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LM2574N-015

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LM2574, NCV2574
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16
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 have 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 LM2574 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 24 and Figure 25). 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 LM2574 regulator was added to this
data sheet (Figures 19 through 23). 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 0.2 A) 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.
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. There are many different styles
of inductors available, 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.
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 LM2574 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 LM2574. Different inductor types have
different saturation characteristics, and this should be kept
in mind when selecting an inductor.
LM2574, NCV2574
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17
HORIZONTAL TIME BASE: 5.0 ms/DIV
VERTRICAL RESOLUTION 200 mADV
Figure 24. Continuous Mode Switching
Current Waveforms
0.5 A
0 A
0.5 A
0 A
Power
Switch
Current
Waveform
Inductor
Current
Waveform
VERTICAL RESOLUTION 100 mADV
HORIZONTAL TIME BASE: 5.0 ms/DIV
Figure 25. Discontinuous Mode Switching
Current Waveforms
0.1 A
0 A
0.1 A
0 A
Power
Switch
Current
Waveform
Inductor
Current
Waveform
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Source of the Output Ripple
Since the LM2574 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 26). 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.
HORIZONTAL TIME BASE: 5.0 ms/DIV
VERTRICAL RESOLUTION 20 mV/DIV
Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor
Figure 26. Output Ripple Voltage Waveforms
Unfiltered
Output
Voltage
Filtered
Output
Voltage
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 35) 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 26
shows the difference between filtered and unfiltered output
waveforms of the regulator shown in Figure 34.
The upper waveform is from the normal unfiltered output
of the converter, while the lower waveform shows the output
ripple voltage filtered by an additional LC filter.
Heatsinking and Thermal Considerations
The LM2574 is available in both 8−pin DIP and SO−16L
packages. When used in the typical application the copper lead
frame conducts the majority of the heat from the die, through
the leads, to the printed circuit copper. The copper and the
board are the heatsink for this package and the other heat
producing components, such as the catch diode and inductor.
For the best thermal performance, wide copper traces
should be used and all ground and unused pins should be
soldered to generous amounts of printed circuit board
copper, such as a ground plane. Large areas of copper
provide the best transfer of heat to the surrounding air. One
exception to this is the output (switch) pin, which should not
have large areas of copper in order to minimize coupling to
sensitive circuitry.
Additional improvement in heat dissipation can be
achieved even by using of double sided or multilayer boards
which can provide even better heat path to the ambient.
Using a socket for the 8−pin DIP package is not
recommended because socket represents an additional
thermal resistance, and as a result the junction temperature
will be higher.
LM2574, NCV2574
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18
Since the current rating of the LM2574 is only 0.5 A, the
total package power dissipation for this switcher is quite
low, ranging from approximately 0.1 W up to 0.75 W under
varying conditions. In a carefully engineered printed circuit
board, the through−hole DIP package can easily dissipate up
to 0.75 W, even at ambient temperatures of 60°C, and still
keep the maximum junction temperature below 125°C.
Thermal Analysis and Design
The following procedure must be performed to determine
the operating junction temperature. 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 LM2574). 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
q
JC
− package thermal resistance junction−case.
5. R
q
JA
− package thermal resistance junction−ambient.
(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R
q
JC
and R
q
JA
values).
The following formula is to calculate the approximate
total power dissipated by the LM2574:
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
LM2574 data sheet,
V
in
is minimum input voltage applied,
V
O
is the regulator output voltage,
I
Load
is the load current.
D1
MBR150
L1
68 mH
Output
7
1
Feedback
8.0 to 25 V
Unregulated
DC Input
C
in
22 mF
5
34ON
/OFFPwr
Gnd
+V
in
−12 V @ 100 mA
Regulated
Output
C
out
680 mF
LM2574−12
2 Sig
Gnd
Figure 27. Inverting Buck−Boost Develops −12 V
(6)
(12)
(4)
(5)
(3)
(14)
The dynamic switching losses during turn−on and
turn−off can be neglected if a proper type catch diode is used.
The junction temperature can be determined by the
following expression:
T
J
= (R
q
JA
)(P
D
) + T
A
where (R
q
JA
)(P
D
) represents the junction temperature rise
caused by the dissipated power and T
A
is the maximum
ambient temperature.
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. At higher power levels the
thermal resistance decreases due to the increased air current
activity.
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. Some of them, like the catch diode or the inductor will
generate some additional heat.
ADDITIONAL APPLICATIONS
Inverting Regulator
An inverting buck−boost regulator using the LM2574−12
is shown in Figure 27. 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 LM2574−12 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.
This circuit configuration is able to deliver approximately
0.1 A to the output when the input voltage is 8.0 V or higher.
At lighter loads the minimum input voltage required drops
to approximately 4.7 V, because the buck−boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck−boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.
This type of buck−boost inverting regulator can also
require a larger amount of startup input current, even for
light loads. This may overload an input power source with
a current limit less than 0.6 A.
Because of the relatively high startup currents required by
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
While using a delayed startup arrangement, the input
capacitor can charge up to a higher voltage before the
switch−mode regulator begins to operate.
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LM2574N-015

Mfr. #:
Buy LM2574N-015
Manufacturer:
ON Semiconductor
Description:
IC REG MULT CONFG 15V 500MA 8DIP
Lifecycle:
New from this manufacturer.
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