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LM2576D2T-3.3G

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LM2576
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16
Catch Diode
Locate the Catch Diode Close to the LM2576
The LM2576 is a step−down buck converter; it requires a
fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close
to the LM2576 using short leads and short printed circuit
traces to avoid EMI problems.
Use a Schottky or a Soft Switching
Ultra−Fast Recovery Diode
Since the rectifier diodes are very significant sources of
losses within switching power supplies, choosing the
rectifier that best fits into the converter design is an
important process. Schottky diodes provide the best
performance because of their fast switching speed and low
forward voltage drop.
They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice
could be Fast−Recovery, or Ultra−Fast Recovery diodes. It
has to be noted, that some types of these diodes with an
abrupt turnoff characteristic may cause instability or
EMI troubles.
A fast−recovery diode with soft recovery characteristics
can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2576
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.
Inductor
The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.
Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply, and the
possibility of core saturation can arise during an abnormal
operational mode. Voltage spikes can cause the
semiconductors to enter avalanche breakdown and the part
can instantly fail if enough energy is applied. It can also
cause significant RFI (Radio Frequency Interference) and
EMI (Electro−Magnetic Interference) problems.
Continuous and Discontinuous Mode of Operation
The LM2576 step−down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period
of time (see Figure 23 and Figure 24). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.
To simplify the inductor selection process, an inductor
selection guide for the LM2576 regulator was added to this
data sheet (Figures 18 through 22). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peak−to−peak inductor
ripple current to be a certain percentage of the maximum
design load current. This percentage is allowed to change as
different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because the
inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak−to−peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter.
Any buck regulator will be forced to enter discontinuous
mode if the load current is light enough.
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 23. Continuous Mode Switching Current
Waveforms
VERTRICAL RESOLUTION 1.0 A/DIV
2.0 A
0 A
2.0 A
0 A
Inductor
Current
Waveform
Power
Switch
Current
Waveform
Selecting the Right Inductor Style
Some important considerations when selecting a core type
are core material, cost, the output power of the power supply,
the physical volume the inductor must fit within, and the
amount of EMI (Electro−Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E−core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.
For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to
the fact that its core is open, and the magnetic flux is not
contained within the core.
LM2576
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When multiple switching regulators are located on the
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E−core (closed magnetic structure) should be
used in such applications.
Do Not Operate an Inductor Beyond its
Maximum Rated Current
Exceeding an inductor’s maximum current rating may
cause the inductor to overheat because of the copper wire
losses, or the core may saturate. Core saturation occurs when
the flux density is too high and consequently the cross
sectional area of the core can no longer support additional
lines of magnetic flux.
This causes the permeability of the core to drop, the
inductance value decreases rapidly and the inductor begins
to look mainly resistive. It has only the DC resistance of the
winding. This can cause the switch current to rise very
rapidly and force the LM2576 internal switch into
cycle−by−cycle current limit, thus reducing the DC output
load current. This can also result in overheating of the
inductor and/or the LM2576. Different inductor types have
different saturation characteristics, and this should be kept
in mind when selecting an inductor.
0.4 A
0 A
0.4 A
0 A
Inductor
Current
Waveform
Power
Switch
Current
Waveform
Figure 24. Discontinuous Mode Switching Current
Waveforms
VERTICAL RESOLUTION 200 mA/DIV
HORIZONTAL TIME BASE: 5.0 ms/DIV
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Source of the Output Ripple
Since the LM2576 is a switch mode power supply
regulator, its output voltage, if left unfiltered, will contain a
sawtooth ripple voltage at the switching frequency. The
output ripple voltage value ranges from 0.5% to 3% of the
output voltage. It is caused mainly by the inductor sawtooth
ripple current multiplied by the ESR of the output capacitor.
Short Voltage Spikes and How to Reduce Them
The regulator output voltage may also contain short
voltage spikes at the peaks of the sawtooth waveform (see
Figure 25). These voltage spikes are present because of the
fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. There are some
other important factors such as wiring inductance, stray
capacitance, as well as the scope probe used to evaluate these
transients, all these contribute to the amplitude of these
spikes. To minimize these voltage spikes, low inductance
capacitors should be used, and their lead lengths must be
kept short. The importance of quality printed circuit board
layout design should also be highlighted.
Unfiltered
Output
Voltage
Filtered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 25. Output Ripple Voltage Waveforms
VERTRICAL
Voltage spikes
caused by
switching action
of the output
switch and the
parasitic
inductance of the
output capacitor
RESOLUTION
20 mV/DIV
Minimizing the Output Ripple
In order to minimize the output ripple voltage it is possible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value output capacitor. There is also another way
to smooth the output by means of an additional LC filter (20
mH, 100 mF), that can be added to the output (see Figure 34)
to further reduce the amount of output ripple and transients.
