Thanks for the answer.
No problem to switch on negative.
Could you explane me a little the principle of control of Ton Toff
time?
If you assume that the current in the inductor never stops, the Ton
and Toff requirements do not depend on the exact value of the
inductor. The average voltage on the inductor is zero so the output
is the duty cycle times the input.
If it wasn't for issues like overloading and start up inrush, you
could just use something like an LM555 to set the duty cycle of the
MOSFET.
When the transistor is on, the current in the inductor rises linearly
(nearly perfectly). This makes it fairly easy to use the current
change as a control on the on time.
When the transistor is off, you don't have any such easy thing to use
so making it simply how long it takes a capacitor to be charged is an
easy way to set that time.
Converters done this way don't run at a fixed frequency. They have a
longer on time when loaded.
The LM339 is a comparator that is fairly low cost. If you load one of
its sections with a capacitor of at least 470p, it will work at a low
quality op-amp. This op-amp can be used to look at the output voltage
and control the current peak value. You need very high values of
resistors because the voltage is high.
+Vout ----/\/\----+----/\/\---!!---+--/\/\--Vcc
! !
GND ----/\/\-----+--!-\ ! Vpeak
! >-------+--+---- To current
-Vout ---/\/\----+---!+/ ! comparator
! ---
+Vref ---/\/\----+ ---
! !
--- GND
---
!
GND
In not very strict cases, the resistor to the Vref can be two
resistors one to the Vcc of the LM339 and the other to ground. The
LM78XX style regulators are within a few %.
Vcc -----/\/\---
!
Isense ----/\/\---+-+-----!-\
! ! >
--- -!+/
--- !
! !
GND !
!
Vpeak ---/\/\----------+
!
GND ---/\/\-----------
The resistors on the Isense assure that the Vpeak can go low enough to
keep the transistor off in the no load case.
A small capacitor is needed because there is a spike on the current
sense just as the transistor turns on.
I suggest you down load LTspice4 from linear's web site and try the
design out before you commit it to real parts.