Regarding LM4780

Hi Suneetha,

It is recommended to set the high pass pole using an input filter comprised of RIN1 and CIN1.

If you set the pole in the feedback loop, (Ri1, Ci1) the minimum gain will always be 1 even at DC.

Since Gain = Rf/Ri + 1.

Ci makes Ri look infinite at the low frequency pole, forcing the gain to 1 at DC. If you set the pole at the input, you can remove any low frequency content at the input and therefore block DC (and low frequency content) entirely.

Regards,

Matt

Hello Sir,

Thanks for giving suggestion.

Is it not a problem if they set at different frequencies (Input filter at 100Hz frequency) and Feedback filter (2.3Hz frequency)?

One more suggestion is required:

LM4780 Output connected to 100V Audio Step-up transformer. I need consider the protection the power amplifier from core saturation of transformer if it happens.Shall I need an extra circuit? or Will Rsn1 (2.7Ω) and Csn1(0.1uF) will act as a snubber ckt to protect the Power amplifier?

With Regards,

Suneetha.V  

Hi Suneetha,

There should be no problem with the input filter set at a different frequency than the feedback filter.

Regarding transformer saturation, the potential issue here is current spiking. At saturation, the primary current of the transformer spikes at the voltage peaks since the primary impedance drops drastically. The amount of which will depend of the transformer and the voltage amplitude during saturation.

You can protect the amplifier by adding series resistance with the output if it is necessary. This will alter performances and efficiency depending on the transformer primary impedance. It will also shift the HF pole in the transformer lower and may be unacceptable if the pole approaches audible frequencies.  

 If the amplifier has sufficient reserve current driving capabilities, then extra current demand will not harm the amplifier.

Regards,

Matt

Hello Sir,

This is my first Audio Project using LM4780.I have doubts which are not clarified yet.I need 60W RMS Output from each channel of LM4780 with supply rail of ±35V.I have a doubt in the Output waveform.

The audio signal which we feed at the input is sinsuiodal.At the output the waveform will be amplified to the level equal to the gain * and the Input signal level.The audio signal will be varying in amplitude and in frequency.

When it will give maximum wattage of 60W at the Output?

With Regards,

Suneetha.V

Hi Suneetha,

The typical power rating, unless otherwise specified, for an amplifier is average power.

Where: Pavg = I-RMS * V-RMS

I-RMS and V-RMS are measured at the output of the amplifier. 60 watts should be achievable for most of the auditable band until the amplifier begins to naturally roll off due to bandwidth limitations.

Regards,

Matt

Hello Sir,

I am using only one channel of LM4780 to get 60W Output to drive to Audio Step-up transformer to 100V RMS. To get 60W Output I need to give Split Supplies of ±35V.From the graph, Fig 22,Supply current was given as 120mA.

Is there any recommended Power Supply circuit to generate the required ±35V for LM4780.Any suggestable Part No of Split Power Supplies?

What Specifications we have to consider for selection of Split Power Supplies?Give me suggestions on this.

Regards,

Suneetha

Suneetha,

Generally you need a power supply that can supply the full 60 watts power + the power consumed by the part. It should genrally be well filterd out to very high frequencys to ensure good noise performance.

There are many ways to generate at bipolar supply and the choice of which has pros and cons. For example you can build linear supply with voltage regulators, transformers, or a  non-linear feedback type switching supply with buck or boost converters.

You may want to contact the power management team for more suggestions.  A lot will also depend on the supply source (ie. AC Outlet vs. battery).

Regards,

Matt

Hello Sir,

Thank you for giving suggestions...LM4780 will only work with Audio signals which are only sinsouidal?I am giving Class-D Output which is a square wave to this IC.It consumes so much current and dissipates so much heat.

We have to filterout this Class-D output by using a low pass filter (LC filter) at the output before giving to LM4780?

Regards,

Suneetha.V

Suneetha,

The LM4780 is a large high power class AB 60 watt amplifier. When driving heavy loads, it will consume significant power and throw a decent amount of heat compared to a large class-D amplifier. However, I would make sure that the amplifier is operating correctly by checking your operating points with the data sheet. This part should be attached to a large heat sink if you expect it to deliver its full output power without overheating and becoming destroyed. Even Class-D amps in this power range should be heat sink mounted.

This amplifier can amplify any type of input signal since it is linear. However, slew rate may be unacceptable for large amplitude high frequency square waves. The output of a class-D amp will most likely have very short pulses for low level signals that may be too quick for the LM4780 to amplify based on its slew rate. You should look at the input and output on a scope to see if the part is working adequately.

