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TC120303EHA Fiches technique(PDF) 5 Page - Microchip Technology |
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TC120303EHA Fiches technique(HTML) 5 Page - Microchip Technology |
5 / 14 page © 2002 Microchip Technology Inc. DS21365B-page 5 TC120 3.0 DETAILED DESCRIPTION The TC120 can be operated as an integrated step- down regulator (using the internal switching transistor); or as a step-down regulator controller (using an external switching transistor). When operating as an integrated regulator, the only required external compo- nents are a Schottky diode, inductor and an output capacitor. Operating in this configuration, the TC120 is capable of supporting output load currents to a maximum of 600mA with operating efficiencies above 85%. Efficiencies at high loads can be further improved by using the on-board charge pump circuit to pull the gate of the internal switching transistor below ground for the lowest possible ON resistance. (For more infor- mation, see Section 3.5 Improving High Load Efficiency in Regulator Operating Mode). Higher output currents are achieved by operating the TC120 with an external P-channel switching transistor (controller mode). In this operating configuration, the maximum output current is determined primarily by the ON resistance of the P-channel switch and the series resistance of the inductor. FIGURE 3-1: TEST CIRCUIT 3.1 Inductor Selection Selecting the proper inductor value is a trade-off between physical size and power conversion require- ments. Lower value inductors cost less, but result in higher ripple current and core losses. They are also more prone to saturate since the coil current ramps faster and could overshoot the desired peak value. This not only reduces efficiency, but could also cause the current rating of the external components to be exceeded. Larger inductor values reduce both ripple current and core losses, but are larger in physical size and tend to increase the start-up time slightly. A 22 µH inductor is the best overall compromise and is recom- mended for use with the TC120. For highest efficiency, use inductors with a low DC resistance (less than 20m Ω). To minimize radiated noise, consider using a toroid, pot core or shielded-bobbin inductor. 3.2 Input Bypass Capacitor Using an input bypass capacitor reduces peak current transients drawn from the input supply, and reduces the switching noise generated by the regulator. The source impedance of the input supply determines the size of the capacitor that should be used. 3.3 Output Capacitor The effective series resistance of the output capacitor directly affects the amplitude of the output voltage ripple. (The product of the peak inductor current and the ESR determines output ripple amplitude.) There- fore, a capacitor with the lowest possible ESR should be selected. Smaller capacitors are acceptable for light loads or in applications where ripple is not a concern. A 47 µF Tantalum capacitor is recommended for most applications. The Sprague 595D series of tantalum capacitors are amongst the smallest of all low ESR surface mount capacitors available. Table 3-1 lists suggested components and suppliers. 3.4 Catch Diode The high operating frequency of the TC120 requires a high-speed diode. Schottky diodes such as the MA737 or 1N5817 through 1N5823 (and the equivalent surface mount versions) are recommended. Select a diode whose average current rating is greater than the peak inductor current; and whose voltage rating is higher than VINMAX. 3.5 Improving High Load Efficiency in Regulator Operating Mode If the TC120 is operated at high output loads most (or all) of the time, efficiency can be improved with the addition of two components. Ordinarily, the voltage swing on the gate of the internal P-channel transistor is from ground to VIN. By adding a capacitor and diode as shown in Figure 3-2, an inverting charge pump is formed, enabling the internal gate voltage to swing from a negative voltage to +VIN. This increased drive lowers the RDSON of the internal transistor, improving efficiency at high output currents. Care must be taken to ensure the voltage measured between VIN and CPC does not exceed an absolute value of 10V. While this is not a problem at values of VIN at (or below) 5V, higher VIN values will require the addition of a clamping mechanism (such as a Zener diode) to limit the voltage as described. While this technique improves efficiency at high output loads, it is at the expense of low load efficiency because energy is expended charging and discharging the charge pump capacitor. This technique is therefore not recommended for applications that operate the TC120 at low output currents for extended time periods. If unused, CPC must be grounded. TC120XX03 VIN EXTW CPC SHDN/SS LX EXT GND SENSE 47 µF/10V Tantalum CSS 4.7nF IN5817 L1 22 µH VOUT VIN COUT 47 µF/10V Tantalum + – + – |
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Description similaire - TC120303EHA |
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