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TC115301ECT Datasheet(PDF) 7 Page - Microchip Technology |
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TC115301ECT Datasheet(HTML) 7 Page - Microchip Technology |
7 / 14 page 2003 Microchip Technology Inc. DS21361C-page 7 TC115 Rewriting in terms of input and output currents and volt- ages: EQUATION 5-5: Solving for input current: EQUATION 5-6: The sawtooth current is centered on the DC current level, swinging equally above and below the DC current calculated in Equation 5-6. The peak inductor current is the sum of the DC current plus half the ac current. Note that minimum input voltage should be used when calculating the ac inductor current (Equation 5-9). EQUATION 5-7: EQUATION 5-8: EQUATION 5-9: Combining the DC current calculated in Equation 5-6, with half the peak ac current calculated in Equation 5-9, the peak inductor current is given by: EQUATION 5-10: 5.3 Internal Transistor Switch The LX pin has a typical ON resistance of 1.4 Ω. Therefore, peak switch current is given by (VIN/1.4). The internal transistor switch has a maximum design rating of 350 mA. An oscillator frequency doubling cir- cuit is an included guard against high switching cur- rents. Should the voltage on the LX pin rise above 1.3V (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 PWM remains in operation. Therefore, the LX input is not internally current-limited and care must be taken never to exceed the 350 mA maximum limit. Failure to observe this will result in damage to the regulator. 5.4 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). Therefore, 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 5-1 lists suggested components and suppliers. 5.5 Board Layout Guidelines As with all inductive switching regulators, the TC115 generates fast switching waveforms which 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. TABLE 5-1: SUGGESTED COMPONENTS AND SUPPLIERS VIN MIN () IIN MAX () VOUT MAX () IOUT MA X () Efficiency --------------------------------------------------- = IIN MA X VOUT MAX () IOUT MAX () Efficiency () VIN MIN () ---------------------------------------------------- = VL Ldi dt ⁄ () = di VL di dt ⁄ () = di VIN MIN VSW – ()tON L ---------------------------------------------- = Where: VSW = The voltage drop across the internal N-channel MOSFET. IPK IIN MA X 0.5 di () + = Type Inductors Capacitors Diodes Surface Mount Sumida® CD54 Series CDR125 Series Coiltronics™ CTX Series Matsuo® 267 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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