LM2576, LM2576HV
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SNVS107C –JUNE 1999–REVISED APRIL 2013
The voltage spikes are present because of the 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 Figure 33) to further
reduce the amount of output ripple and transients. A 10 × 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 kΩ 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 +VIN 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 connections must be low-impedance (see Figure 21 and
Figure 22). For the 5-lead TO-220 and DDPAK/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
temperature approximately 15°C cooler than the maximum temperatures should be selected.
4. LM2576 package thermal resistances θJA and θJC.
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 TYPICAL PERFORMANCE CHARACTERISTICS shown
previously,
•
•
VIN is the applied minimum input voltage, VO is the regulated output voltage,
and ILOAD is the load current.
(3)
(4)
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:
ΔTJ = (PD) (θJA)
To arrive at the actual operating junction temperature, add the junction temperature rise to the maximum ambient
temperature.
TJ = ΔTJ + TA
(5)
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.
Copyright © 1999–2013, Texas Instruments Incorporated
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