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LM2575T-5.0 Datasheet(PDF) 18 Page - National Semiconductor (TI) |
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LM2575T-5.0 Datasheet(HTML) 18 Page - National Semiconductor (TI) |
18 / 27 page Application Hints INPUT CAPACITOR (C IN) To maintain stability, the regulator input pin must be by- passed with at least a 47 µF electrolytic capacitor. The capacitor’s leads must be kept short, and located near the regulator. If the operating temperature range includes temperatures below −25˚C, the input capacitor value may need to be larger. With most electrolytic capacitors, the capacitance value decreases and the ESR increases with lower tempera- tures and age. Paralleling a ceramic or solid tantalum ca- pacitor will increase the regulator stability at cold tempera- tures. For maximum capacitor operating lifetime, the capacitor’s RMS ripple current rating should be greater than INDUCTOR SELECTION All switching regulators have two basic modes of operation: continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulator per- formance and requirements. The LM2575 (or any of the Simple Switcher family) can be used for both continuous and discontinuous modes of opera- tion. The inductor value selection guides in Figure 3 through Figure 7 were designed for buck regulator designs of the continuous inductor current type. When using inductor val- ues shown in the inductor selection guide, the peak-to-peak inductor ripple current will be approximately 20% to 30% of the maximum DC current. With relatively heavy load cur- rents, the circuit operates in the continuous mode (inductor current always flowing), but under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a period of time). This discontinuous mode of operation is perfectly acceptable. For light loads (less than approximately 200 mA) it may be desirable to operate the regulator in the discontinuous mode, primarily because of the lower inductor values required for the discon- tinuous mode. The selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value chosen is prohibitively high, the designer should investigate the possibility of discontinuous operation. The computer design software Switchers Made Simple will provide all component values for discontinuous (as well as continuous) mode of operation. Inductors are available in different styles such as pot core, toriod, E-frame, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. The least expensive, the bobbin core type, consists of wire wrapped on a ferrite rod core. This type of construction makes for an inexpensive inductor, but since the magnetic flux is not com- pletely contained within the core, it generates more electro- magnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect scope readings because of induced voltages in the scope probe. The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for Pulse Engineering, and ferrite bobbin core for Renco. An inductor should not be operated beyond its maximum rated current because it may saturate. When an inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance of the winding). This will cause the switch current to rise very rapidly. Different inductor types have different saturation characteristics, and this should be kept in mind when select- ing an inductor. The inductor manufacturer’s data sheets include current and energy limits to avoid inductor saturation. INDUCTOR RIPPLE CURRENT When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage, the peak-to- peak amplitude of this inductor current waveform remains constant. As the load current rises or falls, the entire saw- tooth current waveform also rises or falls. The average DC value of this waveform is equal to the DC load current (in the buck regulator configuration). If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation. Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough. OUTPUT CAPACITOR An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor should be located near the LM2575 using short pc board traces. Standard aluminum electrolytics are usually adequate, but low ESR types are recommended for low output ripple voltage and good stability. The ESR of a capacitor depends on many factors, some which are: the value, the voltage rating, physi- cal size and the type of construction. In general, low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR numbers. The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output ca- pacitor and the amplitude of the inductor ripple current ( ∆I IND). See the section on inductor ripple current in Applica- tion Hints. The lower capacitor values (220 µF–680 µF) will allow typi- cally 50 mV to 150 mV of output ripple voltage, while larger- value capacitors will reduce the ripple to approximately 20 mV to 50 mV. Output Ripple Voltage = ( ∆I IND) (ESR of COUT) To further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a higher-grade capacitor may be used. Such capacitors are often called “high-frequency,” “low-inductance,” or “low-ESR.” These will reduce the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode, reducing the ESR below 0.05 Ω can cause instability in the regulator. www.national.com 18 |
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