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LM2576T-5.0 Datasheet(PDF) 17 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM2576T-5.0
Description  SIMPLE SWITCHER 3A Step-Down Voltage Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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 17 page
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Application Hints (Continued)
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 LM2576 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
capacitor. (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 LM2576 (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 LM2576
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 Figure 2). For the
5-lead TO-220 and TO-263 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.
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, only a small heat sink is required to keep the
LM2576 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 (125˚C for the
LM2576). For a safe, conservative design, a tempera-
ture approximately 15˚C cooler than the maximum tem-
peratures should be selected.
4.
LM2576 package thermal resistances
θ
JA and
θ
JC.
Total power dissipated by the LM2576 can be estimated as
follows:
P
D =(VIN)(IQ)+(VO/VIN)(ILOAD)(VSAT)
where I
Q (quiescent current) and VSAT 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 =(PD)(
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 +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.
When using a heat sink, the junction temperature rise can be
determined by the following:
∆T
J =(PD)(
θ
JC +
θ
interface +
θ
Heat sink)
The operating junction temperature will be:
T
J =TA +
∆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).
Included on the Switcher Made Simple design software is a
more precise (non-linear) thermal model that can be used to
determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operat-
ing temperature.
Additional Applications
INVERTING REGULATOR
Figure 10 shows a LM2576-12 in a buck-boost configuration
to generate a negative 12V output from a positive input
voltage. This circuit bootstraps the regulator’s ground pin to
the negative output voltage, then by grounding the feedback
pin, the regulator senses the inverted output voltage and
regulates it to −12V.
For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 700 mA.
At lighter loads, the minimum input voltage required drops to
approximately 4.7V.
www.national.com
17




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