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LM4991 THD+N & Stability Time

Sadanori Kato
Guru 36500 points
Other Parts Discussed in Thread: LM4991

Hi,

We have obtained sevral questions from our customers.
When we increased the gain of the amplifier by changing a feedback resistor,
the THD+N has increased.
For example, the gain is 44 V/V, the THD+N is 0.7%.
We think that this reason is the deterioration of S/N characteristics by increasing the gain.
Is this correct?
Moreover, the stability time of start-up is 10s, is so longer.
We think that these reason are the decreasing of cut-off frequency by increasing the gain, the thermal problem because the LM4991 does not have the thermal pad.
Is this correct?
Could you please tell us about them as soon as possible?


Best Regards,
Kato

  • 0
    TI__Intellectual 1240 points

    Hi Kato-san,

    I just checked Twake-up with given condition that Vdd=5V, Rin=10k, Rf=220k, Cin=1uF, Cb=1uF.

    I didn't have so long wake-up time with above condition.

    I attached screen shot as below.

     

    As you can see the screen shot, the wake-up time is around 120ms.

    Please confirm your csutomer's connection espetially Cb cap value because the wake-up time is decided by internal cirrent source and Cb value in

    ramp up portion and after output reach Vdd/2 and then internal timer starts and count about 30ms then output signal will appear on the outputs.

    If your customer has no change on Cb value, wake-up time shouldn't change.

    I will suggest to change Cin from 1uF to 0.47uF or 0.39uF to suppress pop and click sound.

    As you can see the screen shot around 120ms, you will see voltage change on both outputs, This voltage change will cause pop and click sound.

    This causes not fully charged on Cin within ramp up + internal timer period. So it needs slightly shorter time constant on Cin and Rin.

    So by reducing Cin value to 0.47 or 0.39uF, you will not see the voltage change on the outputs.

    Best regards

    T NAOKAWA

  • 0 in reply to Toyojiro Naokawa
    Guru 36500 points

    Hi, Nanokawa-san,

    Thank you for your cooperation quickly.
    We will confirm capacitor's value of the Cb to our customers.
    Moreover, we will let you know the results that our customers evaluated.
    Thank you so much.

    Best Regards,
    Kato

  • 0 in reply to Sadanori Kato
    Guru 36500 points

    Hi, Nanokawa-san,

    We have obtained a reply from our customers.
    The evaluation condition of our customers is Vdd=4.3V, Load Resistance=4ohm.
    They thought that because the generation of heat is large, it is reason.
    They will measure the temperature finally, increase than 4ohm load resistance.
    We have obtained questions from our customers.
    If they change Cin from 1uF to 0.39uF, is it possible to set the gain of about 30dB?
    For example, this condition is Rin=10kohm, Rf=330kohm.
    Moreover, should they set Cin=0.39uF, Rin=10kohm as the time constant?
    Although we think that you are busy, please advise to us.

    Best Regards,
    Kato

  • 0 in reply to Sadanori Kato
    TI__Intellectual 1240 points

    Hi Kato-san,

    Are you talking about long wake-up time?

    As I sent my screen shot in earlier response, the wake-up time is set by internal current source and Cb value, not by thermal.

    And the gain setting with Rin=10k, Rf=330k, the gain in BTL is, about 36dB. and low side cutoff freq will be about 40.8Hz.

    Which pkg type does customer use? SOP ? or LLP?

    If SOP, the pkg thermal resistance is relativelly large like 140deg C/W and when Vdd=4.3V, RL=4 Ohms, the max PD will be about 940mW.

    So with SOP pkg is not suitable for RL=4 ohms. because the device will be easy to get into thermal shutdown mode.

    Please check the pkg type.

    Best regards

    T NAOKAWA

  • 0 in reply to Toyojiro Naokawa
    Guru 36500 points

    Hi, Nanokawa-san,

    Thank you for your cooperation.
    Today, we have obtained the information about Cb from our customers.
    They forgot our question.
    The Cb value is used 1uF, it is the same value as the reference design.
    Although we are worried about the wake-up time, they are worried about the generation of heat.
    They have requested the gain of about 30dB.
    We told the incorrect information to you.
    For example, this condition is Rin=10kohm, Rf=160kohm.
    Moreover, they have requested the package of SOP.
    Therefore, we suggested products with the thermal pad to them.
    Because of the generation of heat, do you think the wake-up time becomes late?

    Best Regards,
    Kato

  • 0 in reply to Sadanori Kato
    TI__Intellectual 1240 points

    Hi Kato-san,

    Regarding the gain setting, new condition with Rin=10k ohms, Rf=160k ohms, this will set the gain of 30.1dB in BTL connection.

    Regarding the package, we recommend to use LD pkg instead of SOP because of 4 ohms load. As I mentioned earlier response, the usage condition at customer is Vdd=4.3V, RL=4 ohms. With this condition, max power dissipation is about 930mW. The thermal resistance of SOP is 140℃/W. This means that even in room temp, the junction temp will reach or exceed 150℃ and easy to get into thermal shut down mode. To avoid this situation, we recommend to use LD pkg which has much lowe thermal resistance compare with SOP.

    Regarding long wake-up time,  It doesn't have relationhip with thermal. Only related to Cb and internal current source and internal clock freq for timer. As I attached screenshot of wake-up time, it is about 120ms with Cb=1uF. So will you get screenshot showing wake-up time with VDD=4.3V, Cb=1uF, and then S/D from "H" to "L". Since it doesn't care about with load or without load, please check with without load that the device will not consume power, meaning no heat.

    Best regards

    T NAOKAWA 

  • 0 in reply to Toyojiro Naokawa
    Guru 36500 points

    Hi, Nanokawa-san,

    Thank you for your cooperation.
    We will inform your suggestion to our customers.
    And our customers will check the wake-up time both with load and without load again.
    We also think that it can be judged by the result of this evaluation whether it is influence of generation of heat.
    Thank you for your advices.

    Best Regards,
    Kato

  • 0 in reply to Sadanori Kato
    Guru 36500 points

    Hi, Nanokawa-san,

    Thank you for your cooperation.
    We have obtained a reply from our customers.
    Our customers understood that LM4991 will be easy to get into thermal shutdown mode by using RL=4ohm,
    said that when they adopt a new set, they would like to evaluate speaker more than 6ohm.
    This project is closed once.
    Thank you so much.

    Best Regards,
    Kato