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TC126503ECT 데이터시트(PDF) 5 Page - Microchip Technology |
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TC126503ECT 데이터시트(HTML) 5 Page - Microchip Technology |
5 / 12 page © 2002 Microchip Technology Inc. DS21372B-page 5 TC125/TC126 4.0 APPLICATIONS 4.1 Input Bypass Capacitors Adding 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. 4.2 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 to a higher value. 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. The recommended inductor value for use with the TC125/ 126 is 100 µH. Inductors with a ferrite core (or equiva- lent) are recommended. For highest efficiency, use an inductor with a series resistance less than 20m Ω. 4.3 Internal Transistor Switch Current Limiting The peak switch current is equal to the input voltage divided by the RDSON of the internal switch. The internal transistor has absolute maximum current rating of 400mA with a design limit of 350mA. A built-in oscillator frequency doubling circuit guards against high switching currents. Should the voltage on the LX pin rise above 1.1V, max while the internal N-channel switch is ON, the oscillator frequency automatically doubles to minimize ON time. Although reduced, switch current still flows because the regulator remains in operation. Therefore, the LX input is not internally current limited and care must be taken never to exceed the 350mA maximum limit. Failure to observe this will result in damage to the regulator. 4.4 Output Diode For best results, use a Schottky diode such as the MA735, 1N5817, MBR0520L or equivalent. Connect the diode between the PS and LX pins (TC125) or SENSE and LX pins (TC126) as close to the IC as possible. (Do not use ordinary rectifier diodes since the higher threshold voltages reduce efficiency.) 4.5 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. The Sprague 595D series of tantalum capacitors are among the smallest of all low ESR surface mount capacitors available. Table 4-1 lists suggested components and suppliers. 4.6 Board Layout Guidelines As with all inductive switching regulators, the TC125/ 126 generate fast switching waveforms that radiate noise. Interconnecting lead lengths should be minimized to keep stray capacitance, trace resistance, and radiated noise as low as possible. In addition, the GND pin, input bypass capacitor, and output filter capacitor ground leads should be connected to a single point. The input capacitor should be placed as close to power and ground pins of the TC125/126 as possible. TABLE 4-1: SUGGESTED COMPONENTS AND SUPPLIERS Type Inductors Capacitors Diodes Surface Mount Sumida CD54 Series CDR125 Series Coiltronics CTX Series Murata LQN6C Series Matsuo 267 Series Murata GRM200 Series Sprague 595D Series Nichicon F93 Series Nihon EC10 Series Matsushita MA735 Series Through-Hole Sumida RCH855 Series RCH110 Series Renco RL1284-12 Sanyo OS-CON Series Nichicon PL Series ON Semiconductor 1N5817 - 1N5822 |
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