Datasheet LT3680 (Analog Devices) - 8

LT3680
OPERATION
The LT3680 is a constant frequency, current mode step- The switch driver operates from either the input or from down regulator. An oscil ator, with frequency set by RT, the BOOST pin. An external capacitor and diode are used enables an RS flip-flop, turning on the internal power to generate a voltage at the BOOST pin that is higher than switch. An amplifier and comparator monitor the current the input supply. This allows the driver to fully saturate the flowing between the VIN and SW pins, turning the switch internal bipolar NPN power switch for efficient operation. off when this current reaches a level determined by the To further optimize efficiency, the LT3680 automatically voltage at VC. An error amplifier measures the output switches to Burst Mode operation in light load situations. voltage through an external resistor divider tied to the Between bursts, all circuitry associated with controlling FB pin and servos the VC pin. If the error amplifier’s out- the output switch is shut down, reducing the input supply put increases, more current is delivered to the output; if current to 75µA in a typical application. it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also The oscillator reduces the LT3680’s operating frequency clamped to the voltage on the RUN/SS pin; soft-start is when the voltage at the FB pin is low. This frequency implemented by generating a voltage ramp at the RUN/ foldback helps to control the output current during startup SS pin using an external resistor and capacitor. and overload. An internal regulator provides power to the control cir- The LT3680 contains a power good comparator which cuitry. The bias regulator normally draws power from the trips when the FB pin is at 91% of its regulated value. The VIN pin, but if the BD pin is connected to an external volt- PG output is an open-collector transistor that is off when age higher than 3V bias power wil be drawn from the the output is in regulation, al owing an external resistor to external source (typically the regulated output voltage). pull the PG pin high. Power good is valid when the LT3680 This improves efficiency. The RUN/SS pin is used to place is enabled and VIN is above 3.6V. the LT3680 in shutdown, disconnecting the output and reducing the input current to less than 0.5µA. Rev C 8 For more information www.analog.com Document Outline Features Applications Description 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