With such a filter it is possible to reduce the output ripple
voltage transients 10 times or more. Figure 25 shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 34.
The lower waveform is from the normal unfiltered output
of the converter, while the upper waveform shows the output
ripple voltage filtered by an additional LC filter.
Heatsinking and Thermal Considerations
The Through−Hole Package TO−220
The LM2576 is available in two packages, a 5−pin
TO−220(T, TV) and a 5−pin surface mount D
2
PAK(D2T).
Although the TO−220(T) package needs a heatsink under
most conditions, there are some applications that require no
heatsink to keep the LM2576 junction temperature within
the allowed operating range. Higher ambient temperatures
require some heat sinking, either to the printed circuit (PC)
board or an external heatsink.
The Surface Mount Package D
2
PAK and its
Heatsinking
The other type of package, the surface mount D
2
PAK, is
designed to be soldered to the copper on the PC board. The
copper and the board are the heatsink for this package and
the other heat producing components, such as the catch
diode and inductor. The PC board copper area that the
package is soldered to should be at least 0.4 in
2
(or 260 mm
2
)
and ideally should have 2 or more square inches (1300 mm
2
)
of 0.0028 inch copper. Additional increases of copper area
LM2576
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18
beyond approximately 6.0 in
2
(4000 mm
2
) will not improve
heat dissipation significantly. If further thermal
improvements are needed, double sided or multilayer PC
boards with large copper areas should be considered. In
order to achieve the best thermal performance, it is highly
recommended to use wide copper traces as well as large
areas of copper in the printed circuit board layout. The only
exception to this is the OUTPUT (switch) pin, which should
not have large areas of copper (see page 8 ‘PCB Layout
Guideline’).
Thermal Analysis and Design
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
1. P
D(max)
maximum regulator power dissipation in the
application.
2. T
A(max
) maximum ambient temperature in the
application.
3. T
J(max)
maximum allowed junction temperature
(125°C for the LM2576). For a conservative
design, the maximum junction temperature
should not exceed 110°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime
of the component is halved.
4. R
qJC
package thermal resistance junction−case.
5. R
qJA
package thermal resistance junction−ambient.
(Refer to Maximum Ratings on page 2 of this data sheet or
R
qJC
and R
qJA
values).
The following formula is to calculate the approximate
total power dissipated by the LM2576:
P
D
= (V
in
x I
Q
) + d x I
Load
x V
sat
where d is the duty cycle and for buck converter
d +
t
on
T
+
V
O
V
in
,
I
Q
(quiescent current) and V
sat
can be found in the
LM2576 data sheet,
V
in
is minimum input voltage applied,
V
O
is the regulator output voltage,
I
Load
is the load current.
The dynamic switching losses during turn−on and
turn−off can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free−Standing)
For a free−standing application when no heatsink is used,
the junction temperature can be determined by the following
expression:
T
J
= (R
q
JA
) (P
D
) + T
A
where (R
qJA
)(P
D
) represents the junction temperature rise
caused by the dissipated power and T
A
is the maximum
ambient temperature.
Packages on a Heatsink
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:
T
J
= P
D
(R
q
JA
+ R
q
CS
+ R
q
SA
) + T
A
where R
qJC
is the thermal resistance junction−case,
R
qCS
is the thermal resistance case−heatsink,
R
qSA
is the thermal resistance heatsink−ambient.
If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still.
Other factors are trace width, total printed circuit copper
area, copper thickness, single− or double−sided, multilayer
board, the amount of solder on the board or even color of the
traces.
The size, quantity and spacing of other components on the
board can also influence its effectiveness to dissipate the heat.
Figure 26. Inverting Buck−Boost Develops −12 V
D1
1N5822
L1
68 mH
Output
2
4
Feedback
12 to 40 V
Unregulated
DC Input
C
in
100 mF
1
53ON/OFFGN
D
+V
in
−12 V @ 0.7 A
Regulated
Output
C
out
2200 mF
LM2576−12
ADDITIONAL APPLICATIONS
Inverting Regulator
An inverting buck−boost regulator using the LM2576−12
is shown in Figure 26. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative
output voltage. By grounding the feedback pin, the regulator
senses the inverted output voltage and regulates it.
In this example the LM2576−12 is used to generate a
−12 V output. The maximum input voltage in this case
cannot exceed +28 V because the maximum voltage
appearing across the regulator is the absolute sum of the
input and output voltages and this must be limited to a
maximum of 40 V.
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LM2576D2T-3.3G

Mfr. #:
Buy LM2576D2T-3.3G
Manufacturer:
ON Semiconductor
Description:
IC REG MULTI CONFG 3.3V D2PAK-5
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