TPS745-Q1: LDO PSRR Test Measurement

Hi Edgar, 

I've already tried to test my LDO PSRR yesterday 

but i found out that my waveform didn't same like the one in the document TI show in the (left below), and mine (right below) :

The yellow part is the Voltage ripple input and the green wave is the voltage ripple output. 

when i did the calculation the result only 15.36dB which the result is really far from the recommendation is 45dB 

can you give me a solution and explain to me what happend ? 

Thank you 

Hi Edgar, 

and i have more question.. after read the datasheet for TPS745-Q1 

is there any capacitor or resistor value we need to consider? 

if you have reference design for  TPS74501PQWDRVRQ1  it will be really helpful !

Thank you 

Hi Ihzany, 

Typically for PSRR measurements Cin should be removed. 

What are your Vin, Vout, and load conditions? Is there a schematic that you can share? How are you coupling the AC+DC signal? 

I highly recommend using a VNA to do this sort of measurements. We commonly use the Bode100 from Omicron which is very simple to use. 

As for a reference design, we have the EVM and it is something we also use to make PSRR measurements. I can also take the measurements if the conditions are given. 

Best,

Edgar Acosta

Hi Edgar, 

This is our design for using the TPS74501PQWDRVRQ1 

is there anything need to be changed? 

Hi Ihzany, 

Thank you for sharing the schematic. 

Before doing any PSRR measurements, has the 1.8 V at Vout been confirmed in steady state? I see CP62, but this shouldn't be there, CP63 is ok, but please remove CP62. This will tend to mess with the FB loop of the LDO and will create instability.

In relation to the PSRR, where both input capacitors removed? 

Also, I see that this device will be powered from a battery. When doing so, batteries tend to have little to no ripple, or as a noiseless source, is the 100mVpp expected to come from some other source? 

Best, 
Edgar Acosta

Hi Edgar, 

1. Yes i've already confirm it and the result is the same Vin 3.3V and Vout 1.8V 

2. CP62 is Not Attached (NA)

3.  When i did a test, i didn't remove both input capacitor, should i removed it when test it? I thought that the ripple output is the problem not the input ripple 

4. I'm powering using power supply for now, this schematic is just a part from previous circuit. but i can't share here due to the private matters. what i can't tell you it's just to powering the other with input 1.8V 

Thank you 

Hi Ihzany, 

Thank you for the clarifications, I just wanted to make sure that CP62 was not installed. 

How is the AC signal being coupled with the DC signal?

When we do our PSRR tests Cin is removed as we want to test the capability of the device to reject ripple, and Cin can attenuate the signal. 

Are both IN and OUT kelvin sensed? What is the load i.e 100mA, 250mA? 

If the shared circuit is the actual circuit, I can make the PSRR measurement on my end if Cout combination can be provided to give some sort of estimation. 

Also, PCB design has a big impact on PSRR measurements, is it possible to share the layer stackup structure or the PCB portion of the LDO? 

You can also send me a private message to maintain sensitive information private. 

Best, 

Edgar Acosta

Hi Edgar, 

Edgar Acosta1 said:

When we do our PSRR tests Cin is removed as we want to test the capability of the device to reject ripple, and Cin can attenuate the signal. 

= Yesterday i've already did a test with Cin removed.. but the result of the ripple output still the same like this but only the ripple input more stable 

Edgar Acosta1 said:

Are both IN and OUT kelvin sensed? What is the load i.e 100mA, 250mA? 

= the Iout is 100mA 

Edgar Acosta1 said:

Also, PCB design has a big impact on PSRR measurements, is it possible to share the layer stackup structure or the PCB portion of the LDO? 

You can also send me a private message to maintain sensitive information private. 

= I don't know how to send the private message.. can you tell me how? 

This is the result of removing Cin (Picture Below) 

as you can see we got the ripple input but not the ripple output.

and btw we don't have network analyzer in our company 

Thank you 

Hi Ihzany, 

Quick follow-up questions. 

On the first scope shot you shared, your input frequency was 48.3kHz, and for the second scope shot I see a 148kHz, what is the actual intended frequency? 

Overall, we can somehow relate Power Supply Rejection Ratio as a filter, therefore, if there is some ripple on the input, we would expect a smaller (or cleaner) signal on the output, which is what you are seeing a smaller amplitude on the output side. 

Here are some recommendations for using the scope method: 

1) Try increasing the amplitude on the input side to 500mV, with a 45dB PSRR you would expect a ~3mVpp on the output, so at 100mV the expected ripple is ~0.5mVpp, which might not be properly captured. 

2) Make sure that you are kelvin sensed on both, input and output, if not, try using the Tip and Barrel method (at least on the output side) and if possible use a 1X probe. 

3) When using the measurement setup from the oscilloscope, change Max to Amplitude and/or use cursors. 

I have reached out to you separately via private message. 

Best, 

Edgar Acosta