LM2576SX-5.0 View Datasheet(PDF) - Unspecified
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LM2576SX-5.0 Datasheet PDF : 23 Pages
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LM2576/LM2576HV
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 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
characteristic may cause instability and EMI problems, A
fast-recovery diode with soft recovery characteristics is a better
choice. Standard 60Hz diodes (e.g., 1N4001 or 1N5400, etc.)
are also not suitable. See Figure 8 for Schottky and “softâ€
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 sawtooth
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 (20μH&100 μF) can be added to
the output (as shown in Figure 15) to further reduce the amount
of output ripple and transients. A 10X 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
100kΩ because of the increased chance of noise pickup.
/OFF INPUT
For normal operation, the /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 /OFF pin can be safely pulled up
to +VIN without a resistor in series with it. The /OFF pin
should not be left open.
GROUNDING
To maintain output voltage stability, the power ground
connections 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 ℃ for the
LM2576). For a safe, conservative design, a temperature
approximately 15 ℃ cooler than the maximum
temperatures should be selected.
4. Lm2576 package thermal resistances JA and JC .
Total power dissipated by the LM2576 can be estimated as
follows:
PD= (VIN)(IQ)+(VO/VIN)(ILOAD)(VSAT)
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.
When no heat sink is used, the junction temperature rise can be
determined by the following:
â–³T J =(PD) ( JA)
To arrive at the actual operating junction temperature, add the
junction temperature rise to the maximum ambient temperature.
T J =â–³TJ+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:
â–³TJ=(PD) ( JC + interface + Heat sink)
The operating junction temperature will be:
TJ=TA +â–³TJ
As above, if the actual operating junction temperature is greater
than the selected safe operating junction temperature, 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 calculate
the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operating
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 700mA. At
lighter loads, the minimum input voltage required drops to
approximately 4.7V.
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