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regulator noise
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Old 13th August 2017, 03:23 PM   #11
Calvin is offline Calvin  Germany
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Hi,

@xx3stkm
Keep in mind that the noise figures of Regs using a reference voltage source apply only to the lowest output voltage of the reg, which is the reference voltage.
Higher voltages require the error amplifier to have some (noise) gain.
Hence the noise for a high output voltage is alot higher than for a low output voltage.

The LT3042 (and the stronger LT3045) generate a 'reference voltage' by the voltage drop of a external reference-setting resistor that is fed by a ccs.
A paralleled cap shunts away noise from the resistor.
Since the ccs's current is small, the setting resistor value is quite large.
Hence even a small parallel cap achieves a low corner frequency for noise filtering.
The 'reference voltage' is then buffered, adding only the lowest noise gain from the error amplifier.
That way even high output voltages can be generated with low noise.
Besides, the buffer has of course a much greater bandwidth than a typical error amplifier.
That makes the LT30xx even more interesting

jauu
Calvin
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Old 14th August 2017, 05:26 AM   #12
xx3stksm is offline xx3stksm  Japan
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Thank you Calvin. I appreciate your reply. I have understood what you mean and the "magic" LTC used. What I couldn't make sense so far is that CCS also has noise (6nArms) and it was amplified by the setting register. So, output noise becomes large according to output voltage even if the buffer gain is always 0dB.

I know bypassing the setting register with a capacitor also decreases the noise voltage. But also this is not so effective enough to cancel the increasing noise voltage from my experience.

What I have overlooked is the source impedance of CCS and the value of the setting register which are much higher than usual voltage reference. It means bypassing effect is very powerful than usual bypass one. I usually use a bypass condenser for voltage source which has low impedance. That's why LTC claims low noise from 1.3V to 15V.
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