LP5907: Display noise while using LP5907-2.9 for low lux testing...

Hi Peter,

The load impedance on the LP5907 looks like it is too low for this device and it may be unstable as a result.  The LP5907 upper limit is 10uF.  If you need a newer device which can handle this load you may want to try the TPS7A20. 

If the TPS7A20 will not work, can you tell me what the LDO requirements are?  It sounds like noise is a major concern - what would that requirement be?

Thanks,

Stephen

Hi Stephen,

As customer feedback, they just used the typical schematic of LP5907 in the datasheet to test but still fail.

About the TPS7A2029 test, since it is not p2p pkg(SOT23-5), is there any influence with jumper wire to test on customer PCB?

Hi Peter,

Please confirm that the customer removed the inductor and shunt capacitor to the right of the inductor, when they used the typical schematic of LP5907.  What is the load current in the application?  I can review my notes to suggest an optimum Cout value and capacitor type for the LP5907.

Yes, using a jumper wire can result in coupled noise into Vout.  Radiated noise can couple into the wire, which can conduct the noise onto Vout.  For very noise sensitive applications this can cause issues.

Can you confirm that the "good" image (black) was obtained in this same hardware, but using a different device?  I would like to confirm that the issue is not noise coupling within the PCB itself.

Thanks,

Stephen

Dear Stephen,

Sorry for late!

The Max. load current is around 54mA while low lux testing and the IC temp. is normal in room temp.

While testing with typical schematics of LP5907, all other components were removed except Cin and Cout on customer board.

Hope we can fix the issue and win the socket.

Thanks.

Hi Peter,

I see the competitor part is something called L55SVBP.  Can we get the datasheet for that device so we can review the noise curves?  It will help us to compete if we know what we are competing against.

For the LP5907 test, please increase the Cout for an effective capacitance between 1.5uF to 10uF with 54mA of load current.  There are X5R capacitors in the 0402 package that may work well here.  For instance, AVX has X5R 0402 10uF capacitor which will lose around 55% of its value with 2.9V applied but this will put it towards the middle of 1.5uF to 10uF. 

Shown below is the noise curve and data for the LP5907:

The TLV758P has the following graphs for noise.  It could be that we need to increase the Cff value to lower the noise.

Thanks,

Stephen

Dear Stephen,

Thanks for your comments.

I will work with customer to test. 

The datasheet as attached.

l55svbp.pdf