I just finished setting up an LM2577 as a boost converter, and have run into some unexpected issues.
For starters, here is the recommended "typical application circuit":
My application circuit is slightly different. At the 5V input, I can actually have anything between a 8V to 14V input as part of a PMOS common source setup so I can turn off the current coming in to the circuit, like so:
However, the output voltage is nothing like what I expected. I had assumed that if I'm drawing at least a few dozen milli-amps, and have a +9V DC rail at the source of the PMOS, I should expect just under 9V at the drain of the PMOS when it's turned on, and that the output voltage of the boost regulator, Vcc, would be approximately 12V. My assumption was also that the output voltage is just a function of the external discrete components connected to the regulator, and not the input voltage (ie: 9V DC) provided the input voltage to the regulator is less than the output voltage it generates. Is this assumption correct? I want the regulator to always output a fixed voltage (ie: 12V) for a wide range of input voltages (ie: 7.5V to 10.0V).
Also, the output voltage I initially measured was much greater than expected, a whopping 41V. When I removed the 0.1uF capacitor that connects the input to GND, it went back down to +15VDC. How can this be happening? Is it because the input to the regulator is from a PMOS drain and not a supply rail?
Best Answer
It turns out that I was using a partially correct application circuit. According to this link, it turns out that when using the fixed output version of this chip, I need to short the feedback (pin 2) to the output of the Schottky diode, and I'm all set.