LM2596 buck converter module

The blue LM2596 step-down module: 4.5 to 40 V in, and a trimmer sets the output from 1.23 V up to about a volt under the input.

Aliases: LM2596S, buck converter, step-down converter, DC-DC converter, switching regulator. The board carries TI's LM2596S adjustable switcher, a 33 uH inductor, an SS34 Schottky diode, a 220 uF capacitor on each side and a blue multiturn trimmer.

A buck converter is not a linear regulator with better numbers. It switches the input on and off 150,000 times a second and the inductor averages it, so it trades volts for amps: 12 V in and 5 V out at 2 A takes about 1 A from the adapter, where a 7805 would take the whole 2 A and burn 14 W doing it.

Pins

Pin	What it does
IN+	The input, 4.5 V to 40 V.
IN-	Input ground.
OUT+	The output, set by the trimmer.
OUT-	Output ground.

IN- and OUT- are one copper pour on the board, so the ground goes straight through: wire either one to ground and the other is ground too.

Properties

volts is where the trimmer is turned to, from 1.23 V to 37 V. The default is 5 V.

A real module comes set to whatever the factory left it at, often well over 5 V. On the bench, set it with a meter on OUT+ before anything is connected to the output.

While it runs

The readout is OUT+ above OUT-. Before Run it shows the trimmer's setting.

What the model gets right

The figures are the typical ones from TI's LM2596 sheet (SNVS124).

Power in is power out plus the losses. The input current is the output's power plus what the switch, the diode, the inductor and the switching edges lose, over the input voltage, plus the chip's 5 mA. From 12 V to 5 V the model is about 86 % efficient at 1 A and 82 % at 3 A; the sheet gives 80 % at 3 A for the fixed 5 V part. From 12 V to 3 V at 3 A it is 75 %, against the sheet's 73 %.

Dropout. The LM2596 can hold its switch on all the time, so near its input the output follows the input down, about the switch's saturation (1.16 V at 3 A) and the inductor's resistance under it. Set to 5 V from 5.5 V, it gives about 4.5 V at an amp.

The current limit. The switch limits at 4.5 A peak, which is about 4.2 A of average output once the inductor's ripple is taken off. The board is sold as a 3 A converter: past 3 A a real one runs hot.

Under 4.5 V in it is off. That is the sheet's lowest input; the sheet does not say what happens below it, and the model simply stops.

What it does not model

The ripple. The output carries a 150 kHz ripple, far faster than the simulator's analog steps, so the output here is its average. At 12 V in and 5 V out the inductor's current swings about 0.6 A peak to peak, and on the board's general purpose 220 uF capacitor (about 0.05 to 0.1 ohm at 150 kHz) that is roughly 30 to 60 mV of ripple peak to peak. A low-ESR capacitor on the output brings it down.

Also not modeled: light-load operation, where the real efficiency falls further than the 5 mA quiescent current alone accounts for; the soft start; the spread of the feedback reference from board to board (1.193 to 1.267 V); line regulation; the ON/OFF pin, which the board ties on; and heat and thermal shutdown.

Common mistakes

Wiring it backwards. Most modules have no reverse protection: 12 V into OUT+ or reversed on IN+ kills the chip.

Not setting the trimmer first. A module set to 12 V on an Arduino's 5V pin kills the Arduino.

Too little headroom. It needs a volt or more across itself at a few hundred milliamps: 5 V out wants 6.5 V in or more.

See it in action

The Arduino on 12 V with an LM2596 project powers an Uno from a 12 V adapter through the module, and reads the adapter's voltage on A0.