TC2575 Datasheet, PDF (15/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
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 TC2575 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
TC2575. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.
GENERAL RECOMMENDATIONS
Output Voltage Ripple and Transients
Source of the Output Ripple
Since the TC2575 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 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
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.
Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor
HORIZONTAL TIME BASE: 10µsec/DIV
UNFILITERED
OUTPUT
VOLTAGE
VERTICAL
RESOLUTION:
20mV/DIV
FILITERED
OUTPUT
VOLTAGE
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
LC filter (20µH, 100µF), that can be added to the output
(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 16.
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 Through-Hole-Package TO-220
The TC2575 is available in a 5-Pin TO-220 package.
There are many applications that require no heatsink to
keep the TC2575 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 heat sinking,
either to the printed circuit (PC) board or an external heatsink.
TC2575-1 3/13/00
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