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TC1303B-1G0EUN Folha de dados(PDF) 21 Page - Microchip Technology |
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TC1303B-1G0EUN Folha de dados(HTML) 21 Page - Microchip Technology |
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21 / 38 page  © 2008 Microchip Technology Inc. DS21949C-page 21 TC1303A/TC1303B/TC1303C/TC1304 4.0 DETAILED DESCRIPTION 4.1 Device Overview The TC1303/TC1304 combines a 500 mA synchronous buck regulator with a 300 mA LDO and a power-good output. This unique combination provides a small, low-cost solution for applications that require two or more voltage rails. The buck regulator can deliver high-output current over a wide range of input- to-output voltage ratios while maintaining high efficiency. This is typically used for the lower-voltage, high-current processor core. The LDO is a minimal parts-count solution (single-output capacitor), providing a regulated voltage for an auxiliary rail. The typical LDO dropout voltage (137 mV @ 200 mA) allows the use of very low input-to-output LDO differential voltages, minimizing the power loss internal to the LDO pass transistor. A power-good output is provided, indicating that the buck regulator output, the LDO output or both outputs are in regulation. Additional features include independent shutdown inputs (TC1303), UVLO, output voltage sequencing (TC1304), overcurrent and overtemperature shutdown. 4.2 Synchronous Buck Regulator The synchronous buck regulator is capable of supplying a 500 mA continuous output current over a wide range of input and output voltages. The output voltage range is from 0.8V (minimum) to 4.5V (maximum). The regulator operates in three different modes, automatically selecting the most efficient mode of operation. During heavy load conditions, the TC1303/TC1304 buck converter operates at a high, fixed frequency (2.0 MHz) using current mode control. This minimizes output ripple and noise (less than 8 mV peak-to-peak ripple) while maintaining high efficiency (typically > 90%). For standby or light load applications, the buck regulator will automatically switch to a power- saving Pulse Frequency Modulation (PFM) mode. This minimizes the quiescent current draw on the battery, while keeping the buck output voltage in regulation. The typical buck PFM mode current is 38 µA. The buck regulator is capable of operating at 100% duty cycle, minimizing the voltage drop from input-to-output for wide input, battery-powered applications. For fixed- output voltage applications, the feedback divider and control loop compensation components are integrated, eliminating the need for external components. The buck regulator output is protected against overcurrent, short circuit and overtemperature. While shut down, the synchronous buck N-channel and P-channel switches are off, so the LX pin is in a high-impedance state (this allows for connecting a source on the output of the buck regulator as long as its voltage does not exceed the input voltage). 4.2.1 FIXED-FREQUENCY PWM MODE While operating in Pulse Width Modulation (PWM) mode, the TC1303/TC1304 buck regulator switches at a fixed, 2.0 MHz frequency. The PWM mode is suited for higher load current operation, maintaining low output noise and high conversion efficiency. PFM-to- PWM mode transition is initiated for any of the following conditions: • Continuous inductor current is sensed • Inductor peak current exceeds 100 mA • The buck regulator output voltage has dropped out of regulation (step load has occurred) The typical PFM-to-PWM threshold is 80 mA. 4.2.2 PFM MODE PFM mode is entered when the output load on the buck regulator is very light. Once detected, the converter enters the PFM mode automatically and begins to skip pulses to minimize unnecessary quiescent current draw by reducing the number of switching cycles per second. The typical quiescent current for the switching regulator is less than 35 µA. The transition from PWM to PFM mode occurs when discontinuous inductor current is sensed or the peak inductor current is less than 60 mA (typical). The typical PWM to PFM mode threshold is 30 mA. For low input-to-output differential voltages, the PWM-to-PFM mode threshold can be low due to the lack of ripple current. It is recommended that VIN1 be one volt greater than VOUT1 for PWM-to-PFM transitions. 4.3 Low Drop Out Regulator (LDO) The LDO output is a 300 mA low-dropout linear regulator that provides a regulated output voltage with a single 1 µF external capacitor. The output voltage is available in fixed options only, ranging from 1.5V to 3.3V. The LDO is stable using ceramic output capacitors that inherently provide lower output noise and reduce the size and cost of the regulator solution. The quiescent current consumed by the LDO output is typically less than 40 µA, with a typical dropout voltage of 137 mV at 200 mA. While operating in Dropout mode, the LDO quiescent current will increase, minimizing the necessary voltage differential needed for the LDO output to maintain regulation. The LDO output is protected against overcurrent and overtemperature conditions. |
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