LP38798: Charge Pump Output Spur

Dajon McGill

Part Number: LP38798

Hello Team,

Could you please help to answer the comments and questions below:

I have a new RF amp design where I would like to use the LP38798. Per the datasheet, I noticed that the part uses a charge pump at ~3.5MHz. LDOs with a charge pump generally exhibit some kind of low level output spur related to that charge pump. I looked in the datasheet and on the web-based forum and did not see mention of the spur and/or its level. So, my question is:

Q). What is the anticipated output level of the spur from the 3.5MHz charge pump?

Best,
Dajon McGill

over 4 years ago

0 Srikanth Lolla Venkata over 4 years ago

TI__Genius 12970 points

Hello,

I am looking into this, I will get back to you by the end of the day tomorrow. Thanks!

Regards,

Srikanth

0 Dajon McGill over 4 years ago in reply to Srikanth Lolla Venkata

TI__Expert 7181 points

Thanks for the update Srikanth!

0 Srikanth Lolla Venkata over 4 years ago in reply to Dajon McGill

TI__Genius 12970 points

Hello Dajon,

You are correct that in general LDO's with charge pump will exhibit some noise spur at the switching frequency. For this device however the charge pump helps power the gate drive of an NMOS pre-regulator inside the Active Ripple Rejection block (as can be seen in section 7.2 of the datasheet). At the output of the Active Ripple Rejection Block we can expect to see the noise spur at 3.5MHz. This is followed by the control loop containing the PMOS pass element, which will help filter out the 3.5MHz noise spur. Hence, as can be seen in Figures 3 & 4 of the datasheet, we do not see any 3.5 MHz noise spur at the output of the LDO. Thanks!

Regards,

Srikanth

Power management

Power management forum

LP38798: Charge Pump Output Spur