Datasheet LT3503 (Analog Devices) - 7

LT3503
U OPERATIO (Refer to Block Diagram)
The LT3503 is a constant frequency, current mode step- An internal regulator provides power to the control cir- down regulator. A 2.2MHz oscillator enables an RS flip- cuitry. This regulator includes an undervoltage lockout to flop, turning on the internal 1.75A power switch Q1. An prevent switching when VIN is less than ~3.4V. The SHDN amplifier and comparator monitor the current flowing pin is used to place the LT3503 in shutdown, disconnect- between the VIN and SW pins, turning the switch off when ing the output and reducing the input current to less than this current reaches a level determined by the voltage at 2µA. VC. An error amplifier measures the output voltage through The switch driver operates from either the input or from an external resistor divider tied to the FB pin and servos the the BOOST pin. An external capacitor and diode are used VC node. If the error amplifier’s output increases, more to generate a voltage at the BOOST pin that is higher than current is delivered to the output; if it decreases, less the input supply. This allows the driver to fully saturate the current is delivered. An active clamp (not shown) on the VC internal bipolar NPN power switch for efficient operation. node provides current limit. The VC node is also clamped to the voltage on the SHDN pin; soft-start is implemented The oscillator reduces the LT3503’s operating frequency by generating a voltage ramp at the SHDN 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 start- up and overload. 3503f 7