Application Hints
(Continued)
Tantalum capacitors can have a very low ESR, and should
be carefully evaluated if it is the only output capacitor. Be-
cause of their good low temperature characteristics, a tan-
talum can be used in parallel with aluminum electrolytics,
with the tantalum making up 10% or 20% of the total capaci-
tance.
The capacitor’s ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.
CATCH DIODE
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.
Because of their fast switching speed and low forward volt-
age drop, Schottky diodes provide the best efficiency, espe-
cially 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 characteristic may cause instability and EMI prob-
lems. A fast-recovery diode with soft recovery characteristics
is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or
1N5400, etc.) are also
not suitable.
See
for Schot-
tky and “soft” fast-recovery diode selection guide.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
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
capacitor. (See the inductor selection in the application
hints.)
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,
special low inductance capacitors can be used, and their
lead lengths must be kept short. Wiring inductance, stray
capacitance, as well as the scope probe used to evaluate
these transients, all contribute to the amplitude of these
spikes.
An additional small LC filter (20 µH & 100 µF) can be added
to the output (as shown in
) to further reduce the
amount of output ripple and transients. A 10 x reduction in
output ripple voltage and transients is possible with this filter.
FEEDBACK CONNECTION
The LM2575 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power
supply. 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 k
Ω
because of the increased chance of
noise pickup.
ON /OFF INPUT
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 +V
IN
without a resistor in series with it.
The ON /OFF pin should not be left open.
GROUNDING
To maintain output voltage stability, the power ground con-
nections must be low-impedance (see
). 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 connection may be used, as they are both part of the
same copper lead frame.
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
inductance connections and good thermal properties.
HEAT SINK/THERMAL CONSIDERATIONS
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:
1.
Maximum ambient temperature (in the application).
2.
Maximum regulator power dissipation (in application).
3.
Maximum allowed junction temperature (150˚C for the
LM1575 or 125˚C for the LM2575). For a safe, conser-
vative design, a temperature approximately 15˚C cooler
than the maximum temperature should be selected.
4.
LM2575 package thermal resistances
θ
JA
and
θ
JC
.
Total power dissipated by the LM2575 can be estimated as
follows:
P
D
= (V
IN
) (I
Q
) + (V
O
/V
IN
) (I
LOAD
) (V
SAT
)
where I
Q
(quiescent current) and V
SAT
can be found in the
Characteristic Curves shown previously, V
IN
is the applied
minimum input voltage, V
O
is the regulated output voltage,
and I
LOAD
is the load current. The dynamic losses during
turn-on and turn-off are negligible if a Schottky type catch
diode is used.
When no heat sink is used, the junction temperature rise can
be determined by the following:
∆
T
J
= (P
D
) (
θ
JA
)
To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.
T
J
=
∆
T
J
+ T
A
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.
When using a heat sink, the junction temperature rise can be
determined by the following:
∆
T
J
= (P
D
) (
θ
JC
+
θ
interface
+
θ
Heat sink
)
The operating junction temperature will be:
T
J
= T
A
+
∆
T
J
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).
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.
LM1575/LM2575/LM2575HV
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