Datasheet LT3980 (Analog Devices) - 8

LT3980
operaTion
The LT3980 is a constant frequency, current mode step- the input supply. This allows the driver to fully saturate the down regulator. An oscillator, with frequency set by RT, internal bipolar NPN power switch for efficient operation. enables an RS flip-flop, turning on the internal power To further optimize efficiency, the LT3980 automatically switch. An amplifier and comparator monitor the current switches to Burst Mode operation in light load situations. flowing between the VIN and SW pins, turning the switch Between bursts, all circuitry associated with controlling off when this current reaches a level determined by the the output switch is shut down, reducing the input supply voltage at VC. An error amplifier measures the output current to 75µA in a typical application. voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier’s output The oscillator reduces the LT3980’s operating frequency increases, more current is delivered to the output; if it when the voltage at the FB pin is low. This frequency decreases, less current is delivered. An active clamp on the foldback helps to control the output current during startup VC pin provides current limit. The VC pin is also clamped to and overload. In addition, the LT3980 monitors the catch the voltage on the RUN/SS pin; soft-start is implemented diode current flowing through the DA pin and pauses the by generating a voltage ramp at the RUN/SS pin using an oscillator during overcurrent conditions to keep inductor external resistor and capacitor. current at safe levels. An internal regulator provides power to the control circuitry. The LT3980 contains a power good comparator which trips The bias regulator normally draws power from the VIN pin, when the FB pin is at 91% of its regulated value. The PG but if the BD pin is connected to an external voltage higher output is an open-collector transistor that is off when the than 3V bias power will be drawn from the external source output is in regulation, allowing an external resistor to pull (typically the regulated output voltage). This improves the PG pin high. Power good is valid when the LT3980 is efficiency. The RUN/SS pin is used to place the LT3980 enabled and VIN is above 3.6V. in shutdown, disconnecting the output and reducing the The LT3980 has an overvoltage protection feature which input current to less than 0.5µA. disables switching action when the VIN goes above 61.5V The switch driver operates from either the input or from typical (58V minimum). When switching is disabled, the the BOOST pin. An external capacitor and diode are used LT3980 can safely sustain input voltages up to 62V. to generate a voltage at the BOOST pin that is higher than 3980fa 8 For more information www.linear.com/LT3980 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts