[time-nuts] Parallel voltage regulators
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
>> 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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