Hi martino,
Here is a circuit I have done for your pulse generator:
spec
Issue 3 of 2016_09_01
NOTES
(1) The circuit has not been optimized and toleranced yet.
(2) The NMOSFET and PMOSFET are in a single pak: type NXP PHC21025.
(3) The output square wave can be set from 0V to supply voltage -1V (0V to 11V with a 12V supply).
(4) Unless otherwise stated, all capacitors are ceramic, X7R dialectic, +-10% or better, 20V working or higher.
(5) C26 is a low ESR aluminum or aluminum polymer electrolytic capacitor of 20V working minimum.
(6) C25 (timing) is a metal film capacitor of 20V working minimum.
(7) R4 and R5 must be mounted on the MOSFET gate terminals.
(8) Unless otherwise stated, all resistors are metal film, 250mW or more, +-5% or better.
(9) The schematic symbol for the LT3080 is not correct. IN and VCONTROL on the LT3080 should be connected together and are represented by IN on the schematic symbol. SET on the LT3080 is represented by GND on the schematic symbol.
DATA SHEETS
(1) LM555
http://www.ti.com/lit/ds/symlink/lm555.pdf
(2) LT3080
http://cds.linear.com/docs/en/datasheet/3080fc.pdf
(3) NMOSFET & PMOSFET
**broken link removed**