What kind of technical documentation is necessary for a patent application

Congratulations on your startup venture - I wish you luck.

A design patent (in the US) protects the ornamental design of an object having practical utility. A design patent does not protect the functional aspects of an invention. In your case, a design patent would protect the design of the device -- what it looks like, the configuration of components, etc. In order to file for the design patent, you would have to submit drawings showing the device from various views which are dependent on the actual device and the points of novelty.

If you wish to protect the actual circuit layouts within the device, you would file for mask work protection which lasts for 10 years and protects "the predetermined, three-dimensional pattern of metallic, insulating, or semiconductor material present or removed from the layers of a semiconductor chip product..." Mask work protection is similar to copyright protection in that it prohibits the unauthorized reproduction of the mask work. In order to secure protection, owners of mask works must apply for registration of their claims with the Copyright Office. Registration must be made within 2 years after the date on which the mask work is first commercially exploited. If not, the opportunity to secure protection is lost.

In filing a mask work copyright application, you would have to submit drawings showing the three-dimensional nature of the circuit diagram(s) that you wish to protect.

There is a significant amount of information on "mask works" on the Internet if you do some searching. Wikipedia is a bit opaque on some of the practical issues - but it is a good starting point. Let me know if you have any follow-up questions. Best, Douglas