Do I have the use the FB (voltage control loop) in the LT3762?

lslonim@our3cats.com said:

From what I can tell, FB is used to detect voltage too high (open) or too low (short). My boards just feed it 0.25V so it will stay out of the way, and LTSpice was fine with that. I am wondering if the chip needs to see that voltage ramp up or something. Does anyone know if I need to use the FB pin?

FB needs to be connected to a sense voltage. 

lslonim@our3cats.com said:

Since I am driving a high current 3V LED from a 3-4V source and I don't have any conditions where I would have an open circuit or a short, I didn't want to use the FB pin. The FB pin also requires several parts for level shifting that I don't want to use if I don't have to because I am extremely tight on space.

If you want to drive the LED with 3V, then you need to connect FB to the voltage divider such that it gives FB 1.25v when output is 3v. The lock diag below shows how FB is used for the amplifiers to control the switching FETs.

Sorry, typo in the original post, I supply FB with 1.25V so it is satisfied. The question is whether it needs to actually reflect the voltage output as the switcher starts up and runs, or can I just drive it at 1.25v and omit the parts to sense it.

Lloyd

lslonim@our3cats.com said:

but in the real world, they don't work. Output very low, output flickers, no switching activity, depending on conditions and voltages.

You could check to see if SHORTLED and OPENLED  pins pulled low (you need to pull this pin up to 3-4v with 1K resistor). if SHORTLED is low then the output voltage will be disabled due to bad FB signal.

lslonim@our3cats.com said:

Sorry, typo in the original post, I supply FB with 1.25V so it is satisfied. The question is whether it needs to actually reflect the voltage output as the switcher starts up and runs, or can I just drive it at 1.25v and omit the parts to sense it.

No you can't tie FB to a const 1.25v and assume the ampliers do magic to give you the proper output of 3v for LED. Here is the spec:

the Vref can be anything from 1.225v to 1.275v and you never know. So you need to the 2 resistor divider to provide the FB. 

How do you expect the circuit will give the output of 3v as you wanted without the sense voltage for FB? What magic mechanism to regulate the output at 3V?

Thanks, I'll try that next week.

Lloyd