LM1117: LM1117DT

Hi Ponraj,

20mVpp is quite a lot of noise. Can you provide a scope shot showing this?

What is supplying the input to the LDO? Is the input also very noisy?

Here is a slideshow that discusses what effects noise for an LDO and how it can be improved upon. Unfortunately, LM1117 does not have an NR/SS pin, but if you are using an adjustable version of the device you can use a feedforward capacitor that will help to reduce higher frequency noise gain. 

8233.Measuring LDO Noise.pptx

Regards,

Nick

hi Sir,

here i attached ripple noise picture.

Hi Ponraj,

Can you please include the input supply with AC coupling and provide a scope shot showing input and output together? This is a lot more noise than is expected.

Regards,

Nick

Hi sir,

I get the same result after including AC coupling (20mVpp- input and output side). I'm using GX650W switch mode power supply. SMPS output voltage directly connect to LDO. Any filter circuit include between SMPS and LDO?.

Hi Pinraj,

If you are seeing this noise at both the input and the output then, assuming the LDO is otherwise properly regulating, this is evidence that the source of the noise is at the output side of the LDO. Can you disconnect the load of the LDO and measure the noise at the input and output again?

Regards,

Nick

Hi sir,

can you suggest a good LDO to out requirement. 

Hi Ponraj,

Can you provide more operating details to guide a search for a replacement? 

As I mentioned, if you are seeing the same noise at the input as at the output, it is likely that the source of the noise is at the output because otherwise the PSRR of the LDO would attenuate the input noise if it were originating at the input, so I'm not sure that a different LDO will help here because LDOs are not capable of attenuating noise from output to input. 

Regards,

Nick