LM2575
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Heatsinking and Thermal Considerations
Source of the Output Ripple
The Through−Hole Package TO−220
Since the LM2575 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.
The LM2575 is available in two packages, a 5−pin
TO−220(T, TV) and a 5−pin surface mount D PAK(D2T).
2
There are many applications that require no heatsink to keep
the LM2575 junction temperature within the allowed
operating range. The TO−220 package can be used without
a heatsink for ambient temperatures up to approximately
50°C (depending on the output voltage and load current).
Higher ambient temperatures require some heatsinking,
either to the printed circuit (PC) board or an external
heatsink.
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 24). 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.
The Surface Mount Package D2PAK and its
Heatsinking
The other type of package, the surface mount D PAK, is
2
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
2
2
package is soldered to should be at least 0.4 in (or 100 mm )
2
and ideally should have 2 or more square inches (1300 mm )
of 0.0028 inch copper. Additional increasing of copper area
2
2
Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor
beyond approximately 3.0 in (2000 mm ) 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.
UNFILTERED
OUTPUT
VOLTAGE
Thermal Analysis and Design
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
VERTICAL
RESOLUTION:
20 mV/DIV
1. P
2. T
3. T
maximum regulator power dissipation in
the application.
maximum ambient temperature in the
application.
maximum allowed junction temperature
(125°C for the LM2575). 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.
D(max)
FILTERED
OUTPUT
VOLTAGE
)
A(max
J(max)
HORIZONTAL TIME BASE: 10 ms/DIV
Figure 24. Output Ripple Voltage Waveforms
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 33) 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 24
shows the difference between filtered and unfiltered output
waveforms of the regulator shown in Figure 33.
4. R
5. R
package thermal resistance junction−case.
package thermal resistance junction−ambient.
qJC
qJA
(Refer to Absolute Maximum Ratings in this data sheet or
and R values).
R
qJC
qJA
The following formula is to calculate the total power
dissipated by the LM2575:
PD = (Vin x IQ) + d x ILoad x Vsat
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.
where d is the duty cycle and for buck converter
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