Voltage Drop across H-Bridge

Using Arduino Motors, Mechanics, Power and CNC
1

I am using the flowing shield: http://mcukits.com/2009/03/12/assembling-the-freeduino-arduino-motor-shield/
Which uses the following H-Bridge: http://www.makingthings.com/resources/datasheets/SN754410NE_slrs007b.pdf

I have a few questions before I put the kit together as I might need to invert the Vin header to supply a different Vin for the board then the Arduino.

  1. what is the maximum current that I can safely pull from vin header when the Arduino is supplied by the barrel jack (want to make sure I don't burn traces on the board)?

  2. From the data sheet, it looks like the motor voltage is likely to be around ~2 volts less then the drive supply voltage (page 4, High-level output voltage & Low-level output voltage). However it is rated at 1A continuous and max dissipation of 2075 mW, which would only allow 0.5 A continues per channel at a 2 volt per channel drop. Am I miss reading one of the specifications?

2

I found the answer to question 1:

Looks like the reverse voltage protection diode is typically rated around 1A - so less then my stall current of my 2 motors, so I need to do separate Vin for the shield.