LM2596 datasheet(8/26 Pages) ONSEMI

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LM2596

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PCB LAYOUT GUIDELINES

As in any switching regulator, the layout of the printed

circuit board is very important. Rapidly switching currents

associated with wiring inductance, stray capacitance and

parasitic inductance of the printed circuit board traces can

generate

voltage

transients

which

can

generate

electromagnetic interferences (EMI) and affect the desired

operation. As indicated in the Figure 15, to minimize

inductance and ground loops, the length of the leads

indicated by heavy lines should be kept as short as possible.

For best results, single−point grounding (as indicated) or

ground plane construction should be used.

On the other hand, the PCB area connected to the Pin 2

(emitter of the internal switch) of the LM2596 should be

kept to a minimum in order to minimize coupling to sensitive

circuitry.

Another sensitive part of the circuit is the feedback. It is

important to keep the sensitive feedback wiring short. To

assure this, physically locate the programming resistors near

to the regulator, when using the adjustable version of the

LM2596 regulator.

DESIGN PROCEDURE

Buck Converter Basics

The LM2596 is a “Buck” or Step−Down Converter which

is the most elementary forward−mode converter. Its basic

schematic can be seen in Figure 16.

The operation of this regulator topology has two distinct

time periods. The first one occurs when the series switch is

on, the input voltage is connected to the input of the inductor.

The output of the inductor is the output voltage, and the

rectifier (or catch diode) is reverse biased. During this

period, since there is a constant voltage source connected

across the inductor, the inductor current begins to linearly

ramp upwards, as described by the following equation:

IL(on) +

VIN * VOUT ton

During this “on” period, energy is stored within the core

material in the form of magnetic flux. If the inductor is

properly designed, there is sufficient energy stored to carry

the requirements of the load during the “off” period.

Figure 16. Basic Buck Converter

Vin

RLoad

Cout

Power

Switch

The next period is the “off” period of the power switch.

When the power switch turns off, the voltage across the

inductor reverses its polarity and is clamped at one diode

voltage drop below ground by the catch diode. The current

now flows through the catch diode thus maintaining the load

current loop. This removes the stored energy from the

inductor. The inductor current during this time is:

IL(off) +

VOUT * VD toff

This period ends when the power switch is once again

turned on. Regulation of the converter is accomplished by

varying the duty cycle of the power switch. It is possible to

describe the duty cycle as follows:

d +

ton

, where T is the period of switching.

For the buck converter with ideal components, the duty

cycle can also be described as:

d +

Vout

Vin

Figure 17 shows the buck converter, idealized waveforms

of the catch diode voltage and the inductor current.

Power

Switch

Figure 17. Buck Converter Idealized Waveforms

Power

Switch

Off

Power

Switch

Off

Power

Switch

Power

Switch

Von(SW)

VD(FWD)

Time

ILoad(AV)

Imin

Ipk

Diode

Power

Switch

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