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MCP1640T-ICHY Datasheet(PDF) 16 Page - Microchip Technology |
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MCP1640T-ICHY Datasheet(HTML) 16 Page - Microchip Technology |
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16 / 32 page  MCP1640/B/C/D DS22234A-page 16 2010 Microchip Technology Inc. 5.5 Inductor Selection The MCP1640/B/C/D is designed to be used with small surface mount inductors; the inductance value can range from 2.2 µH to 10 µH. An inductance value of 4.7 µH is recommended to achieve a good balance between inductor size, converter load transient response and minimized noise. Several parameters are used to select the correct inductor: maximum rated current, saturation current and copper resistance (ESR). For boost converters, the inductor current can be much higher than the output current. The lower the inductor ESR, the higher the efficiency of the converter, a common trade-off in size versus efficiency. Peak current is the maximum or limit, and saturation current typically specifies a point at which the induc- tance has rolled off a percentage of the rated value. This can range from a 20% to 40% reduction in induc- tance. As inductance rolls off, the inductor ripple cur- rent increases as does the peak switch current. It is important to keep the inductance from rolling off too much, causing switch current to reach the peak limit. 5.6 Thermal Calculations The MCP1640/B/C/D is available in two different packages (SOT23-6 and 2x3 DFN8). By calculating the power dissipation and applying the package thermal resistance, ( JA), the junction temperature is esti- mated. The maximum continuous junction temperature rating for the MCP1640/B/C/D is +125oC. To quickly estimate the internal power dissipation for the switching boost regulator, an empirical calculation using measured efficiency can be used. Given the measured efficiency, the internal power dissipation is estimated by Equation 5-3. EQUATION 5-3: The difference between the first term, input power, and the second term, power delivered, is the internal MCP1640/B/C/D power dissipation. This is an estimate assuming that most of the power lost is internal to the MCP1640/B/C/D and not CIN, COUT and the inductor. There is some percentage of power lost in the boost inductor, with very little loss in the input and output capacitors. For a more accurate estimation of internal power dissipation, subtract the IINRMS 2*L ESR power dissipation. 5.7 PCB Layout Information Good printed circuit board layout techniques are important to any switching circuitry and switching power supplies are no different. When wiring the switching high current paths, short and wide traces should be used. Therefore it is important that the input and output capacitors be placed as close as possible to the MCP1640/B/C/D to minimize the loop area. The feedback resistors and feedback signal should be routed away from the switching node and the switching current loop. When possible, ground planes and traces should be used to help shield the feedback signal and minimize noise and magnetic interference. TABLE 5-2: MCP1640/B/C/D RECOMMENDED INDUCTORS Part Number Value (µH) DCR (typ) ISAT (A) Size WxLxH (mm) Coiltronics® SD3110 4.7 0.285 0.68 3.1x3.1x1.0 SD3112 4.7 0.246 0.80 3.1x3.1x1.2 SD3114 4.7 0.251 1.14 3.1x3.1x1.4 SD3118 4.7 0.162 1.31 3.8x3.8x1.2 SD3812 4.7 0.256 1.13 3.8x3.8x1.2 SD25 4.7 0.0467 1.83 5.0x5.0x2.5 Part Number Value (µH) DCR (max) ISAT (A) Size WxLxH (mm) Wurth Elektronik® WE-TPC Type TH 4.7 0.200 0.8 2.8x2.8x1.35 WE-TPC Type S 4.7 0.105 0.90 3.8x3.8x1.65 WE-TPC Type M 4.7 0.082 1.65 4.8x4.8x1.8 WE-TPC Type X 4.7 0.046 2.00 6.8x6.8x2.3 Part Number Value (µH) DCR (max) ISAT (A) Size WxLxH (mm) Sumida® CMH23 4.7 0.537 0.70 2.3x2.3x1.0 CMD4D06 4.7 0.216 0.75 3.5x4.3x0.8 CDRH4D 4.7 0.09 0.800 4.6x4.6x1.5 EPCOS® B82462A2 472M000 4.7 0.084 2.00 6.0x6.0x2.5 B82462G4 472M 4.7 0.04 1.8 6.3x6.3x3.0 V OUT I OUT Efficiency ------------------------------- V OUT I OUT – P Dis = |
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