SHDN bedeutet Shut down. Du musst diesen PIN unbedingt belegen, sonst kann es sein das es während des betriebs abschaltet. Zitat Polulu webseite:
The SHDN pin can be driven low (under 0.3 V) to turn off the output and put the board into a low-power state that typically draws 20 μA, and it can be driven high (above 2.3 V) to enable the board. If you do not need to use the shutdown feature, the SHDN pin can be directly connected to VIN to permanently enable the board. You should not leave this pin disconnected as this can result in unpredictable behavior.
Quelle:
http://www.pololu.com/product/2106