[time-nuts] Parallel voltage regulators

J.D. Bakker jdb at lartmaker.nl
Fri Oct 26 17:38:56 EDT 2007

>Poul-Henning Kamp wrote:
>>  Considering the efficiency and easy availability of switchmode
>>  supplies these days, I would never bother with a linear regulator
>>  in a new design.
>>  For instance national has a series of switch mode regulators (LM25xx)
>>  which just requires a coil and a diode more than the usual LM78xx
>>  types.
>>  If you are worried about noise from the swichmode, you can add more
>>  filtering to the output.
>That is a recipe for disaster if one wants a really low noise oscillator.
>This is particularly true if one is a beginner.
>Reducing the output noise of a switching regulator to 100uV rms or less
>is neither easy nor simple.


In this document, Linear Technology's Application Note 70, a 
switchmode power supply design is presented with less than 100uV 
output noise, and little RFI. This is achieved by using a switch 
controller which has controlled output switch slewing, allowing a 
trade-off between EMI/RFI and efficiency. Methods to measure and 
control PSU noise are also discussed (IMHO, the EMI sniffer probe in 
Section J is particularly interesting).

BTW: contrary to popular belief, linear PSUs are NOT inherently 
noise-free. The cap charging current spikes through the rectifier 
diodes can produce plenty of EMI/RFI.

[not claiming it's easy, just pointing out one route that could be taken]
LART. 250 MIPS under one Watt. Free hardware design files.

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