Datasheet LT3502, LT3502A (Analog Devices) - 9

LT3502/LT3502A
operation
The LT3502/LT3502A are constant frequency, current The switch driver operates from either VIN or from the mode step-down regulators. An oscillator enables an RS BOOST pin. An external capacitor and the internal diode flip-flop, turning on the internal 500mA power switch Q1. are used to generate a voltage at the BOOST pin that is An amplifier and comparator monitor the current flowing higher than the input supply. This allows the driver to between the VIN and SW pins, turning the switch off when fully saturate the internal bipolar NPN power switch for this current reaches a level determined by the voltage at efficient operation. VC. An error amplifier measures the output voltage through A comparator monitors the current flowing through an external resistor divider tied to the FB pin and servos the catch diode via the DA pin and reduces the LT3502/ the VC node. If the error amplifier’s output increases, more LT3502A’s operating frequency when the DA pin current current is delivered to the output; if it decreases, less exceeds the 650mA valley current limit. This frequency current is delivered. An active clamp (not shown) on the VC foldback helps to control the output current in fault node provides current limit. The VC node is also clamped conditions such as shorted output with high input volt- to the voltage on the SHDN pin; soft-start is implemented age. The DA comparator works in conjunction with the by generating a voltage ramp at the SHDN pin using an switch peak current limit comparator to determine the external resistor and capacitor. The SHDN pin voltage maximum deliverable current of the LT3502/LT3502A. The during soft-start also reduces the oscillator frequency to peak current limit comparator is used in normal current avoid hitting current limit during start-up. mode operations and is used to turn off the switch. The DA An internal regulator provides power to the control cir- valley current comparator monitors the catch diode current cuitry. This regulator includes an undervoltage lockout to and will delay switching until the catch diode current is prevent switching when VIN is less than ~3V. The SHDN below the 650mA limit. Maximum deliverable current to pin is used to place the LT3502/LT3502A in shutdown, the output is therefore limited by both switch peak current disconnecting the output and reducing the input current limit and DA valley current limit. to less than 2µA. 3502fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts