LM2596 buck regulator: pinout, ratings, a 12 V to 5 V design and the blue module

The suffix after LM2596 gives the package and the output. T is the 5-pin TO-220, S the 5-pin TO-263 surface-mount package. Then comes -3.3, -5.0, -12 or -ADJ. The fixed versions have the divider inside and pin 4 goes straight to the output; the ADJ version needs two resistors and covers 1.2 V to 37 V. TI makes all four. onsemi's datasheet covers the ADJ only, in two TO-220 lead forms (T and TV) and a D2PAK.

The blue "LM2596 module" is a different question. It is a small board with a TO-263 chip, an inductor, a diode, two electrolytics and a trimmer, and the listing rarely says who made the chip. Neither datasheet can tell you what is on a module. The bench check further down tells you whether it behaves like an LM2596.

Both makers rate the input at 45 V absolute and 4.5 V to 40 V in use, and both allow 3 A out. They differ on heat: TI gives 50 °C/W from the junction to the air for a vertical TO-220 with its leads on about a square inch of copper, onsemi 65 °C/W. With a part of unknown origin, use 65 °C/W.

The ON/OFF pin is rated differently too. TI's pin table allows up to 25 V on it; onsemi allows anything from −0.3 V up to VIN. Driving it from a 5 V or 3.3 V logic pin is fine for both.

The reference and the 150 kHz clock agree between the makers, but the switch does not: onsemi's saturation voltage at 3 A is 1.5 V typical and 1.8 V maximum, against TI's 1.16 V and 1.4 V. That drop is most of the chip's own heat, so the onsemi figure is the one to design to. The current limit also differs: TI's can trip as low as 3.6 A at 25 °C and 3.4 A over temperature, onsemi's at 4.2 A and 3.5 A. Hot, either one leaves only about half an amp above a 3 A load.

onsemi notes one more behaviour: if the output is shorted or overloaded so that it falls about 40 % below its setting, the oscillator drops to about 30 kHz to cut the heat in the chip.

• Stepping 12 V or 24 V down to 5 V or 3.3 V at 1 to 2 A for a microcontroller board, a Raspberry Pi-class computer or LED strips, where a linear regulator would burn several watts.
• A pre-regulator ahead of a linear regulator, cutting the drop the linear part has to absorb.
• Battery-powered gear that has to sleep: pulling ON/OFF high drops the input current to about 80 µA.

It is not the right part for a tiny board (150 kHz needs a large inductor), for loads near 3 A without a good heat sink, or for inputs below about 7 V when you want 5 V out. TI now points new designs at its newer synchronous converters. The switching regulators lesson explains how a buck converter works.

An LM2596T-ADJ makes 5 V at 2 A from a 12 V adapter. Every step below follows the datasheet's own procedure; the values are the ones in Figure 2.

A bare TO-220 at 40 °C ambient rises by the chip's heat times its junction-to-air resistance. With TI's typical figures that is 1.2 W × 50 °C/W = 59 °C of rise, a junction at 99 °C. With onsemi's typical drop and 65 °C/W it is 141 °C, past the 125 °C operating limit; with its maximum drop, 168 °C. TI's own advice is that the TO-220 needs a heat sink under most conditions. On the Thermalloy #7020 sink TI tested, the rise at 12 V in, 5 V out was about 38 °C even at 3 A.

The surface-mount version gets its cooling from the board. TI measured a TO-263 on 9 square inches of copper at 2 A, 12 V in and 5 V out: about 46 °C above ambient. A module board has far less copper than that. The regulator thermal design lesson shows how to size a heat sink from these numbers.

• Keep the high-current loop tight: input capacitor, pin 1, pin 3 and the diode as close together as the parts allow. TI asks for short, heavy traces and a ground plane.
• Keep the copper on pin 2 small. It swings 12 V at 150 kHz and couples into anything nearby; TI says so in the pin table.
• Route the feedback wire away from the inductor, and use a closed-core (toroid or shielded) inductor if the output ripple or regulation looks wrong. TI shows how an open bobbin coil couples into nearby traces.
• For a 5 V output, keep the input above about 7 V. TI specifies its 5 V version from 7 V up, and onsemi's dropout curve puts the input-to-output difference near 1.8 V at 3 A.
• Do not use an output capacitor above 820 µF; TI's design procedure says so. TI also warns that an output capacitor with extremely low ESR can make the loop unstable.

The same LM2596 from another maker, or another output version from the same one, drops into the same five holes. Parts with other numbers, even similar ones, use other frequencies and pins; check their own datasheets before swapping.

1. Read the chip. A TO-263 part from TI or onsemi carries LM2596 and its suffix (S-ADJ, S-5.0 and so on) with the maker's logo. Note what is there; an unmarked or oddly marked chip tells you to test more carefully.
2. Power it from 12 V with a current-limited supply and no load. On an ADJ module, measure pin 4 to ground: it should read between 1.193 V and 1.267 V, whatever the trimmer is set to. Then set the output to 5.0 V.
3. With 0.5 A of load, put a scope probe on pin 2 or on the diode. The square wave should repeat at 127 kHz to 173 kHz. A very different frequency means the chip is not working like an LM2596.
4. Load it to 2 A with a 2.5 Ω, 10 W resistor for ten minutes. The output should stay within 4 % of 5 V, the adjustable version's tolerance over line and load, and the input current at 12 V should be close to 1 A to 1.1 A. Much more means the module is wasting power.
5. Measure the tab with a thermocouple, not a finger. If it passes about 100 °C at 2 A, the junction is close to its 125 °C limit: derate the module or add copper and airflow.

The bench power supply lesson covers current-limited testing.

For 5 V at up to 2 A from 12 V or 24 V, build the LM2596T-ADJ circuit above with a 68 µH inductor rated past 2.3 A, a 1N5822 (or a 1N5825 if shorts are possible) and a small heat sink on the TO-220, and design the heat to onsemi's 1.8 V switch drop. Treat a blue module as a 1.5 to 2 A part until it has passed the bench check above, whatever its listing says.

Indian sellers stock the blue LM2596 module widely and cheaply, and the bare LM2596T-ADJ in TO-220 less often. A listing that names the chip maker is the exception. Buy a few, run the bench check on each, and keep the ones that pass for real loads. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.