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DC to DC followed by LDO and Inductor in series

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tiwari.sachin

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I am using a Buck converter (9V to 5V)

and from 5V I am using 1117 3.3V LDO.

The output seems fine and everything works but the MCU connected to 3V3 sometimes goes bad.

I checked on multimeter and the voltages are proper.

I am using 47uF at input of 1117 and 10uF and output.

I donot see a very constant output when i check the LDO output on CRO.

How can I make the output more constant. Should I be using a inductor in series at output of LDO, if so what value. How do I calculate this?
 

I would start by looking at the input to the 1117 on a scope to see if the buck regulator is somehow misbehaving. Then check your connections, such as the polarity of the filter caps on the input and the output. What about the length of the wires from the caps to the 1117? Are they extraordinarily long? As for using an inductor, don't! The 1117 is not designed to be used with a series inductor. That will just complicate the situation, and you really want to keep it simple.
 

Everything is fine and its perfectly working. The caps are ceramic... Input reads 5V on multimeter. But I do see some ringing when I check on CRO but I got that solved (Almost)

I have two 10uF caps at output and 1 47uF at input...

What other possible measures can I take. Seems relatively straight forward
 

Try reducing your capacitor values. An okay ringing frequency is 1/10 of your switching frequency.

The goal is to reduce ringing amplitude. A small capacitor is associated with smaller Ampere flow. It carries less energy for the same amplitude. The ringing should be easier to dampen.
 

Hi,

Show use the schematic of both regulators.
And show us a picture of the pcb layout.

Klaus
 

The problem is the ceramic output capacitor. Read the section in the datasheet under Application Information / Output Capacitor. They require between 0.3 Ohms and 22 Ohms effective series resistance for stability. They recommend tantalums for this very reason. Ceramics are too "good". Either switch to tantalum on the output cap or add 1 Ohm in series with the ceramic cap.
 

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