0.5A Step-Down Switching Regulator
TC2574
these transients, all these contribute to the amplitude of
these spikes. To minimize these voltage spikes, low induc-
tance 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.
within the core, it generates less EMI, reducing noise prob-
lems in sensitive circuits. The least expensive is the bobbin
core type, which consists of wire wound on a ferrite rod core.
ThistypeofinductorgeneratesmoreEMIduetothefactthat
itscoreisopen, andthemagneticfluxisnot containedwithin
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.
Voltage spikes caused by switching action
of the output switch and the parasitic inductance
of the output capacitor
Unfiltered
Output
Voltage
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 TC2574 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
TC2574. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.
Filtered
Output
Voltage
HORIZONTAL TIME BASE: 5.0µsec/DIV
Figure 7. 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
LCfilter(20µH, 100µF), thatcanbeaddedtotheoutput(see
Figure 16) 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 7
shows the difference between filtered and unfiltered output
waveforms of the regulator shown in Figure 15.
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Source of the Output Ripple
The upper waveform is from the normal unfiltered
outputoftheconverter, whilethelowerwaveformshowsthe
output ripple voltage filtered by an additional LC filter.
Since the TC2574 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.
Heatsinking and Thermal Considerations
The TC2574 is available in both 8-Pin PDIP (narrow)
and16-PinSOIC(wide)packages. Whenusedinthetypical
application the copper lead frame conducts the majority of
theheatfromthedie, throughtheleads, totheprintedcircuit
copper. The copper and the board are the heatsink for this
packageandtheotherheatproducingcomponents,suchas
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
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 7). 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
TC2574-1 1/6/00
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