Hi,
we built a driver based on the AN-1839. We added transistor+resistor to the LM3404 to keep the current constant over input voltage changes.
This basciall works, but it turns out that not all drivers do. Our intense investigation did not lead to a clear conclusion so far. The problem might be that while the LEDs are shorted by the MOSFET (LEDs off) the voltage at the RON is about 750mV. By the datasheet, this should be enough to power up the regulator looking at the typ values for the min RON voltage. But could be also 1V.
If the regulator is powered, it does not start up normally. Instead it tries to power up every few seconds, but does only one ON-Time, then stops again.Seems the on-time is 5us or so. CS reaches 300mV.
Finally, we used the IC from the eval PCB: it also does work normally, but instead of making only one on-time, it makes many with the correct calculated on-time of 560ns. But then stops also after some 10s us.
The circuit does this even it totall isolated, only powered and output short.
Vin=24V
Vout (to got to the error) = 0V, otherwise 9V.
I = 1A
L = 100uH
Ron = 100k
Rsns = 200mR
Everything else like on the eval PCB.
I would be happy about any suggestions.
If the "modified COT application circuit" is replaced by a simple RON resistor, then everything works normally.
Thanks,
Simon