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Hi, Daniel,

60 kHz? You're not driving a speaker, are you? Or, maybe a hi-fi set for dogs?

-d2

Daniel,

Can you provide more details, what is the load, how much power/current/voltage output do you need?  We have high power AB amps that could eliminate the transformer, depending on what you're trying to drive.

Regards,

royce

First thanks for the responses I appreciate it.

Don Dapkus

60 kHz? You're not driving a speaker, are you? Or,maybe a hi-fi set for dogs?

60kHz its a requirement, I'm not driving a speaker... I guess the best way to describe it is a variable load.

Royce Higashi

Can you provide more details, what is the load, how much power/current/voltage output do you need? We have high power AB amps that could eliminate the transformer, depending on what you're trying to drive.

The load needs a constant current supply of 9mA from 30 ohms to 280 ohms, and from 281 ohms to 10k ohms a constant voltage source of 2.6 volts max.My battery supply is a minimum of 3.1V. My requirements are very basic that's why I know there has to be a way to simplify this.

I'd also like to add that I'd like the sinewave to be as clean as possible.

Again thanks for all the responses.

Can any part meet my requirments?

Daniel,

I'm a little confused about your application.  Where does the sinusoidal input/output come into play?  Is the variable load changing in a sinusoidal pattern?  Do you essentially want a 9mA constant current source that reaches saturation at 2.6V?

-Clancy

Hi Daniel,

If I understand your topology correctly, you have:

1. a Signal Generator stage converting a square wave to sine wave wia a tuned filtering circuit.

2. a control stage with a digipot that controls the sine wave amplitude.

3. an amplification stage using the LM4951 driving an auto-transformer.

You mentioned that you would like to update the amplification circuit, saying that it is limited. I have reviewed your output constant current/voltage requirements and it looks as if the amp would work just fine. Could you elaborate more on how the amplifier is limited? ie what would you like it to do that it is presently not doing.

You also mention that you would like to simplify the circuit. I am not aware of a circuit that encompasses all three stages. There are synthesizer chips in the market, but I don't believe they have drive capability. It is difficult to really be able to help simplify without seeing your present circuit.

You also mention that you would like the sine wave to be as clean as possible. Do you know what the circuit's present THD level is and what you would like it to be?

If you would like to discuss the technical details of your solution "off line" and witih complete confidentiality, just let me know.

Best Regards,

JD

Clancy Soehren

Where does the sinusoidal input/output come into play?

As John said below it comes from a simple signal generator that takes a square wave and turns it into a sine ave through a filter.

Clancy Soehren

Is the variable load changing in a sinusoidal pattern? 

No it is a pure resistive load, that changes variably.

Clancy Soehren

Do you essentially want a 9mA constant current source that reaches saturation at 2.6V?

Correct

John De Celles

You mentioned that you would like to update the amplification circuit, saying that it is limited. I have reviewed your output constant current/voltage requirements and it looks as if the amp would work just fine. Could you elaborate more on how the amplifier is limited? ie what would you like it to do that it is presently not doing.

Well limited was bad wording on my part, rather the circuit as a whole is to big, I was aware that the LM4951 can do what I need as a stand alone but I was told that it would distort to much. I have not tested it myself but I was hoping to get at least a few options before I got to the prototyping stage.

John De Celles

You also mention that you would like the sine wave to be as clean as possible. Do you know what the circuit's present THD level is and what you would like it to be?

I don't know what the present THD is and we don't really have any criteria to be honest, which was upsetting when I found out.

Here is a url for the circuit itself to help show you what I'm working with.

http://www.freeimagehosting.net/kv7ju

Hi Daniel,

First of all, sorry for the delayed reply. Thanks for providing your circuit, this has been helpful in understanding your present implementation.

