LM2776: 30kHz spurs with LM2776 design

Jonathan Geronga

Part Number: LM2776
Other Parts Discussed in Thread: TPS60403

Hi Team,

I would like to get your comment on why our customer is getting 30kHz spurs with LM2776 design below:

Please let me know if you have any questions.

Thanks,

Jonathan

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 316440 points

That is the switching frequency. It is lower than 2 MHz in PFM mode (see section 7.3.2 of the datasheet).

There are voltage inverters with a fixed switching frequency, e.g., TPS60403.

0 Febin over 4 years ago in reply to Clemens Ladisch

TI__Mastermind 25430 points

Hi Jonathan,

I agree with the answer above. This is due to the PFM mode operation.

Have you also tried to reduce the output voltage and compare the noise performance?

Regards,

Febin

0 Jonathan Geronga over 4 years ago in reply to Febin

TI__Expert 7930 points

Thank you for the support in this thread Clemens and Febin,

Upon sharing your inputs here with the customer, here's the answer I get:

They did not reduce the voltage. But they loaded the device so it can draw more than 40 mA. So the spurs frequency changed to 2 MHz, the switching speed of the converter.

Is there a way to stop it without using external filter?

Regards,

Jonathan

0 Febin over 4 years ago in reply to Jonathan Geronga

TI__Mastermind 25430 points

Hi Jonathan,

Firstly, a 5V to -5V rail is not achievable with this device. You need to try to set it to a lower output voltage. What is the max output load?

Are the noise spurs seen both in Vin and Vout? Please also provide all the measurement plots for analyzing further.

Could you share a complete schematic and layout as well?

Regards,

Febin

0 Clemens Ladisch over 4 years ago in reply to Jonathan Geronga

Guru 316440 points

Either use a voltage inverter with a fixed switching frequency (e.g., TPS60403), or reduce the size of the flying capacitor (see Optimizing Output Voltage Ripple for the REG‍710).

0 Febin over 4 years ago in reply to Clemens Ladisch

TI__Mastermind 25430 points

Hi Jonathan,

I hope you could solve your problem. Due to inactivity, I will go ahead and close this thread. Please reach out to us if you have any further questions.

Regards,

Febin

0 Jonathan Geronga over 4 years ago in reply to Febin

TI__Expert 7930 points

Hi Febin,

I did not hear anymore from the customer so I assume that this thread is now resolved.

Thank you for your support Febin and Clemens

Regards,

Jonathan

Power management

Power management forum

LM2776: 30kHz spurs with LM2776 design