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LM2574 Datasheet(PDF) 20 Page - Texas Instruments

Part No. LM2574
Description  LM2574/LM2574HV SIMPLE SWITCHER™ 0.5A Step-Down Voltage Regulator
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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LM2574 Datasheet(HTML) 20 Page - Texas Instruments

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LM2574, LM2574HV
SNVS104C – JUNE 1999 – REVISED APRIL 2013
www.ti.com
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 LM2574 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 60 Hz diodes
(e.g., 1N4001 or 1N5400, etc.) are also not suitable. See Table 2 for Schottky 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 sawtooth ripple current multiplied by the ESR of the output
capacitor. (See INDUCTOR SELECTION in 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 Figure 36) 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 LM2574 (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 LM2574 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 +VIN without a resistor in series with it. The ON /OFF pin should not be
left open.
GROUNDING
The 8-pin molded PDIP and the 14-pin SOIC package have separate power and signal ground pins. Both ground
pins should be soldered directly to wide printed circuit board copper traces to assure low inductance connections
and good thermal properties.
THERMAL CONSIDERATIONS
The 8-pin PDIP (P) package and the 14-pin SOIC (NPA) package are molded plastic packages with solid copper
lead frames. The copper lead frame conducts the majority of the heat from the die, through the leads, to the
printed circuit board copper, which acts as the heat sink. For best thermal performance, wide copper traces
should be used, and all ground and unused pins should be soldered to generous amounts of printed circuit board
copper, such as a ground plane. Large areas of copper provide the best transfer of heat (lower thermal
resistance) to the surrounding air, and even double-sided or multilayer boards provide better heat paths to the
surrounding air. Unless the power levels are small, using a socket for the 8-pin package is not recommended
because of the additional thermal resistance it introduces, and the resultant higher junction temperature.
20
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Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links: LM2574 LM2574HV


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