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LM2595
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 LM2595 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 LM2595. Different inductor types have
different saturation characteristics, and this should be kept
in mind when selecting an inductor.
0.05 A
Inductor
Current
Waveform
0A
0.05 A
Power
Switch
Current
Waveform
0A
HORIZONTAL TIME BASE: 2.0 m s/DIV
Figure 21. Discontinuous Mode Switching Current
Waveforms
GENERAL RECOMMENDATIONS
Output
Voltage
Output Voltage Ripple and Transients
Source of the Output Ripple
Since the LM2595 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 22). 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.
Voltage spikes
caused by
switching action
Filtered of the output
switch and the
parasitic
inductance of the
output capacitor
Unfiltered
Output
Voltage
HORIZONTAL TIME BASE: 5.0 m s/DIV
Figure 22. 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 (3 m H,
100 m F), that can be added to the output (see Figure 31) 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 22 shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 31.
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.
The Surface Mount Package D2PAK and its
Heatsinking
The other type of package, the surface mount D2PAK, 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
100 mm 2 ) and ideally should have 2 or more square inches
(1300 mm 2 ) of 0.0028 inch copper. Additional increasing of
copper area beyond approximately 3.0 in 2 (2000 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.
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.
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