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Schematic please.
What voltage/current is the light bulb.
The LM2577 does regulate the output voltage so something is wrong.

 

What are the values of your feedback resistors?

And what, exactly are you using as an inductor (most likely culprit).

 

Hello,

Wich version of the LM2577 are you using?
There are the fixed and adjustable versions.

Bertus

 

Is your 9V battery a PP3 type? If so, it will be limiting the input current and hence the output current.

 

Breadboarding a switching regulator is almost always a mistake. The capacitor cannot supply the current, or inductor cannot handle the current demand of a cold filament, or the source cannot supply at least twice the instantaneous current demand, that is most likely why the voltage drops.

Your output power is 1.3 watts, so I estimate your input power requirement as 1.62 watts. At 9V this is .18 Amperes from the batteries. Are they capable of supplying that much power?

 

Light bulb is 2.5 volt/0.35A? And you are connecting this to what is nominally a 14 volt output?

The bulb is going to try to draw much more than 350 mA. And your input current is going to be almost 3x the current draw of the bulb. The AA batteries may not be able to supply sufficient current.

When the output drops to 3.5 volts, what does the input voltage measure?

 

Try the regulator with a fixed resistor load.

 

Maybe the inductor has too much resistance, but we won't know unless that detail is posted.

 

Measure it's resistance.

You also must know the max current for the inductor. If exceeded it will saturate and not perform correctly.

Bob