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Old 18th December 2011, 08:48 PM   #1
FOXYE is offline FOXYE  Israel
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Default 3.3v regulator

Hello,

What do you think about the following circuit?
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Old 18th December 2011, 10:35 PM   #2
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Tracking preregulator.
Remove LED4, it is fighting against Q1.
With R2 = 100R, you are running 12.5 mA over Q1. Increase R2 to 1K to decrease current to 1.25 mA. R5 and C6 are forming LPF, you can increase R5 to improve filtering. C6 should be film capacitor or low leakage electrolytic.

And don't forget to load down reg output with 56 - 100R 1W (or more W) resistor. It will give you lower output impedance and you get free shuntie parallel to your load.

If LED4 is removed, then your output voltage will be ~ 3.7V Increase R8 to 13K to dial output voltage to ~ 3.3V

Improving the LM3x7 regulator circuit

Using 3-pin regulators off-piste: part 4
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Old 18th December 2011, 10:40 PM   #3
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Or use a 7805 followed by a ADP150/151 + necessary caps and experience a very low noise 3.3 V supply (max. 150/200 mA). Just 9 uV noise, less parts, less hassle. Given the fact that a BC547 is used you will not exceed 100 mA of current anyway so ADP150/151 should be OK.

Are those real measured values or calculated ones ? I can't imagine picoVolts of noise with LM117.
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Last edited by jean-paul; 18th December 2011 at 10:45 PM.
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Old 19th December 2011, 12:23 AM   #4
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For powering what? All in context, it could be an unneeded part count, size and complexity for many 3.3V loads. Is it for a DAC?
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Old 19th December 2011, 09:53 AM   #5
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Thanks for the suggestions.

The bc547 transistor is limited to 100 mA, is there a popular substitute that could provide 200 mA?

In the circuit with transistor output that reduces the noise, different load greatly increased noise.

Is not it better to use the second circuit with the leds?
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Old 19th December 2011, 02:06 PM   #6
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Somebody asked you before: what is the purpose of the circuit ? When you need a low noise low power 3.3 V reg there are quite some types now that will bring you low noise and low parts count/small size. Probably with better specs as well.
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Old 19th December 2011, 02:08 PM   #7
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For digital receiver and dac chip.
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Old 19th December 2011, 02:21 PM   #8
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OK, the schematic makes not much sense to me. Why use a noisy reg that can handle 1.5 A preceded by a BC547 that can handle 100 mA ?

You can build the circuit but don't be surprised that for instance a plain Jane 3.3 V reg like LF33AB has better specs. Some of the newer SMD LDO 3.3 V regs like the ones mentioned before are even way better.
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Last edited by jean-paul; 19th December 2011 at 02:23 PM.
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Old 19th December 2011, 02:26 PM   #9
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Using 3-pin regulators off-piste: part 2
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Old 19th December 2011, 04:29 PM   #10
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Aha, there is an explanation how standard 317 or 78xx 3 pin regs can perform better. Also the PFM Flea is mentioned which is an excellent reg. I use those on a regular basis and they perform very good, also on 3.3 V but max. current is quite low (enough for a lot of modern DAC chips but not for the older bipolar chips).

You can choose to improve a standard LM317 circuit but choosing another reg may be simpler.
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Last edited by jean-paul; 19th December 2011 at 04:32 PM.
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