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SC194A Datasheet(PDF) 7 Page - Semtech Corporation |
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SC194A Datasheet(HTML) 7 Page - Semtech Corporation |
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7 / 17 page  7 © 2006 Semtech Corp. www.semtech.com SC194A PRELIMINARY POWER MANAGEMENT Applications Information SC194A Detailed Description The SC194A is a synchronous step-down Pulse Width Modulated (PWM), DC-DC converter utilizing a 1MHz fixed-frequency current mode architecture. The device is designed to operate in a fixed-frequency PWM mode across the full load range and can enter Power Save Mode (PSAVE) utilizing Pulse Frequency Modulation (PFM) at light loads to maximize efficiency. Operation During normal operation the PMOS MOSFET is activated on each rising edge of the internal oscillator. Current feedback for the switching regulator uses the PMOS current path, and it is amplified and summed with the internal slope compensation network. The voltage feedback loop uses an internal feedback divider. The on- time is determined by comparing the summed current feedback and the output of the error amplifier. The period is set by the onboard oscillator or by an external clock attached to the SYNC/PWM pin. The SC194A has an internal synchronous NMOS rectifier and does not require a Schottky diode on the LX pin. Programmable Output Voltage The SC194A has four pre-determined output voltage values which can be individually selected by the correct programmingoftheVID0andVID1pins(see Programmable Output Voltage table on Page 4). This eliminates the need for external programming resistors saving PCB area and inventory. The VID pins can be statically tied to GND or VIN for fixed output configurations or they may be driven by a microprocessor enabling the possibility of dynamic voltage adjustment for host equipment "sleep" states. Continuous Conduction & Oscillator Synchronization The SC194A is designed to operate in continuous conduction, fixed-frequency mode. When the SYNC/ PWM pin is tied high the part runs in PWM mode using the internal oscillator. The part can be synchronized to an external clock by driving a clock signal into the SYNC/ PWM pin. The part synchronizes to the rising edge of the clock. Protection Features The SC194A provides the following protection features: • Thermal shutdown • Current limit • Over-voltage protection • Soft-start Thermal Shutdown The device has a thermal shutdown feature to protect the SC194A if the junction temperature exceeds 145°C. In thermal shutdown the on-chip power devices are disabled, tri-stating the LX output. Switching will resume when the temperature drops by 10°C. During this time if the output voltage decreases by more than 60% of its programmed value, a soft-start will be invoked. Current Limit The PMOS and NMOS power devices of the buck switcher stage are protected by current limit functions. In the case of a short to ground on the output, the part enters frequency foldback mode, that causes the switching frequency to divide by a factor determined by the output voltage. This prevents the inductor current from "stair- casing". Over-Voltage Protection Over-voltage protection is provided on the SC194A. In the event of an over-voltage on the output in switcher mode, the PWM drive is disabled, tri-stating the LX output. The part will not resume switching until the output voltage has fallen below 2% of the regulation voltage. Soft-Start The soft-start mode is enabled after every shutdown cycle to limit in-rush current. In conjunction with the frequency foldback, this controls the maximum current during start- up. The PMOS current limit is stepped up through seven soft-start levels to the full value by a timer driven from the internal oscillator. During soft-start, the switching frequency is stepped by 1/8, 1/4, and 1/2 of the internal oscillator frequency up to the full value, under control of three output voltage thresholds. As soon as the output voltage is within 2% of the regulation voltage, soft-start mode is disabled. Power Save Mode Operation The PSAVE mode may be selected by tying the SYNC/PWM pin to GND. Selecting PSAVE mode will enable the SC194A to automatically activate/deactivate operation at light loads maximizing efficiency across the full load range. The SC194A automatically detects the load current at which it should enter PSAVE mode. The SC194A is optimized to track maximum efficiency with respect to V IN. |
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