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TPS81256SIP Datasheet(PDF) 11 Page - Texas Instruments |
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TPS81256SIP Datasheet(HTML) 11 Page - Texas Instruments |
11 / 23 page TPS81256 www.ti.com SLVSAZ9 – JUNE 2012 DETAILED DESCRIPTION OPERATION The TPS8125x is a stand-alone, synchronous, step-up converter. The converter operates at a quasi-constant 4- MHz frequency pulse width modulation (PWM) at moderate to heavy load currents. At light load currents, the TPS8125x converter operates in power-save mode with pulse frequency modulation (PFM). During PWM operation, the converter uses a novel quasi-constant on-time valley current mode control scheme to achieve excellent line/load regulation and allows the use of a small ceramic inductor and capacitors. Based on the VIN/VOUT ratio, a simple circuit predicts the required on-time. At the beginning of the switching cycle, the low-side N-MOS switch is turned-on and the inductor current ramps up to a peak current that is defined by the on-time and the inductance. In the second phase, once the on-timer has expired, the rectifier is turned-on and the inductor current decays to a preset valley current threshold. Finally, the switching cycle repeats by setting the on timer again and activating the low-side N-MOS switch. In general, a dc/dc step-up converter can only operate in "true" boost mode, i.e. the output “boosted” by a certain amount above the input voltage. The TPS8125x device operates differently as it can smoothly transition in and out of zero duty cycle operation. Therefore the output can be kept as close as possible to its regulation limits even though the converter is subject to an input voltage that tends to be excessive. In this operation mode, the output current capability of the regulator is limited to ca. 150mA. Refer to the typical characteristics section (DC Output Voltage vs. Input Voltage) for further details. The current mode architecture with adaptive slope compensation provides excellent transient load response while requiring only one external tiny capacitor for output filtering and loop stability purposes. Internal soft-start and loop compensation simplifies the application design process. POWER-SAVE MODE The TPS8125X integrates a power-save mode to improve efficiency at light load. In power save mode the converter only operates when the output voltage trips below a set threshold voltage. It ramps up the output voltage with several pulses and goes into power save mode once the output voltage exceeds the set threshold voltage. The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM mode. Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Link(s): TPS81256 |
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