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TC105303ECT Datasheet(PDF) 4 Page - Microchip Technology |
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TC105303ECT Datasheet(HTML) 4 Page - Microchip Technology |
4 / 14 page TC105 DS21349B-page 4 © 2002 Microchip Technology Inc. 3.0 DETAILED DESCRIPTION The TC105 is a PFM/PWM step-down DC/DC controller for use in systems operating from two or more cells, or in line-powered applications. It uses PWM as the primary modulation scheme, but automat- ically converts to PFM at output duty cycles less than approximately 10%. The conversion to PFM provides reduced supply current, and therefore higher operating efficiency at low loads. The TC105 uses an external switching transistor, allowing construction of switching regulators with output currents of up to 1A. The TC105 consumes only 102 µA, max, of supply current when VIN = 5V and VOUT = 3.3V, and can be placed in a 0.5 µA shutdown mode by bringing the shutdown input (SHDN) low. The regulator remains disabled while in shutdown mode, and output voltage discharges to zero through the load. Normal operation resumes when SHDN is brought high. Other features include a built-in undervoltage lockout (UVLO) and externally programmable soft start time. 3.1 Low Power Shutdown Mode The TC105 enters a low power shutdown mode when SHDN is brought low. While in shutdown, the oscillator is disabled and the output switch is shut off. Normal regulator operation resumes when SHDN is again brought high. SHDN may be tied to the input supply if not used. 3.2 Soft Start Soft start allows the output voltage to gradually ramp from 0 to rated output value during start-up. This action minimizes (or eliminates) overshoot, and in general, reduces stress on circuit components. Figure 4-1 shows the circuit required to implement soft start (values of 470K and 0.033 µFfor RSS and CSS respectively, are adequate for most applications). 3.3 Undervoltage Lockout (UVLO) The TC105 is disabled when VIN is below the under- voltage lockout threshold. This threshold is equal to the guaranteed minimum operating voltage for the TC105 (i.e., 2.2V). When UVLO is active, the TC105 is completely disabled. 3.4 Input Bypass Capacitors 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. 3.5 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. 3.6 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. A22 µH inductor is recommended as the best overall compromise. For highest efficiency, use inductors with a low DC resistance (less than 20 m Ω). To minimize radiated noise, consider using a toroid, pot core or shielded-bobbin inductor. 3.7 Output Diode The high operating frequency of the TC105 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 VDDMAX. |
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