Datasheet LT3682 (Analog Devices) - 10

LT3682
OPERATION
The LT3682 is a constant frequency, current mode To further optimize effi ciency, the LT3682 automatically step-down regulator. An oscillator, with frequency set by switches to Burst Mode operation in light load situations. RT, enables an RS fl ip-fl op, turning on the internal power Between bursts, all circuitry associated with controlling switch. An amplifi er and comparator monitor the current the output switch is shut down, reducing the input supply fl owing between the VIN and SW pins, turning the switch current to 75μA in a typical application. off when this current reaches a level determined by the The oscillator reduces the LT3682’s operating frequency voltage at VC. An error amplifi er measures the output when the voltage at the FB pin is low. This frequency voltage through an external resistor divider tied to the FB foldback helps to control the output current during startup pin and servos the VC pin. If the error amplifi er’s output and overload conditions. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the Internal circuitry monitors the current fl owing through the V catch diode via the DA pin and delays the generation of C pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented new switch pulses if this current is too high (above 1.6A by generating a voltage ramp at the RUN/SS pin using an nominal). This mechanism also protects the part during external resistor and capacitor. short-circuit and overload conditions by keeping the cur- rent through the inductor under control. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the V The LT3682 contains a power good comparator which trips IN pin, but if the BD pin is connected to an external voltage higher when the FB pin is at 90% of its regulated value. The PG than 3V, bias power will be drawn from the external source output is an open-collector transistor that is off when the (typically the regulated output voltage). This improves output is in regulation, allowing an external resistor to pull effi ciency. The RUN/SS pin is used to place the LT3682 the PG pin high. Power good is valid when the LT3682 is in shutdown, disconnecting the output and reducing the enabled and VIN is above the minimum input voltage. input current to less than 1μA. The LT3682 has an overvoltage protection feature which The switch driver operates from either the input or from disables switching action when the VIN goes above 39V the BOOST pin. An external capacitor and the internal boost typical (36V minimum) during transients. When switching diode are used to generate a voltage at the BOOST pin that is disabled, the LT3682 can safely sustain transient input is higher than the input supply. This allows the driver to voltages up to 60V. fully saturate the internal bipolar NPN power switch for effi cient operation. 3682f 10