PWM chip with constant impulse time ?

[Drak]

forest,

Ah I see. I'm only familiar with the AVR family, If I were wanting PFM I'm not sure how I could get high frequency PFM but I would use an adjustable external clock source to run the MCU (such as an ad9850). Its what I'm doing to run the sinewave to the ATTiny.

http://draksplace.com/PowerSupply.png

Sorry I can't be of much more help.

[forest]

Well...if you need pure sinewave , there is cheap ready to go H-bridge driver with all functionality to build DC-AC pure sinewave inverter on ebay . Below 10$ with free shipping. But of course doing it own way is a fun too.

[MarkE]

Hi


I have a question for experienced electronic gurus here :


Do you know any chip which is working similiar to SG3525 or any other PWM , but with fixed (which can be set by user) ON time , while managing output voltage (like in DC-DC converters using voltage divider feedback) but with variable OFF time (and thus variable frequency) ?
Is there a known method of such kind of building DC converters ? How is that kind called ?


MarkE ? TinselKoala ? Somebody ?

There are lots of PWM chips designed to have constant on time or constant off time.  TI in particular offers a bunch to the automotive market.  Many of those converters use hysteretic control.  One buck converter example that works very nicely is the LM25017.  http://www.ti.com/lit/ds/symlink/lm25017.pdf

[MarkE]

Thanks picowatt


Do you know any simple popular pure PFM chip ?
Do you know if voltage regulation feedback loop in  PFM mode can be made using software code inside such microcontrollers as Attiny or Atmega ? Any code for that ? Mainly I wonder how to implement load regulation in case somebody switched off/on most of load and overvoltage on output capacitor should be prevented.

PFM can easily be implemented with a counter timer in AVR's.  I would use an external comparator though, because the comparator in the AVRs is not very good.  The main problem with using a uC based device for DC-DC is that the dedicated IC's are really, really good and some are very cheap.  The limited counter frequency in the parts without PLLs and the power consumption in the parts with PLLs makes an external design superior in most cases.  I have done some pretty tricky designs in AVRs including designs that transition from fixed on time to fixed off time to minimize ripple.  I have found that the savings in absolute cost and real estate of the energy storage components favor dedicated PWM hardware.

[forest]

Thanks MarkE


Can I use output of this lm25017 for driving external mosfet ? Seems that output can go below zero ?  Any other chip dedicated for external mosfet switch using constant ON-time, variable off- time with integrated comaparator and maybe hysteresis loop ?