While I presume this is not your highest priority, I have evaluated the THD+N of various portions of the circuit. Depending upon the cleanliness of your oscillator, I was able to obtain about 0.2% THD+N from a lab oscillator at 60kHz into your filter with a 50k ohm load (the digipot). So, based on this I don't presume the THD+N to be any better than this. The LMV321 has about 0.1% THD+N at 60kHz from the datasheet and I tested the LM4951 at 60kHz under various loads and don't see the THD+N being above 0.1%. If your circuit needs a lower THD value then a little more effort might be required to reduce harmonics with the filter.

One question I have is: how do you control the signal amplitude (digipot) with respect to the changing load impedance? Is this done based on some panel/GUI adjustment or is there a feedback mechanism that is not shown in the schematic? I'm just trying to get an idea of the control mechanism.

I will need a little more time to investigate whether there is a more elegant solution to simplify your circuit, but so far I don't have a solution. We have analog subsystems with volume controls, and power amplifiers, so there could be some simplification there, but these guys have many more features than what you might require. Is PCB area savings your number one goal?

Best Regards,

JD

John De Celles

First of all, sorry for the delayed reply. Thanks for providing your circuit, this has been helpful in understanding your present implementation.

No problem, I appreciate the help.

John De Celles

I tested the LM4951 at 60kHz under various loads and don't see the THD+N being above 0.1%.

Can you share your circuit with me, I recently orderd some samples, and when I get them I'm going to bread board it and measure the THD, I want to see if I can drive it directly from the digipot and bypass the transformer.

John De Celles

If your circuit needs a lower THD value then a little more effort might be required to reduce harmonics with the filter.

Actually I've done a little bit of digging, and have been testing some of our existing products, and have found out that a THD of about 5% would be acceptable.

John De Celles

how do you control the signal amplitude (digipot) with respect to the changing load impedance? Is this done based on some panel/GUI adjustment or is there a feedback mechanism that is not shown in the schematic? I'm just trying to get an idea of the control mechanism.

The signal amplitude is changed by a micro that gets feedback from two peak detectors that monitor voltage and current, that ironically was another circuit that i was looking at to improve.

John De Celles

Is PCB area savings your number one goal?

PCB space is my number one priority then low power requirments.

John thanks a lot I appreciate all the help. Let me know if you have any more questions.

Hi Daniel,

Wrt LM4951 THD tests, I used the circuit you provided previously; ie feedback resistor = 18.7k so that the gain was the same. I drove load impedances of 8 ohms and 30 ohms.

Thanks for supplying the info on your control mechanism. Based on this I presume that the optimum thing we can do is replace the digipot, the LMV321 buffer and the LM4951 power amp with one IC; assuming we can find the right part. This way, your sensing circuitry can remain the same with your micro controlling an integrated volume control. The code would have to be changed, but I'm assuming that since you are trying to reduce ICs and PCB area that this is on the table.

In order to converge onto the right IC can I confirm the supply voltage range? You had mentioned 3.1V min, but was is the nominal and maximum supply voltage?

I am looking at the LM48100Q or the LM4865 or LM4875. Can you take a look at these and let me know what you think. I will continue looking at options on my side and if you have any other details that you feel are pertinent that would be much appreciated.

Best Regards,

JD

John De Celles

Based on this I presume that the optimum thing we can do is replace the digipot, the LMV321 buffer and the LM4951 power amp with one IC; assuming we can find the right part. This way, your sensing circuitry can remain the same with your micro controlling an integrated volume control.

Thats an interesting proposal. I was primarily looking to remove the transformer since it takes up a pretty big chunk or real-estate. I was hoping I could push the LM4951 to a higher gain to compensate for the absence of the transformer, but as far as alternatives go thats not bad, it would remove a few components and overall might be a little more efficient.

John De Celles

You had mentioned 3.1V min, but was is the nominal and maximum supply voltage?

To answer your question the battery is about 3.7V fully charged, and about 3.4V nominal, once it falls below 3.1V the device is designed to automatically turn itself off.

