Datasheet LT3481 (Analog Devices) - 8

LT3481
OPERATION
The LT3481 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 fl owing between the VIN and SW pins, turning the switch the internal bipolar NPN power switch for effi cient off when this current reaches a level determined by the operation. voltage at VC. An error amplifi er measures the output To further optimize effi ciency, the LT3481 automatically voltage through an external resistor divider tied to the FB switches to Burst Mode operation in light load situations. pin and servos the VC pin. If the error amplifi er’s output Between bursts, all circuitry associated with controlling increases, more current is delivered to the output; if it the output switch is shut down reducing the input supply decreases, less current is delivered. An active clamp on the current to 50μA in a typical application. VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented The oscillator reduces the LT3481’s operating frequency by generating a voltage ramp at the RUN/SS pin using an when the voltage at the FB pin is low. This frequency external resistor and capacitor. foldback helps to control the output current during startup and overload. An internal regulator provides power to the control cir- cuitry. The bias regulator normally draws power from the The LT3481 contains a power good comparator which trips V when the FB pin is at 91% of its regulated value. The PG IN pin, but if the BIAS pin is connected to an external voltage higher than 3V bias power will be drawn from the output is an open-collector transistor that is off when the external source (typically the regulated output voltage). output is in regulation, allowing an external resistor to pull This improves effi ciency. The RUN/SS pin is used to place the PG pin high. Power good is valid when the LT3481 is the LT3481 in shutdown, disconnecting the output and enabled and VIN is above 3.6V. reducing the input current to less than 1μA. 3481fc 8