Q: Can TPS659xx devices be powered ON with a USB charger only (no battery connected)
A: No. This is not a valid configuration. This feature called constant voltage is supported only with AC chargers and not with USB chargers. For details you can see the battery management interface chapter in the correcponding TRM document. Documents are available at - http://focus.ti.com/docs/prod/folders/print/tps65950.html