There are no App notes that I'm aware of. Can you be a bit more specific about what you're trying to achieve? Are you a USB host or device? Are you using a USB stack? Baremetal or RTOS?
As a device, for example, you could use Freescale's baremetal stack, and it will handle suspend for you. Stack version 4.1.1 added an example implementation for the TWR_K20D50M to the hid_keyboard example, that will put the device into STOP mode after the host issues a suspend.
