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New Low-Noise LDO Regulators Are Here

For many types of analog and RF circuits, low-noise voltage supplies are important for achieving the proper performance. A high-noise floor on RF, audio, or other ADC/DAC and analog circuits can doom a project. Whether it's a precision analog data-acquisition system (like reading transducers to 10 bits and above), HDTV, a radio, or a music system, good voltage regulation is critical. To that end TI has come out with a new series of low-noise, low-drop-out (LDO) voltage regulators.

Consider that you typically use an LDO in circuit locations where there is already electrical noise on the power bus, so a device with a good power supply rejection ratio (PSRR) is important. With the PSRR, it's important to see at what frequencies the measurements were made. Older technology LDOs would show a good PSRR at 120Hz, but not one so good at the higher frequencies. Also of importance is the inherent noise of the device.

From the LP38798 landing page on the TI site and from the data sheet, we get some interesting pieces of info. The device has very low output noise — 5 μV in the 10 Hz to 100 kHz band. It has high a PSRR — 90 dB at 10 kHz. Even at 100 kHz the PSRR is still at 60 dB. Those specs meet the important criteria mentioned above.

Also worth noting: TI has fabricated the device on a medium-voltage process that can operate up to 20 V. That means this part is good for systems with an intermediate bus voltage of 12 V. There are other parts that are probably a little lower cost than this part, but they are fabricated on a low-voltage process. They can't operate much above 6 V. Regarding cost, it sells for $1.30 in lots of 1,000 pieces. It's not dirt cheap, but not outrageously expensive either.

Stability is important with any regulator. This one will work properly with tantalum or ceramic output caps, so that gives you more flexibility regarding component cost, size, and availability.

If you need a little better PSRR at high frequencies, you can add a bit more external filtering. That's not a perfect solution of course, but it's not too bad (two more caps and a small inductor).

You would probably use an LDO like this after a switch-mode power supply (SMPS) to provide clean DC for VCOs, PLLs, and ADCs. These are also good where there can be transients on the SMPS output due to other loads stepping/changing with time. They are also able to solve other issues, such as susceptibility to power line-conducted noise (noise rejection issues).

Note that if you have a 16-bit data-acquisition system, you need 96 dB of noise-free range to get a full 16 bits. With today’s lower voltage rails, this noise level can be trouble, even with (for example) an op-amp with a 60 dB PSRR spec. Many other analog and RF applications will benefit from an LDO like this.

What uses do you see for an LDO like this? Can an LDO like this replace older multi-part discrete filters and give an equivalent or better performance in your application?

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13 comments on “New Low-Noise LDO Regulators Are Here

  1. RedDerek
    August 28, 2013

    I have an instrumentation application that could use a cleaner output supply. My input supply rails are operating at 1MHz and have a large ripple at 800 Hz. Thus two frequency components to filter out. I am always keeping an eye out for better parts to improve my system and give me more leeway with the rest of the performance.

  2. BillWM
    August 28, 2013

    http://www.ti.com/product/tps78833   — SOT-23-5 Low Noise LDO — due to the higher lead inductance of the SOT-23-5 and different die construction the performance differs — This is good for 13.5V max in, and 150-350ma of current Max out — noise is 56uV rms.

  3. Vishal Prajapati
    August 29, 2013

    It was nice info for LDO. I have not worked on the Analog RF systems so I didn't know the importance of the PSRR of LDO. I have just improved filter in my ckt and it has worked fine till now.

    By the way my question is

    Does value of load current effect the PSRR?

  4. samicksha
    August 29, 2013

    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.

  5. BillWM
    August 29, 2013

    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.

  6. Vishal Prajapati
    August 29, 2013

    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.

  7. BillWM
    August 29, 2013

    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 —

  8. RedDerek
    August 29, 2013

    @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.

  9. BillWM
    August 29, 2013

    @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

  10. RedDerek
    August 29, 2013

    @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.

  11. BillWM
    August 29, 2013

    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 — 

  12. Hughston
    September 5, 2013

    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.

  13. Hughston
    September 5, 2013

    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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