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ADP3050ARZ-5 Arkusz danych(PDF) 11 Page - Analog Devices |
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ADP3050ARZ-5 Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 20 page Data Sheet ADP3050 Rev. C | Page 11 of 20 APPLICATIONS INFORMATION ADIsimPower DESIGN TOOL The ADP3050 is supported by the ADIsimPower design tool set. ADIsimPower is a collection of tools that produce complete power designs optimized for a specific design goal. The tools enable the user to generate a full schematic, bill of materials, and calculate performance in minutes. ADIsimPower can optimize designs for cost, area, efficiency, and parts count while taking into considera- tion the operating conditions and limitations of the IC and all real external components. For more information about ADIsimPower design tools, refer to www.analog.com/ADIsimPower. The tool set is available from this website, and users can request an unpopulated board through the tool. The complete process for designing a step-down switching regulator using the ADP3050 is provided in the following sections. Each section includes a list of recommended devices. These lists do not include every available device or manufacturer. They contain only surface-mount devices. Equivalent through- hole devices can be substituted if needed. In choosing components, keep in mind what is most important to the design, for example, efficiency, cost, and size. These ultimately determine which compo- nents are used. It is also important to ensure that the design specifications are clearly defined and reflect the worst-case conditions. Key specifications include the minimum and maximum input voltage, the output voltage and ripple, and the minimum and maximum load current. INDUCTOR SELECTION The inductor value determines the mode of operation for the regulator: continuous mode, where the inductor current flows continuously; or discontinuous mode, where the inductor current reduces to zero during every switch cycle. Continuous mode is the best choice for many applications. It provides higher output power, lower peak currents in the switch, inductor, and diode, and a lower inductor ripple current, which means lower output ripple voltage. Discontinuous mode allows the use of smaller magnetics, but at a price: lower available load current and higher peak and ripple currents. Designs with a high input voltage or a low load current often operate in discontinuous mode to minimize inductor value and size. The ADP3050 is designed to work well in both modes of operation. Continuous Mode The inductor current in a continuous mode system is a triangular waveform (equal to the ripple current) centered around a dc value (equal to the load current). The amount of ripple current is determined by the inductor value, and is usually between 20% and 40% of the maximum load current. To reduce the inductor size, ripple currents between 40% and 80% are often used in continuous mode designs with a high input voltage or a low output current. The inductor value is calculated using the following equation: ) ( ) ( 1 MAX IN OUT SW RIPPLE OUT MAX IN V V f I V V L × × − = (2) Where VIN(MAX) is the maximum input voltage, VOUT is the regulated output voltage, and fSW is the switching frequency (200 kHz). The initial choice for the amount of ripple current may seem arbitrary, but it serves as a good starting point for finding a standard off-the-shelf inductor value, such as 10 μH, 15 μH, 22 μH, 33 μH, and 47 μH. If a specific inductance value is to be used, simply rearrange Equation 2 to find the ripple current. For an 800 mA, 12 V to 5 V system, and a ripple current of 320 mA (40% of 800 mA) is chosen, the inductance is μH 45.5 12 5 10 200 1 0.32 5 12 3 = × × × − = L A 47 μH inductor is the closest standard value that gives a ripple current of about 310 mA. The peak switch current is equal to the load current plus one-half the ripple current (this is also the peak current for the inductor and the catch diode). A 95 . 0 155 . 0 8 . 0 2 1 ) ( ) ( = + = + = RIPPLE MAX OUT PK SW I I I (3) Pick an inductor with a dc (or saturation) current rating about 20% larger than ISW(PK) to ensure that the inductor is not running near the edge of saturation. For this example, 1.20 × 0.95 A = 1.14 A, use an inductor with a dc current rating of at least 1.2 A. The maxi- mum switch current is internally limited to 1.5 A, and this limit, along with the ripple current, determines the maximum load current the system can provide. If the load current decreases to below one-half the ripple current, the regulator operates in discontinuous mode. Discontinuous Mode For load currents less than approximately 0.5 A, discontinuous mode operation can be used. This allows the use of a smaller inductor, but the ripple current is much higher (which means a higher output ripple voltage). If a larger output capacitor must be used to reduce the output ripple voltage, the overall system may take up more board area than if a larger inductor is used. The operation and equations for the two modes are quite different, but the boundary between these two modes occurs when the ripple current is equal to twice the load current (when IRIPPLE = 2 × IOUT). From this, Equation 2 is used to find the minimum inductor value needed to keep the system in continuous mode operation (solve for the inductor value with IRIPPLE = 2 × IOUT). ) ( ) ( 1 2 MAX IN OUT SW OUT OUT MAX IN DIS V V f I V V L × × × − = (4) Using an inductor below this value causes the system to operate in discontinuous mode. |
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