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LT3480IDD Datasheet(PDF) 8 Page - Linear Technology |
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LT3480IDD Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  LT3480 8 3480fb OPERATION The LT3480 is a constant frequency, current mode step- down regulator. An oscillator, with frequency set by RT, enables an RS flip-flop, turning on the internal power switch. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at VC. An error amplifier measures the output voltage through an external resistor divider tied to the FB pin and servos the VC pin. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the VC pin provides current limit. The VC pin is also clamped to the voltage on the RUN/SS pin; soft-start is implemented by generating a voltage ramp at the RUN/SS pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the VIN pin, but if the BD pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. The RUN/SS pin is used to place the LT3480 in shutdown, disconnecting the output and reducing the input current to less than 1μA. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. To further optimize efficiency, the LT3480 automatically switches to Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 70μA in a typical application. TheoscillatorreducestheLT3480’soperatingfrequencywhen the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload. The LT3480 contains a power good comparator which trips when the FB pin is at 86% of its regulated value. The PG output is an open-collector transistor that is off when the output is in regulation, allowing an external resistor to pull the PG pin high. Power good is valid when the LT3480 is enabled and VIN is above 3.6V. The LT3480 has an overvoltage protection feature which disables switching action when the VIN goes above 38V typical (36V minimum). When switching is disabled, the LT3480 can safely sustain input voltages up to 60V. |
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