LM2575T-5.0 National Semiconductor, LM2575T-5.0 Datasheet - Page 17
![Voltage Regulator IC](/photos/18/98/189891/4243351_sml.jpg)
LM2575T-5.0
Manufacturer Part Number
LM2575T-5.0
Description
Voltage Regulator IC
Manufacturer
National Semiconductor
Specifications of LM2575T-5.0
Input Voltage
40V
Output Current
1A
Output Voltage
5V
No. Of Outputs
1
No. Of Pins
5
Operating Temperature Range
-40°C To +125°C
Termination Type
Through Hole
Mounting Type
Through Hole
Dc
0848
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
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Application Hints
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 tanta-
lum can be used in parallel with aluminum electrolytics, with
the tantalum making up 10% or 20% of the total capacitance.
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 Figure 8 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 ca-
pacitor. (See the inductor selection in the application hints.)
The voltage spikes are present because of the the fast
switching action of the output switch, and the parasitic induc-
tance 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 evalu-
ate 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 Figure 15 ) 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 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 k
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
The ON /OFF pin should not be left open.
because of the increased chance of
IN
without a resistor in series with it.
(Continued)
17
GROUNDING
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 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 in-
ductance 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
4. LM2575 package thermal resistances
Total power dissipated by the LM2575 can be estimated as
follows:
where I
Characteristic Curves shown previously, V
minimum input voltage, V
and I
turn-on and turn-off are negligible if a Schottky type catch di-
ode is used.
When no heat sink is used, the junction temperature rise can
be determined by the following:
To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.
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:
The operating junction temperature will be:
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.
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 best thermal performance, the ground pins and all the
unconnected pins should be soldered to generous amounts
LM1575 or 125˚C for the LM2575). For a safe, conserva-
tive design, a temperature approximately 15˚C cooler
than the maximum temperature should be selected.
LOAD
Q
P
(quiescent current) and V
D
is the load current. The dynamic losses during
T
= (V
J
= (P
IN
) (I
D
) (
Q
T
T
T
) + (V
J
JC
J
J
O
= T
= T
= (P
+
is the regulated output voltage,
O
A
interface
D
/V
J
+ T
) (
+ T
IN
) (I
JA
SAT
A
J
)
LOAD
+
can be found in the
Heat sink
) (V
IN
JA
is the applied
SAT
and
www.national.com
)
)
JC
.