Micro has a SY6280 , which you can enable disable from PIN18/USB_EN , use that.

Also do make sure what is the startup current of the modem, most cheap modems draw huge amounts of current when reliably starting up. For ex SIM 300 can source 2A when starting ( USB Will not support it ).

[flavigny]

Thanks,
I know GPIO17 for shudown the Usb socket.
My interrogation is:
If I use that to disable Usb, I leave O'Micro powered by PWR 5V jack,
so my idea was to open the 5V circuit externaly.

About the power consumed by modem, I experience it and already added a powered hub on the path.
(so the modem continue to be powered by the hub even usb disabled).

here is why I plan a relay to power-down: O'Micro PWR jack, usb hub and modem.
I even dream about using Usb 5V out as trigger for the relay.

Regards
PO.