LM2574-12 Motorola Inc, LM2574-12 Datasheet - Page 17

• Manufacturer Part Number: LM2574-12
• Description: EASY SWITCHERE 0.5 A STEP-DOWN VOLTAGE REGULATOR
• Manufacturer: Motorola Inc

Output Voltage Ripple and Transients
Source of the Output Ripple
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
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.
Minimizing the Output Ripple
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 H,
100 F), 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 converter, while the lower waveform shows the output
ripple voltage filtered by an additional LC filter.
Heatsinking and Thermal Considerations
used in the typical application the copper lead frame conducts
MOTOROLA ANALOG IC DEVICE DATA
Since the LM2574 is a switch mode power supply
The regulator output voltage may also contain short
Unfiltered
In order to minimize the output ripple voltage it is possible to
The upper waveform is from the normal unfiltered output of
The LM2574 is available in a 8–pin DIP package. When
Voltage
Filtered
Voltage
Output
Output
Figure 26. Output Ripple Voltage Waveforms
Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor
HORIZONTAL TIME BASE: 5.0 s/DIV
GENERAL RECOMMENDATIONS
LM2574
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.
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.
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.
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 operating junction temperature. First determine:
1. P D(max) – maximum regulator power dissipation in the
2. T A(max) – maximum ambient temperature in the
3. T J(max) – maximum allowed junction temperature
4. R JC
5. R JA
(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R JC and R JA values).
power dissipated by the LM2574:
where d is the duty cycle and for buck converter
LM2574 data sheet,
For the best thermal performance, wide copper traces
Since the current rating of the LM2574 is only 0.5 A, the
The following procedure must be performed to determine
The following formula is to calculate the approximate total
I Q (quiescent current) and V sat can be found in the
V in is minimum input voltage applied,
V O is the regulator output voltage,
I Load is the load current.
application.
Additional improvement in heat dissipation can be
– package thermal resistance junction–case.
– package thermal resistance junction–ambient.
P D = (V in x I Q ) + d x I Load x V sat
application.
(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.
d
+
t on
T
+
V
V
O
in
,
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