Fake LM2596?

[holex]

I have bought an LM2596 on Ebay. I have checked its output by an oscilloscope, but its frequency is only 55 kHz, not 150. Have I bought a fake item?

 

[davenn]

hi there
welcome

ummm first question .... was it regulating your voltage correctly ?

The 150 kHz osc is all internal, chances are you are not going to be able to measure it any way

 

[holex]

Hi!
Yes, voltage is correct, but I have to use other inductor and capacitor value if frequency is lower.

I should be able to measure a 150 kHz square wave on the output if load is high enough. Or I'm wrong?

 

[holex]

This picture is from the datasheet. Here the period time is 7us (-> frequency is 143 kHz).
If you see my oscilloscope screeshot, the period time is 18us.

 

[davenn]

I would be surprised if you can ... the osc signal doesn't appear directly at the output

 

[holex]

Okay, thank you. But why is the frequency 143 kHz in the picture from the datasheet?

 

[davenn]

Okay, thank you. But why is the frequency 143 kHz in the picture from the datasheet?

I don't know which pic you are referring to ??

 

[holex]

See the #4 answer.
Or check the datasheet on page 8.

 

[(*steve*)]

Check to see if the frequency increases with load. The regulator may be operating in burst, or stop-start, or non-continuous mode if it has a relatively low load on it. What this means is that a single "on" cycle raises the voltage above the set point and it takes more than the remainder of the cycle for it to drop below the set point again, so the regulator skips cycles to prevent the voltage from rising too high.

It's still possible that you have a clone chip, but it may not necessarily be a fake.