Datasheet LT3685 (Analog Devices) - 8

LT3685
OPERATION
The LT3685 is a constant frequency, current mode step- The switch driver operates from either the input or from down regulator. An oscillator, with frequency set by RT, the BOOST pin. An external capacitor and diode are used enables an RS fl ip-fl op, turning on the internal power to generate a voltage at the BOOST pin that is higher than switch. An amplifi er and comparator monitor the current the input supply. This allows the driver to fully saturate the fl owing between the VIN and SW pins, turning the switch internal bipolar NPN power switch for effi cient operation. off when this current reaches a level determined by the The oscillator reduces the LT3685’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. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the The LT3685 contains a power good comparator which trips V when the FB pin is at 89% of its regulated value. The PG C pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented output is an open-collector transistor that is off when the by generating a voltage ramp at the RUN/SS pin using an output is in regulation, allowing an external resistor to pull external resistor and capacitor. the PG pin high. Power good is valid when the LT3685 is enabled and V An internal regulator provides power to the control circuitry. IN is above 3.6V. The bias regulator normally draws power from the V The LT3685 has an overvoltage protection feature which IN pin, but if the BD pin is connected to an external voltage higher disables switching action when the VIN goes above 38V than 3V bias power will be drawn from the external source typical (36V minimum). When switching is disabled, the (typically the regulated output voltage). This improves LT3685 can safely sustain input voltages up to 60V. effi ciency. The RUN/SS pin is used to place the LT3685 in shutdown, disconnecting the output and reducing the input current to less than 1μA. 3685fb 8