Do you need a square wave or sinusoidal output from this part?

A square wave in = square wave out and sine in = sine out.

If you are expect to drive a transformer with a sine wave then you will most certainly have to filter the output of the preceding driver stage if it is class-D.

Why did you choose a class-D driver stage for this design?

Regards,

Matt

Hello Sir,

My application is audio announcement in trains related to the data of Present station,Next coming station...like that.

In Railway spec it was given as CODEC ---> 1.2W Mono Preamplifier --->Power Amplifier (signal should be1.8V ±0.2V)--> Distribution network ---> Loud Speakers.

I am working on Power amplifier.Earlier they used VS1053 CODEC IC in one of audio project,whose ouput is sinsuoidal.The peak-to peak specifications of the audio signal from CODEC are

Min - 1.64 (Vpp)

Typ - 1.85(Vpp)

Max - 2.06V (Vpp)

As the Peak signal from CODEC is only 1.03V,we require 1.8V (±0.2V) for Power amplifier ,they used the preamplifier which is Class-D with out filter at the output.

Just I will explain my audio chain once again.

Audio CODEC (VS1053) --> Preamplifier (Class-D type MAX98302) with out LC filter at output ---> Power amplifier (LM4780 presently working on this) --->Audio Step up transformer (100V Audio Line) ---> Loud Speakers (with built-in step down transformers).

I isolated Preamplifier (MAX98302)and I connected CODEC to LM4780,it is drawing only little current.But when connected to Preamplifier LM4780 is drawing so much current.

One option is I have to put LC filter at the output of MAX98302 to get the sinsuoidal signal.

With Regards,

Suneetha.V

Hi Suneetha,

The class-D preamplifier should be filtered before the LM4780. This will reduce the current demand and heat generated by the LM4780 significantly. This would explain your excessive heat issue. However, the part will most likely still need a heat sink if you want 60 or 120 watts from it.

On that note, I would recommend removing the class-D amplifier altogether, unless you have a specific reason for using it. Class-D makes for a poor preamplifier due to the additional part count from the added LC filter and, it converts a signal to a PWM which you then have to convert back to a sinusoidal signal. It will also add noise and distortion that can be easily removed.

If you need more output from the LM4780, instead of using the Class-D for for a stronger input signal, the LM4780 can be configured with gain using feedback resistors. You can scale the output of the LM4780 even if the input is not the target value of 1.8V.

Or, if you must have an input of 1.8V to the LM4780, you can add an op-amp gain stage after the CODEC, or better yet, proved a 2 chip solution with one of TI’s CODECs into the LM4780. Many of our codecs have adjustable output gain that can swing up to 2.1V RMS or 2.97V Peak.

Let me know if you are interested in one of our CODEC's as I can help you choose the correct part for your application.

Regards,

Matt

Hello Sir,

Thanks for giving me so many suggestions.But I have to fix to this design only as PCBs for that design came,and they are in testing phase.

This LM4780 Power amplifier project was started recently and moreover I have less time to complete this Project.If possible suggest Lowpass filter LC at the output of Class-D (MAX98302),so that I can drive to our LM4780 without any problem.

With Regards,

Suneetha.

Suneetha,

See the app note HERE to aid in filter design. With your system, the load impedance for the class-D amp will be the input impedance to the LM4780 which should make the filter smaller.

Matt

Hello Sir,

What is the Maximum Input Impedance of LM4780 to calculate the LC filter to MAX98302?

The application note is is so much calculative.

I asked the Maxim support people,they are suggesting the LC values which are in the MAX98302 Evaluation kit schematic.Those values are calculated for 4Ω,8Ω speakers. But our impedance is different ?

Suneetha.V

Suneetha,

Class -d amplifiers are usually used to power loudspeakers, usually 4 or 8 ohms impedance. Since your class-d amplifier is driving the input of the LM4780 amplifier, it is driving the input impedance of this amplifier. If you are using the configuration in Fig. 39 of the datasheet, the input impedance will be roughly the value of RIN or 15K ohms according to the figure.  This is why a preamp is recommended not a power amp to drive the input of any amplifier. Power amps drive low impedance loads, preamps drive high impedance loads.

-Matt

Hello Sir,

Finally can you suggest any Audio preamplifier IC,that will support LM4780 or any OP-AMP IC that can be used as a preamplifier.?

With Regards,

Suneetha.V

Hi Suneetha,

Any decent opamp will work. In the audio product line the TL971 would be a good choice, or for better perfomance and wider oporating ranges, try the LME49710.

Regards,

Matt

?