Cheaper and less noisy too although the latter does not matter that much when used as a pre regulator. It is funny to have to use a noisy non LDO regulator to be able to use an ultra low noise LDO
I avoid pre regulators and choose stuff so that it works out OK. In practice this means a maximum output voltage of 15V when using selected 12V transformers, Schottky diodes or LT4320 rectifiers and large filter caps and all within reasonable limits/safe areas.
Even a 2 x 7V transformer with windings in series already fries the LT3045.
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Yes, but only while it is fast charging. When it gets close to the intended voltage, it switches to its normal mode with its normal noise.
It surprises me that you can get 15 V output voltage that way. A 15.8 V to 20 V input voltage range is awfully tight with +/- 10 % mains voltage tolerance and ripple.
So no effect when it stop fast charging. Thank you.
Here i did some maths, what dou you think MarcelvdG and Jean-paul ?
With the lt3045 if I use a 12VAC (24V center tapped + bridge rectifier of 4 schottky).
With perfect components : 12VAC*1.44=17.28V (-10%=15.55V) (+10%=19.01V)
With real life components : 12VAC*1.39=16.68V (-10%=15.01V) (+10%=18.35V)
Worst case scenario : 12VAC*1.33=15.96V (-10%=14.36V) (+10%=17.56V)
Or will it vary more than that with mains voltage +-10% change ?
Suppose you had an ideal centre-tapped 24 V transformer. You would then get 10.8 V RMS at -10 % mains tolerance and 13.2 V at +10 %. The peak values would be sqrt(2) times as high, so 15.27350647 V and 18.66761902 V, respectively. Assuming you lose 0.5 V across your Schottky diode, the peak rectified voltage becomes 14.77350647 V and 18.16761902 V, respectively. The minimum voltage will drop further due to the ripple voltage across the reservoir capacitor, how much depends on how you size the capacitor. Assuming a fairly optimistic 0.5 V peak-peak ripple, the momentary voltage at the regulator input will be something in between 14.27350647 V and 18.16761902 V. Clearly not enough for a 15 V regulator; taking into account the tolerance and drop-out voltage of the regulator, you would need something like 15.8 V as the minimum voltage. (The LT3045 set current has a +/- 2 % inaccuracy, the external set resistor is not perfectly accurate and the dropout voltage is 330 mV max. up to 50 mA, 350 mV max. at 300 mA and 450 mV max. at 500 mA.)
A real transformer has winding resistance and spreading inductance, and an open-circuit voltage that is deliberately made a bit higher than the nominal voltage to compensate for the voltage drop across the winding resistance and spreading inductance. How much higher varies a lot depending on the transformer type; it is only a few percent for large toroidal transformers and can be as much as 40 % for small PCB-mounted EI transformers.
If you use an oversized transformer that gives 9.8158259 % more voltage with the load that you have than an ideal centre-tapped 24 V transformer would, then it more or less fits. Assuming that the voltage drop across the diodes remains 0.5 V, which is again a bit optimistic, the maximum voltage would just remain below the LT3045's maximum recommended input voltage of 20 V. The minimum peak voltage would become 16.77272727 V instead of 15.27350647 V. Subtracting 0.5 V for the diode and 0.5 V for the ripple, it becomes 15.77272727 V, which is quite close to that 15.8 V. It probably fits as long as everything is not at the edge of its tolerance interval simultaneously.
So all in all, jean-paul is correct. If you really do your best, it might just fit without pre-regulator when you choose the right oversized transformer.
A real transformer has winding resistance and spreading inductance, and an open-circuit voltage that is deliberately made a bit higher than the nominal voltage to compensate for the voltage drop across the winding resistance and spreading inductance. How much higher varies a lot depending on the transformer type; it is only a few percent for large toroidal transformers and can be as much as 40 % for small PCB-mounted EI transformers.
If you use an oversized transformer that gives 9.8158259 % more voltage with the load that you have than an ideal centre-tapped 24 V transformer would, then it more or less fits. Assuming that the voltage drop across the diodes remains 0.5 V, which is again a bit optimistic, the maximum voltage would just remain below the LT3045's maximum recommended input voltage of 20 V. The minimum peak voltage would become 16.77272727 V instead of 15.27350647 V. Subtracting 0.5 V for the diode and 0.5 V for the ripple, it becomes 15.77272727 V, which is quite close to that 15.8 V. It probably fits as long as everything is not at the edge of its tolerance interval simultaneously.
So all in all, jean-paul is correct. If you really do your best, it might just fit without pre-regulator when you choose the right oversized transformer.
First thank you for the perfect and very detailled explanation. Thank you, really.
From this I come to 2 conclusions. I am better of using the 3045 without pre-reg at +-12V (not +-15V). Or I need a pre reg before it to have a confortable margin.
I was told to design for mains of 85V to 250V mains. So it will not work well without a preregulator.
Will it create a big difference if I set it at 12V instead of 15V to most opamps ? On the forum they always tell to power opamp as high voltage possible for better performances ?
Thank you.
From this I come to 2 conclusions. I am better of using the 3045 without pre-reg at +-12V (not +-15V). Or I need a pre reg before it to have a confortable margin.
I was told to design for mains of 85V to 250V mains. So it will not work well without a preregulator.
Will it create a big difference if I set it at 12V instead of 15V to most opamps ? On the forum they always tell to power opamp as high voltage possible for better performances ?
Thank you.
Hi I really should refrain myself from posting too much but I have so much question sorry guys if I bother you.
Here, should I put the lt3042/lt3094 close to the opamp, will it not create some EMI ?
Bridge + CRC + Cin + LT3045 + Cout ----------CABLE 30cm----------> OPamp
or
Bridge + CRC ----------CABLE 30cm----------> Cin + LT3045 + Cout > OPamp
I am planning to put the lt3045/lt3094 on the same pcb as the Opamp, and very close. Good idea ?
Thank you.
Here, should I put the lt3042/lt3094 close to the opamp, will it not create some EMI ?
Bridge + CRC + Cin + LT3045 + Cout ----------CABLE 30cm----------> OPamp
or
Bridge + CRC ----------CABLE 30cm----------> Cin + LT3045 + Cout > OPamp
I am planning to put the lt3045/lt3094 on the same pcb as the Opamp, and very close. Good idea ?
Thank you.