John De Celles

I am looking at the LM48100Q or the LM4865 or LM4875. Can you take a look at these and let me know what you think. I will continue looking at options on my side and if you have any other details that you feel are pertinent that would be much appreciated.

I'll take a look at the specs, hopefully I'll have a response by Monday for you. I was hoping I could find something similar to the audio amp but something that possibly had an internal up-converter so I could get the output I needed inorder to remove the transformer but if I can't find something or the LM4951 is to noisy then I'll have to continue to work to see how small I can make the transformer, or optimize it all together.

Let me know if you see any issues with me pushing the LM4951 to a higher gain.

Thanks for all your help, I appreciate it.

Hi Daniel,

Can we clarify the end-load requirements?

I presently have 9mA from 30 ohms to 280 ohms and 2.6Vmax from 281 ohms to 10k ohms. Is this on the output of the transformer?

Some possible options I see are:

1. Use the LM4951 in its true bridge-tied load configuration, this will give you more voltage swing; the IC has plenty of current drive. I presume that this may not be possible with your detection circuitry for your feedback mechanism; please confirm. Would a bridged output work for your solution?

2. Try to use a simple analog subsystem like the LM48100Q to integrate volume control buffer and power amp into one IC; different programming would be required for attenuation control. I presume transformer would still be required. This is probably the most work on your end.

3. Keep all circuitry the same, but replace the transformer with a DC-DC boost to power the power amplifier. Once again, please confirm end load requirements. Please review attached quick sim from web-bench. This would probably be the easiest, smallest and probably cheapest solution.

Let me know what you think of these options. I'm sorry that I'm still not 100% clear on the end load requirements; I wasn't able to find the transformer turns on the web.

Best Regards,

JD

4857.Boost_for_LM4951.pdf

Hi Daniel,

Thank you for clearing up the loading requirements. Based on that information the worst-case loading requirement occurs at 280 ohms with 9mA of current for 23mW of output power. The 2.6Vpk output voltage also becomes an issue for your minimum 3.1V supply voltage. Hence the need for the transformer.

What I was suggested previously was possibly using the LM4951 in its intended operation as a bridged output; ie the load is connected across Vo1 and Vo2 of the IC. These outputs are 180 degrees out of phase, creating twice the peak voltage across the load each half cycle. This is a great way to get more output voltage using small power supply voltages. So, by using the 3.1V min supply voltage we should be able to obtain 3Vpk into the load without using the transformer.

I connected up the LM4951 using a 3.4V nominal supply with 18.7k ohm feedback resistor as you have in your schematic. The closed-loop gain of the amplifier is 1.87 x 2 or 3.74 due to the bridged output operation. So, with a 700mVpk 60kHz input signal I obtained a 2.6Vpk output across the 280 ohm load. This is shown in the attached scope photo.

I then proceeded to check the THD of the amplifier at this frequency, as this was also one of your concerns. The harmonic analyzer stated 1.8mV/1.85V x 100% = 0.097% THD at 60kHz into 280 ohms at the 2.6Vpk. I also quickly ran an FFT showing the 2nd harmonic at -60dB or again 0.1% THD. The analyzer is limited in how high the harmonics can be stated, but this gives you the idea that it is not completely terrible.

If you can utilize the bridged output topology (example as in Figure 1 of the datasheet) then I would recommend at this point that it is the simplest way of eliminating the transformer, thus reducing some of your PCB area requirements.

I hope this helps a little.

Best Regards,

JD

2161.LM4951_BTL_Output_061312.TIF

0601.LM4951_THD_061312.TIF

John,

I'm going to try what you're suggesting, my only other concern related to this project is the feedback that this signal creates. At the following link is the circuit we use to measure current and voltage feedback, these circuits are half peak detectors, however their output is not linear, I was hoping i could find an amplifier, preferably in a small dual package that could detect those low current peaks and voltages and report them linearly any thoughts?

http://www.freeimagehosting.net/b3i1a