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William Murray

New Low-Noise LDO Regulators Are Here

William Murray
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Hughston
Hughston
9/5/2013 11:10:34 AM
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Newbie
Re: Got it in a SOT23-5?
It may not be your supply but you grounding or power distribution. Better power and ground distribution would be hard to explain in a short blog.

There are a few alternatives to a low noise regulator if you can spend the money. You can lower noise further by using another linear regulator in front of the low noise regulator. You can preceed the regulator with a very good passive filter. You can make a low noise regulator even quieter by paralleling two of them. There is a Maxim app note on how to do that but the idea goes back further than that. You can use separate regulators for sensitive circuits, just beware of sequencing issues.

If noise is an issue for the power then so are other things.  Such as PSRR, line regulation, load regulation, voltage drift, output filtering capacitors, power distribution, grounding, input filtering of high frequency, component selection, etc. Besides the regulator, what are you other sources of noise?  They have to be controlled.

Sometimes regulators that go from a heavy load to a very light load. You can load it down to reduce the load variation and tht might improve the noise.

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Hughston
Hughston
9/5/2013 10:57:07 AM
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Newbie
Re: Got it in a SOT23-5?
PSRR is important for battery powered systems because the supply voltage is changing with the load. For example: an RF transmission burst or modulating a display backlight can cause the power to sag.  A quiet supply voltage is needed for the PLL and the receiver especially. The input line regulation is the starting point for good PSRR on a regulator, so always look at that spec. For a high frequency load, I think you would want to choose a regulator that is stable with ceramic capacitors due to their better characteristics at high frequency. Kemet and AVX have SPICE simulators to show you the impedance characteristics versus frequency to help you with capacitor selection. Keep in mind the harmonics of your fundamental noise frequencies because you want to filter them too.

The regulator noise is not the only issue in a system that needs to be quiet. The way you distribute the power and ground also matters. You may want a star ground and power distribution because of common impedance coupling on the power and ground.

There is always a practical limit to the high frequency input rejection of a regulator.  Passive filters can improve the high frequency rejection but not the low frequency rejection. That's where line regulation comes in. In a battery powered system, a large tantalum capacitor in parallel with the battery helps lower the source impedance for rapidly changing loads.

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BillWM
BillWM
8/29/2013 1:00:23 PM
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Blogger
Re: Got it in a SOT23-5?
Very well could be -- In that case one would be looking at a board spin to stack two of the 2.5V parts, or use one of them in an adjustible configuration -- Lots of work compared to a drop-in --  For some analog there are a few better 3.3V op-amps so one might be able to convert the rail to 3.3V and swap a few other parts -- 

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RedDerek
RedDerek
8/29/2013 12:01:23 PM
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Master
Re: Got it in a SOT23-5?
@William - I went back and did my search based on the circuit criteria and I still get the same parts as I have had 3 years ago. Looks like nothing new has hit the market to improve my product.

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BillWM
BillWM
8/29/2013 11:06:21 AM
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Blogger
Re: Got it in a SOT23-5?
@Derek--- Not sure on what to try in your situation -- TI and Analog Devices, and LTC, and Maxim all make parts and all have parametric part selector tools on the Web -- Digikey and Mouser also have good parametric part selector tools on their Web sites -- one can narrow down the options this way -- Octopart and Chipfind can also help locate parts parametrically. On many fixed output regulators it is possible to convert these into an adjustable provided there is a Vsense pin --this also might be worth a look in the app-notes

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RedDerek
RedDerek
8/29/2013 10:41:47 AM
User Rank
Master
Re: Got it in a SOT23-5?
@William - Looked at the datasheet and compared to what I am currently using and my application. The TPS does have about half of the LDO voltage than my current part. However I do need to generate + and - 5 Volts in the end and the TPS only provides 2.5 and 3.3. Also, the TPS has a limit of 13.5 Volts for the input; I need to operate with a maximum of 20 Volts. My current is very low. Now I filter the output quite a bit to help knock down the output ripple.

If you have any other suggested parts, I would be glad to look at them for replacement in the application.

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BillWM
BillWM
8/29/2013 9:11:21 AM
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Blogger
Re: Got it in a SOT23-5?
The PSRR us usually specified at one or at most a few Process, Voltage, Temperature, Load and Input Voltage points -- and they may or may not be as demanding as yours -- use the datasheet to estimate, but Verify in actuality in some way --

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Vishal Prajapati
Vishal Prajapati
8/29/2013 9:07:59 AM
User Rank
Master
Re: Got it in a SOT23-5?
I also thought so. But what about PSRR when the Load current is 100%. I think it will be in the datasheet defined limit. It should be.

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BillWM
BillWM
8/29/2013 8:36:53 AM
User Rank
Blogger
Re: Got it in a SOT23-5?
Load Current and PSRR -- yes load current can affect PSRR -- often the heavier the load, the lower the PSRR figure -- but this is not always 100% the case -- test in your application.

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samicksha
samicksha
8/29/2013 5:52:30 AM
User Rank
Artist
Re: Got it in a SOT23-5?
I read the link, and it reminded me that it is important to keep thermal considerations in mind when using a low drop-out linear regulator. Having too wide a differential between input and output voltage could lead to large power dissipation.

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