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LM1771 Datasheet(PDF) 9 Page - Texas Instruments |
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LM1771 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 31 page FB VIN ON TIMER High-Side Driver 0.8V Q Vin LM1771 REGULATION COMPARATOR Q OFF TIMER GND HG LG Level Shift and Shoot Through Protection Low-Side Driver SHORT CIRCUIT PROTECTION 0.55 V /Soft-Start UVLO UVLO SD SD Q Q R S Q Q R S 1.2 V EN ENABLE COMPARATOR LM1771 www.ti.com SNVS446D – JUNE 2006 – REVISED JANUARY 2016 7 Detailed Description 7.1 Overview The LM1771 synchronous buck controller has a control scheme that is referred to as adaptive ON-time control. This topology relies on a fixed switch ON-time to regulate the output voltage. This ON-time is internally set by EEPROM and is available with three different set-points to allow for different frequency options. The LM1771 automatically adjusts the ON-time during operation inversely with the input voltage (VIN) to maintain a constant frequency. Therefore the switching frequency during continuous conduction mode is independent of the inductor and capacitor size unlike hysteretic switchers. At the beginning of the cycle, the LM1771 turns on the high-side PFET for a fixed duration. This ON-time is predetermined (internally set by EEPROM and adjusted by VIN) and the switch does not turn off until the timer has completed its period. The PFET then turns off for a minimum predetermined time period. This minimum TOFF of 150 ns is internally set and cannot be adjusted. This is to prevent false triggering from occurring on the comparator due to noise from the SW node transition. After the minimum TOFF period has expired, the PFET remains off until the comparator trip-point has been reached. Upon passing this trip-point (set at 0.8 V at the feedback pin), the PFET turns back on and the process repeats, thus regulating the output. The NFET control is complementary to the PFET control with the exception of a short dead-time to prevent shoot-through from occurring. 7.2 Functional Block Diagram Copyright © 2006–2016, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM1771 |
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