The LM2577 integrated circuit from National Semiconductor is a useful IC that provides all of the power and control functions for a simple boost (step-up), flyback, and forward converter switching regulator. The IC has a wide input voltage range (3.5V to 40V) and is available in different output voltage versions: 12V, 15V, and adjustable.
The chip includes a 3A NPN switch transistor with its associated protection circuitry, consisting of current and thermal limiting, and undervoltage lockout. Other features: 52kHz fixed-frequency internal oscillator that requires no external components, soft start mode to reduce in-rush current during start-up and current mode control for improved rejection of input voltage and output load transients.
The LM2577 is available in different packages: 5-Lead TO-220 (T), 16-Lead DIP (N), 24-Lead Surface Mount (M) and TO-263 (S) 5-Lead Surface-Mount. There is also a 4-Lead TO-3 variant of this IC - the LM1577.
The pinout for the TO-220 variant (LM2577T-12, LM2577T-15, or LM2577T-ADJ) is presented on the right. For other packages see the LM2577 datasheet.
The schematic is simple, easy to build and cost effective, producing 12V from a 5V unregulated supply with a maximum output current of 800mA. Pin numbers shown are for the TO-220 package (LM2577T-ADJ).
The design uses the adjustable version of LM2577, but the 12V fixed-voltage version (LM2577T-12) will also work if you remove R1 and R2 and connect the feedback pin directly to the regulator's output. The UC2577 - a pin to pin compatible replacement available from Texas Instruments can also be used.
By selecting different values for R1 and R2, you can use the LM1577-ADJ/LM2577-ADJ to produce different voltages. The output voltage is given by the following formula:
VOUT = 1.23V (1 + R1/R2)
The input capacitor (C1) should be a good quality, low ESR, 0.1 µF capacitor with the leads cut as short as possible. If the IC is located far from the supply source filter capacitors, an additional larger electrolytic capacitor (e.g. 10µF-100µF) is required.
Select a low ESR type capacitor for the output filter (C3), with a working voltage at least 20% higher than the output voltage. Low values of ESR can be achieved by using larger value capacitors or by paralleling multiple smaller-value capacitors.
You can design and make your own inductor using a suitable core recuperated from old equipment if you have the knowledge, practical experience and equipment for this task. But the inductor is one of the most critical components in these types of circuits, so it's much safer and simpler to or use readymade off-the shelf parts: 415-0930 (Inductive Technologies), 67127000 (Schott), PE-92108 (Pulse Engineering) or RL2444 (Renco) or equivalents.
The 415-0930 part from Pulse has the following specifications:
Typical inductance: 100µH, ETop: 90VµSec, Rated current: 2.6A, DCR: 0.1ohm.