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LM2575T-5.0 数据表(PDF) 19 Page - National Semiconductor (TI) |
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LM2575T-5.0 数据表(HTML) 19 Page - National Semiconductor (TI) |
19 / 27 page Application Hints (Continued) 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 tan- talum can be used in parallel with aluminum electrolytics, with the tantalum making up 10% or 20% of the total capaci- tance. 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 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 (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 supply. 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 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 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 inductance 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 LM1575 or 125˚C for the LM2575). For a safe, conser- vative design, a temperature approximately 15˚C cooler than the maximum temperature should be selected. 4. LM2575 package thermal resistances θ JA and θ JC. Total power dissipated by the LM2575 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). 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. www.national.com 19 |
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