LM4951A: Pop noise

Masayoshi Takahashi

Intellectual 1530 points

Part Number: LM4951A

Hi team,

My customer faced pop noise issue when turn on the shut down.

Please see the schematic and waveform below.

Could you give your advice how to prevent the pop noise?

Regards,

Yoshi

over 4 years ago

0 Luis Fernando Rodriguez S. over 4 years ago

TI__Guru* 84915 points

Hi, Yoshi,

There are many recommendations for this device when pop issues appear.

Basically, the input capacitor is the component associated to this problem. Its charging time must be regulated in order to reduce pop issues. So, I would recommend to try with a low capacitor value in order to allow a faster charging time. Or another alternative would be to short the 1Kohm resistor of CCHG pin. However, if this resistor is shorted/removed, it is important to ensure that the input signal won't be less than VDD+0.3V and greater than -0.3V, otherwise it may result in a circuit damage.

Please let me know if you have additional questions or comments on this.

Best regards,
Luis Fernando Rodríguez S.

0 Masayoshi Takahashi over 4 years ago in reply to Luis Fernando Rodriguez S.

TI__Intellectual 1530 points

Hi Luis,

Thanks for your quick reply.

When the customer change the Cin like 0.22uF, the pop noise decreased than using 1uF. There is small pop noise. So, should the customer use smaller one?

Could you show the CCHG resistor connection as alternative solution that you mentioned?

Regards,

Yoshi

0 Luis Fernando Rodriguez S. over 4 years ago in reply to Masayoshi Takahashi

TI__Guru* 84915 points

Hi, Yoshi,

Yes, the customer can use smaller resistors. This capacitor value is not mandatory, so the customer can use the value that results in a better performance for his circuit.

Regarding the CCHG resistor connection, I just recommended to replace this resistor by a 0-ohm resistor or a single wire. As mentioned in section "Selecting Value A For Rc":

http://www.ti.com/lit/ds/snas453c/snas453c.pdf#page=13

Best regards,
Luis Fernando Rodríguez S.

0 Masayoshi Takahashi over 4 years ago in reply to Luis Fernando Rodriguez S.

TI__Intellectual 1530 points

Hi Luis,

Thanks for your suggestion.

When the customer using 0.1uF of Cin, the pop noise is gone. However cut off frequency is changing.

So, to keep the cut off frequency, The customer need to use Ri and RF higher value like below.

Ri : 10k ohm → 100k ohm

Rf : 100k ohm → 1M ohm

Is this correct action? Is there any consideration about using higher value?

And also, they are seeing bias voltage is increase in output pin. Is it correct behavior?

Regards,

Yoshi

0 Luis Fernando Rodriguez S. over 4 years ago in reply to Masayoshi Takahashi

TI__Guru* 84915 points

Hi, Yoshi,

Yes, unfortunately there will be a trade off on this and the cutoff frequency will need to be adjusted with the input capacitor and resistor values. So, the action that the customer performed is correct.

The bias level modification is expected since the bias control portion bases the output on the input components as you may see in the block diagram.

Best regards,
Luis Fernando Rodríguez S.

0 Masayoshi Takahashi over 4 years ago in reply to Luis Fernando Rodriguez S.

TI__Intellectual 1530 points

Hi Luis,

Thanks for your suggestion. I understood now.

Is there any cons by using higher value of resistor?

I'd like to know is there any issue when bias level increasing or not.

Regards,

Yoshi

0 Luis Fernando Rodriguez S. over 4 years ago in reply to Masayoshi Takahashi

TI__Guru* 84915 points

Hi, Yoshi,

The consequences on using a higher input resistor would be seen in the frequency response (as you correctly mentioned in your previous response) and the output gain. The output gain depends of Rf and Ri as mentioned in the datasheet:

http://www.ti.com/lit/ds/snas453c/snas453c.pdf#page=12

However, there won't be any issue by changing the input resistor and the bias level.

Best regards,
Luis Fernando Rodríguez S.

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LM4951A: Pop noise