be used in parallel with aluminum electrolytics, with the tan-
talum making up 10% or 20% of the total capacitance.
With the N or M packages, all the pins labeled ground, power
ground, or signal ground should be soldered directly to wide
printed circuit board copper traces. This assures both low in-
ductance connections and good thermal properties.
The capacitor's ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
HEAT SINK/THERMAL CONSIDERATIONS
CATCH DIODE
In many cases, no heat sink is required to keep the LM2575
junction temperature within the allowed operating range. For
each application, to determine whether or not a heat sink will
be required, the following must be identified:
Buck regulators require a diode to provide a return path for
the inductor current when the switch is off. This diode should
be located close to the LM2575 using short leads and short
printed circuit traces.
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
Because of their fast switching speed and low forward voltage
drop, Schottky diodes provide the best efficiency, especially
in low output voltage switching regulators (less than 5V). Fast-
Recovery, High-Efficiency, or Ultra-Fast Recovery diodes are
also suitable, but some types with an abrupt turn-off charac-
teristic may cause instability and EMI problems. A fast-recov-
ery diode with soft recovery characteristics is a better choice.
Standard 60 Hz diodes (e.g., 1N4001 or 1N5400, etc.) are
also not suitable. See Figure 8 for Schottky and “soft” fast-
recovery diode selection guide.
3. Maximum allowed junction temperature (150°C for the
LM1575 or 125°C for the LM2575). For a safe,
conservative design, a temperature approximately 15°C
cooler than the maximum temperature should be
selected.
4. LM2575 package thermal resistances θJA and θJC
.
Total power dissipated by the LM2575 can be estimated as
follows:
PD = (VIN) (IQ) + (VO/VIN) (ILOAD) (VSAT
)
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
where IQ (quiescent current) and VSAT can be found in the
Characteristic Curves shown previously, VIN is the applied
minimum input voltage, VO is the regulated output voltage,
and ILOAD is the load current. The dynamic losses during turn-
on and turn-off are negligible if a Schottky type catch diode is
used.
The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.
The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output ca-
pacitor. (See the inductor selection in the application hints.)
When no heat sink is used, the junction temperature rise can
be determined by the following:
The voltage spikes are present because of the fast switching
action of the output switch, and the parasitic inductance of the
output filter capacitor. To minimize these voltage spikes, spe-
cial low inductance capacitors can be used, and their lead
lengths must be kept short. Wiring inductance, stray capaci-
tance, as well as the scope probe used to evaluate these
transients, all contribute to the amplitude of these spikes.
ΔTJ = (PD) (θJA
)
To arrive at the actual operating junction temperature, add the
junction temperature rise to the maximum ambient tempera-
ture.
TJ = ΔTJ + TA
If the actual operating junction temperature is greater than the
selected safe operating junction temperature determined in
step 3, then a heat sink is required.
An additional small LC filter (20 μH & 100 μF) can be added
to the output (as shown in Figure 15) to further reduce the
amount of output ripple and transients. A 10 × reduction in
output ripple voltage and transients is possible with this filter.
When using a heat sink, the junction temperature rise can be
determined by the following:
FEEDBACK CONNECTION
ΔTJ = (PD) (θJC + θinterface + θHeat sink
The operating junction temperature will be:
TJ = TA + ΔTJ
)
The LM2575 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power sup-
ply. When using the adjustable version, physically locate both
output voltage programming resistors near the LM2575 to
avoid picking up unwanted noise. Avoid using resistors
greater than 100 kΩ because of the increased chance of noise
pickup.
As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).
ON /OFF INPUT
When using the LM2575 in the plastic DIP (N) or surface
mount (M) packages, several items about the thermal prop-
erties of the packages should be understood. The majority of
the heat is conducted out of the package through the leads,
with a minor portion through the plastic parts of the package.
Since the lead frame is solid copper, heat from the die is
readily conducted through the leads to the printed circuit
board copper, which is acting as a heat sink.
For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +VIN without a resistor in series with it. The
ON /OFF pin should not be left open.
GROUNDING
For best thermal performance, the ground pins and all the
unconnected 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 sur-
rounding air. Copper on both sides of the board is also helpful
in getting the heat away from the package, even if there is no
direct copper contact between the two sides. Thermal resis-
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see Figure 2). For the TO-3
style package, the case is ground. For the 5-lead TO-220 style
package, both the tab and pin 3 are ground and either con-
nection may be used, as they are both part of the same copper
lead frame.
19
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