PWM chip with constant impulse time ?

[forest]

Found some Ti chip here http://www.ti.com/lit/ds/symlink/lm25085-q1.pdf but it has PFET gate driver. Is the output reversed then ? Can I connect a N Channel FET with a mosfet driver or totem pole or whatever to drive nfet in low side mode here ?

[forest]

This one looks nice http://www.ti.com/lit/ds/symlink/lm5006.pdf What do you think ?

[MarkE]

Found some Ti chip here http://www.ti.com/lit/ds/symlink/lm25085-q1.pdf but it has PFET gate driver. Is the output reversed then ? Can I connect a N Channel FET with a mosfet driver or totem pole or whatever to drive nfet in low side mode here ?

It would be easier to help you if you would describe what you are trying to build:  Buck or boost, positive or negative input voltage, positive or negative output voltage.

[forest]

It looks more like a simple flyback converter. Can't describe precisely . All I need is the chip with PFM and output voltage loop to control voltage on output capacitor. Chip cannot be used directly as per datasheet but only to drive external N-Channel mosfet, because  I need to switch 300-400VDC. The output should also be 300-400VDC. :-) and that's why   feedback is required to change OFF-time.
The most important is constant ON-time, variable OFF time.
I will try to adapt my Attiny45 code for that. So far I learned how to merge two timers to work together  to get constant ON-time pulse and a period between, the problem I have is how to change timer0 period on the fly without glitch (timer0 is working in simple CTC mode while timer1 is doing the ON time pulse).

[MarkE]

It looks more like a simple flyback converter. Can't describe precisely . All I need is the chip with PFM and output voltage loop to control voltage on output capacitor. Chip cannot be used directly as per datasheet but only to drive external N-Channel mosfet, because  I need to switch 300-400VDC. The output should also be 300-400VDC. :-) and that's why   feedback is required to change OFF-time.
The most important is constant ON-time, variable OFF time.
I will try to adapt my Attiny45 code for that. So far I learned how to merge two timers to work together  to get constant ON-time pulse and a period between, the problem I have is how to change timer0 period on the fly without glitch (timer0 is working in simple CTC mode while timer1 is doing the ON time pulse).

To use a part like an ATTINY25/45/85 use timer1 in pwm mode.  Use either OC1A or OC1B for your output using the COM1xy bits and the associated compare register for the fixed pulse width.  That will give you hardware controlled timing for the on pulse.  Use OC1C to set the period.  If you use an AVR with a 16 bit timer1 make sure that you update the 16 bit registers atomically.