LM2596
http://onsemi.com
8
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:
I
L(on)
+
ǒ
V
IN
* V
OUT
Ǔ
t
on
L
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
DV
in
R
Load
L
C
out
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:
I
L(off)
+
ǒ
V
OUT
* V
D
Ǔ
t
off
L
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 +
t
on
T
, where T is the period of switching.
For the buck converter with ideal components, the duty
cycle can also be described as:
d +
V
out
V
in
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
On
Power
Switch
On
V
on(SW)
V
D
(FWD)
Time
Time
I
Load
(AV)
I
min
I
pk
Diode Diode
Power
Switch
Diode VoltageInductor Current
