Datasheet LT3480 (Analog Devices) - 8

LT3480
BLOCK DIAGRAM
VIN VIN 4 C1 – INTERNAL 0.79V REF + BD 1 RUN/SS 5 Σ SLOPE COMP SWITCH BOOST LATCH 2 R C3 RT OSCILLATOR Q 10 200kHz–2.4MHz S SW L1 R V T 3 OUT SYNC DISABLE 6 C2 D1 Burst Mode SOFT-START DETECT PG 7 ERROR AMP VC CLAMP + 0.7V + VC – – 9 CC CF RC GND FB 11 8 R2 R1 3480 BD
OPERATION
The LT3480 is a constant frequency, current mode step- (typically the regulated output voltage). This improves down regulator. An oscillator, with frequency set by RT, efficiency. The RUN/SS pin is used to place the LT3480 enables an RS flip-flop, turning on the internal power in shutdown, disconnecting the output and reducing the switch. An amplifier and comparator monitor the current input current to less than 1µA. flowing between the VIN and SW pins, turning the switch The switch driver operates from either the input or from off when this current reaches a level determined by the the BOOST pin. An external capacitor and diode are used voltage at VC. An error amplifier measures the output to generate a voltage at the BOOST pin that is higher than voltage through an external resistor divider tied to the FB the input supply. This allows the driver to fully saturate the pin and servos the VC pin. If the error amplifier’s output internal bipolar NPN power switch for efficient operation. increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the To further optimize efficiency, the LT3480 automatically VC pin provides current limit. The VC pin is also clamped to switches to Burst Mode operation in light load situations. the voltage on the RUN/SS pin; soft-start is implemented Between bursts, all circuitry associated with controlling by generating a voltage ramp at the RUN/SS pin using an the output switch is shut down, reducing the input supply external resistor and capacitor. current to 70µA in a typical application. An internal regulator provides power to the control circuitry. The oscillator reduces the LT3480’s operating frequency when The bias regulator normally draws power from the VIN pin, the voltage at the FB pin is low. This frequency foldback helps but if the BD pin is connected to an external voltage higher to control the output current during startup and overload. than 3V bias power will be drawn from the external source 3480fe 8 For more information www.linear.com/LT3480 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts