LP5907: Lp5907

Eric Liu(SZ-FAE)

Intellectual 950 points

Part Number: LP5907

Hi team,

Do we have a ldo can decrease the ripple from the dcdc to uV?if the psrr is 60db when 200khz ,the fsw is 200khz and the ripple of dcdc is 40mV ,can the ldo ripple acheive 40uV？

7 months ago

0 Andres Sagredo 7 months ago

TI__Expert 4380 points

Hi Eric,

Given that the formula for PSRR is the following:

We can rearrange the formula to figure out the ripple at the output:

Using the output ripple formula and using 60dB PSRR when frequency = 200kHz and Iout = 200mA, the formula tells us the output ripple should be 40uV. For more information on PSRR please see the references attached below, they will also cover parameters that could affect your PSRR.

References:

https://www.ti.com/lit/an/slyt202/slyt202.pdf?ts=1701793937397

https://www.ti.com/lit/eb/slyy151a/slyy151a.pdf?ts=1701794095570&ref_url=https%253A%252F%252Fwww.google.com%252F

Regards,

Andres

0 Eric Liu(SZ-FAE) 7 months ago in reply to Andres Sagredo

TI__Intellectual 950 points

Hi Andres,

Thanks for your response.

for the ldo, how can we test the noise and ripple in such low amplitude?for the oscillscop, it may just the signal exceed 100uV.

0 Andres Sagredo 7 months ago in reply to Eric Liu(SZ-FAE)

TI__Expert 4380 points

Hi Eric,

I'll add one of our applications engineers to this conversation to help explain how to test ripple and noise at these low levels of amplitude.

Regards,

Andres

0 Eric Liu(SZ-FAE) 7 months ago in reply to Andres Sagredo

TI__Intellectual 950 points

Hi team,is there any update for this？

0 Stephen Ziel 7 months ago in reply to Eric Liu(SZ-FAE)

TI__Mastermind 32600 points

Hi Eric,

I would refer to these links from Picotest on measuring the PSRR. Keep in mind that you'll want to minimize the input capacitor (Cin) or remove it entirely (which is preferred) as this capacitor inhibits the ability to inject AC voltage onto Vin used to conduct the PSRR measurement.

https://www.picotest.com/measurements/PSRR.html

If you have the switching converter connected to the LDO, and you just need to confirm the DC switching frequency is sufficiently attenuated, then you'll want to use a frequency response analyzer (or something that gives you a measurement in the frequency domain). You can refer to your oscilloscope to see if it is capable of taking frequency domain measurements, and that it is also capable of taking this measurement.

Thanks,

Stephen

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LP5907: Lp5907