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TC110333ECT bảng dữ liệu(PDF) 4 Page - Microchip Technology |
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TC110333ECT bảng dữ liệu(HTML) 4 Page - Microchip Technology |
4 / 16 page TC110 DS21355B-page 4 © 2002 Microchip Technology Inc. 3.0 DETAILED DESCRIPTION The TC110 is a PFM/PWM step-up DC/DC controller for use in systems operating from two or more cells, or in low voltage, line-powered applications. It uses PWM as the primary modulation scheme, but automatically converts to PFM at output duty cycles less than approximately 25%. The conversion to PFM provides reduced supply current, and therefore higher operating efficiency at low loads. The TC110 uses an external switching transistor, allowing construction of switching regulators with maximum output currents of 300mA. The TC110 consumes only 70 µA, typical, of supply current 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 normal operation resumes when SHDN is brought high. Other features include start-up at VIN = 0.9V and an externally programmable soft start time. 3.1 Operating Mode The TC110 is powered by the voltage present on the VDD input. The applications circuits of Figure 3-1 and Figure 3-2 show operation in the bootstrapped and non-bootstrapped modes. In bootstrapped mode, the TC110 is powered from the output (start-up voltage is supplied by VIN through the inductor and Schottky diode while Q1 is off). In bootstrapped mode, the switching transistor is turned on harder because its gate voltage is higher (due to the boost action of the regulator), resulting in higher output current capacity. The TC110 is powered from the input supply in the non- bootstrapped mode. In this mode, the supply current to the TC110 is minimized. However, the drive applied to the gate of the switching transistor swings from the input supply level to ground, so the transistor’s ON resistance increases at low input voltages. Overall efficiency is increased since supply current is reduced, and less energy is consumed charging and discharging the gate of the MOSFET. While the TC110 is guaran- teed to start up at 0.9V the device performs to specifications at 2.0V and higher. 3.2 Low Power Shutdown Mode The TC110 enters a low power shutdown mode when SHDN is brought low. While in shutdown, the oscillator is disabled and the output switch (internal or external) is shut off. Normal regulator operation resumes when SHDN is brought high. SHDN maybetiedtothe input supply if not used. Note: Because the TC110 uses an external diode, a leakage path between the input voltage and the output node (through the inductor and diode) exists while the regulator is in shutdown. Care must be taken in system design to assure the input supply is isolated from the load during shutdown. 3.3 Soft Start Soft start allows the output voltage to gradually ramp from 0V to rated output value during start-up. This action minimizes (or eliminates) overshoot, and in general, reduces stress on circuit components. Figure 3-3 shows the circuit required to implement soft start (values of 470K and 0.1 µFfor RSS and CSS , respectively, are adequate for most applications). 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. |
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