Other Parts Discussed in Thread: LM3409HV, LM3409
I modified an LM3409HV EVM, but it's not working as I would expected it to. The changes I made to the EVM are as follows:
L1=68uH (3.6A); Rsns = 82mR, Roff (R6) = 18.2kR, load= string of 10 x 3.5V LEDs. All other components were kept the same.
At a first glance, it seems to work correctly, since I can vary the intensity by applying a 0-1.24V input to Iadj input and enable/disable the output
However, when I probe the gate of the pmos, I find a SW of 1.6MHz, instead of the ~500kHz (considering Coff and Roff).
When I probe the node between Coff and Roff I observe the same frequency of 1.6MHz
This frequency is fairly constant when Iadj = 0V and decreases when I increase Iadj, but never reaches 500kHz. Moreover, when Iadj = 0V, the waveform at the node is a continuous sawtooth wave without any gaps between the "tooths". As I increase Iadj, gaps start appearing in the waveform. It's as if there are "missing" sawtooths in the waveform.
Any ideas as to why this might be happening?
Could this be a "damaged" IC?
I only ask this because I had to replace the original IC due to accidental damage. I made sure to solder the IC in the correct orientation and checked that there were no solder bridges and/or poor soldering.
As I said, the circuit seems to be operating correctly, just not how I expected it to. It could also be that my expectation is incorrect.