How can I separate Android's encryption and lockscreen passwords?

[oily]

I just encrypted my external microSD card, but before l was able to do it l was asked for a minimum 6 character alphanumerical password (which is the lockscreen password). This is good because l know my data is secured but it's an annoyance to enter a long password each time I need to use my phone.

Now I only have the 'Password' option enabled for unlock the phone's screen (Settings > My Device > Screen lock) all the other options such as PIN, face recognition, pattern, etc, are grayed out and disabled.

How can I go back to using a PIN instead of a password ?