A switcher at 2.2MHz does not have single frequency spikes in its power spectrum, but a rather wideband distribution. It is not a fixed frequency, fixed duty cycle oscillator, but a load-dependent feedback loop that will change duty cycle or frequency (depending on device) to maintain a target output. Check for yourself.
Try to get >90% efficiency for instance, from 24V down to 3.5V 5A. Unlikely you can work at 2MHz. -----Original Message----- From: time-nuts-boun...@febo.com [mailto:time-nuts-boun...@febo.com] On Behalf Of saidj...@aol.com Sent: Monday, November 28, 2011 3:06 PM To: time-nuts@febo.com Subject: Re: [time-nuts] Low noise power supplies - dont' use Electrolytics Hi Attila, I like >2MHz switchers because they use small components (inductors and capacitors) and are easier to filter out at our usual frequencies of interest. The LT3502A for example works at 2.2MHz, which gives harmonics at 8.8MHz and 11MHz, far enough away from 10.0MHz to avoid beating and causing a spur issue. The 1.1MHz part uses a 15uH inductor, the 2.2MHz part only a 4.7uH inductor for the same 1.8V, 500mA application example, a huge difference in size and cost. The capacitor is also 22uF vs. 47uF, again a large size and cost savings for the 2.2MHz part. Not much of an efficiency difference between the 1MHz and 2MHz parts anymore, they are usually around 80% to 85% at 2.2MHz and one or two percent better at 1.1MHz. With proper layout these parts are extremely well behaved and don't have any noticeable ringing etc on the switch as older units did, and they don't create a lot of noise when using shielded inductors. bye, Said In a message dated 11/28/2011 14:22:58 Pacific Standard Time, att...@kinali.ch writes: > Use ceramic caps where vibration is not an issue. Use high frequency > (>2MHz) switchers wherever efficiency is required, otherwise use linear > regulators. Not really. If you want to have good efficiency, then you want to stay below 1MHz, otherwise switching loses get too high. But the disadvantage is that you have big and bulky capacitors and inductors. Staying somewhere between 1MHz and 1.5 is usually a good compromise if you can tolerate PFM (aka pulse skip) for low load conditions. Going up to 2MHz (and beyond) is only recomended if you are severly space limited. Attila Kinali _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.