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Original question:

PLIMIT Calcluation for TPA3111D1

TPA3111D1: How to Set PLIMIT

Carissa Pocock
Prodigy 120 points
Part Number: TPA3111D1

Tool/software:

I am struggling to understand how to calculate the Plimit voltage based on the datashetet. The values in Table 2 do not make sense to me. I've looked at a few other forum posts, and didn't get any clarity, and the supporting documentation and graphs seem to contradict each other.

If I follow the above equation, for the first set of table values:
PLIMIT = 1.92V
Vp = 4* PLIMIT = 7.68V

RL = 4ohm as given.
Rs =~240mohm as given in the datasheet

Pout for unclipped power using those values yields Pout=5.88W

Pout =1.25*Pout unclipped = ~7.4W

This is far less than the 10W shown in the table, and no where close to the 15Vpp.

If I instead start with 1VRMS input and 20db gain => factor of 10.

Then I get 10Vrms output =>~14.14Vpeak output voltage. This is closer to the 15Vpp shown in the table, but a little bit less. I'm not sure how 15Vpp is calculated in either case. 
Using 14.14Vpeak amplitude, the output power is closer to 20W



Ultimately, I want to power the device off 24V for PVCC, and I will use an 8ohm speaker, and want to limit the output power to 3W. I plan to set the gain to 32dB

Based on the graph for Figure 7, I expect the voltage for Plimit would be near 1V if I had a 20dB gain. I need to understand how to adjust this equation for a gain of 32dB. Any help you can provide would be appreciated.

  • 0
    TI__Intellectual 1930 points

    Hi Carissa Pocock,

    the formula is the calculate method, and the table2 is test results of some settings. usually the calculate data is a little different to the real results, it is used for reference.

    the output power also related to the input signal and the gain setting, refer to the figure 15, the power limit will results in the distortion in some cases, so you have to define the target power and the THD level. or use the max input signal to calculate the unclip output signal, use 1.25Pout (10% THD) as your final results.

    for example, max input signal 0.15Vrms, and the gain is 32dB, so unclipped output is 6v rms, in this case, the unclipped power on 8 ohm load is around 4W unclipped ((8/ (8+2*0.24)*(6*1.414))^2/2/8.  if you want to limit this signal to 3W( in this case, maybe THD is aournd10%), use the pout=1.25P unclip, the calculate output voltage is 6.57Vp, you can set the plimit to 1.64V for init value, and test it and adjust it according to the results.

    but if the input signal is much higher than 0.15Vrms, such like 1V rms, and the gain setting is still 32dB, the unclipped output can reach 40vrms (ignore the power supply's limit), in this case, if still set the plimit to 1.64V, and the output voltage is limited to around 6.57V, the THD will be much higher than 10% , so the output power will be larger than 3W, the plimit have to be reduced.

    generally, the plimit is related to the input signal, gain, target power, load resistance, output signal THD@ target power etc.. the calculated value can be used as reference but further test is needed.

    Br,

    Wenbin

  • 0 in reply to Wenbin Li1
    Prodigy 120 points

    Hello Wenbin-

    Thank you for the information. I went ahead and grabbed the Tina TI simulation for this part from the website.
    https://www.ti.com/lit/tsc/slam054

    I simulated your example, with Plimit = 24VDC as originally designed in the simulation. I altered the other parameters as described in the text box. I get about 4.4W output power to the speaker as expected.





    If I connect to a 6.9V source, such as that would be output by GVDD, where I would expect NO POWER LIMITING to occur based on the Datasheet Page 4. However, the output voltage OUTPN appears to be limited to the voltage at the PLIMIT pin (~6.9V). Since 8.4Vp is more than 6.9V peak.  Is the simulation incorrect? Does the PLIMIT pin in the simulation actually represent 4*the Voltage at PLIMIT for the part?



  • 0 in reply to Carissa Pocock
    Prodigy 120 points

    To clarify, I connected a 6.9V source to PLIMIT pin like shown below. I did not change the PVCC pin voltage. That is still 24VDC

  • 0 in reply to Carissa Pocock
    TI__Intellectual 1930 points

    Hi Carissa Pocock,

    It's better you can test on the board. This virtual rail is 4 times the voltage at the PLIMIT pin. the simulation is really confusing. anyway, the results should be verified on board. and adjusted based on the test.

    Br,

    Wenbin