Input

to 40 Volts (60 Volts on the high voltage LM2576HV-XX), and you get more current out (up to three Amperes) at a regulated lower voltage. As well as the 12 Volt output version (LM2576-12), the chips are available in 3.3 volts, 5.0 Volts, 15 Volts, and an adjustable 1.23V to 37V version (LM2576-ADJ).

For the basics on how buck converters work, see "Using Magnetic Fields to Change Voltages" on page 40 in this issue. In this circuit, Pin 4 on the LM2576-12 senses the output voltage, and any time it falls below 12 Volts, the chip allows current to pass from pin 1 to pin 2. Current from pin 2 passes through the inductor, building up its magnetic field, and arrives at the output, raising the output voltage. When the voltage rises above the 12 Volt threshold, pin 2 is shut off, and the inductor's collapsing magnetic field induces current through the Schottky Diode, supplying the output. The capacitors provide filter input and output to within ±3% of 12 Volts.

The chip contains internal frequency compensation, current limit protection, and thermal shutdown. It works great! I used the circuit to power a PowerStar P0W200 inverter from a 24 Volt battery pack. Using a variety of ac powered loads from the inverter, and taking data with a recording Fluke 87 DMM, I measured efficiencies from 82% (239 mA output), to 93% (980 mA output). At

1.7 Amperes output the efficiency was 87%. To test the thermal shut-down and overcurrent protection, I powered a 60 Watt ac incandescent bulb through the inverter. This meant my 3 Ampere buck converter was supplying 5.0 Amperes DC! The light turned on (a little dim) and ran for 15 seconds before shutting down. Ten seconds later, it fired up again for another 15 seconds. No damage to the chip. Efficiency out here: 79%.

When you build the circuit, keep wire leads short. Since the switcher operates at 52 kHz, I wouldn't recommend using it to power radio equipment which operates near this band. But I noticed no interference on a FM (~100 MHz) radio powered by the regulator/inverter.

Access

Author: Chris Greacen, Rt. 1 Box 2335B, Lopez, WA 98261 • 206-468-2838. Thanks to Dave Gardner at Advanced Electronics (see advertiser's index) for turning me onto this chip.

Parts: Finding components for these circuits isn't easy. DigiKey has the LM2575T-5.0 which is a 1 Ampere, 5 Volt version of the chip. It sells for $6.55. For most folks this won't do. National Semiconductor was not able to recommend any other retailer of these chips on a piece by piece basis, and big distributers sell in lots of 45. But National Semiconductor says they will accommodate individual customers building prototype circuits. What this means is they'll send you one if you ask nicely and use words like "sample", "engineer" and "prototype". Their support center is 1-800-272-9959. If you're interested in this stuff, their "1993 Power IC's Databook" is excellent.

Schottky diodes are available from DigiKey • 1-800-DIGI-KEY. Their PBYR745PH-ND is good for 7.5 A, and costs $1.29. Also you might try All Electronics • 800-826-5432. Hosfelt Electronics, 2700 Sunset Boulevard, Steubenville, OH 43952 • 614-264-6464 has a 160 ^H inductor for a buck, part number 18-123.

I'll build you one of these circuits, and put it in a nice little box, for $75. Tell me your input and output voltage choice. Or I'll send you all the parts, including the box, for $60. Allow six weeks for delivery. ^

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