Datasheet LT3592 (Analog Devices) - 9

LT3592
OPERATION
The LT3592 is a constant frequency, current mode step- error amplifi er output means less output current. Current down LED driver. An internal oscillator that is programmed limit is provided by an active clamp on the VC node, and by a resistor from the RT pin to ground enables an RS this node is also clamped to the SHDN pin. Soft-start is fl ip-fl op, turning on the internal 1.25A power switch Q1. implemented by ramping the SHDN pin using an external An amplifi er and comparator monitor the current fl owing resistor and capacitor. between the VIN and SW pins, turning the switch off when An internal regulator provides power to the control this current reaches a level determined by the voltage at circuitry and also includes an undervoltage lockout to VC. An error amplifi er that servos the VC node has two prevent switching when V inputs, one from a voltage measurement and one from a IN is less than 3.25V. If SHDN is low, the output is disconnected and the input current current measurement. is less than 2μA. An instrumentation amplifi er measures the drop across The switch driver operates from the input of the BOOST an external current sense resistor between the CAP and pin. An external capacitor and internal diode are used to OUT pins and applies a gain of 60 (BRIGHT low for dim generate a voltage at the BOOST pin that is higher than the mode) or 6 (BRIGHT high for bright mode) to this signal input supply, which allows the driver to fully saturate the and presents it to one negative error amp input. The output internal bipolar NPN power switch for effi cient operation. of a external resistor divider between OUT and ground is An external diode can be used to make the BOOST drive tied to the VFB pin and presented to a second negative error more effective at low output voltage. amp input. Whichever input is higher in voltage will end up controlling the loop, so a circuit in which current control The oscillator reduces the LT3592’s operating frequency is desired (as for driving a LED) will be set up such that when the voltage at the OUT pin is low. This frequency the output of the instrumentation amp will be higher than foldback helps to control the output current during startup the V and overload. FB pin at the current level that is desired. The voltage feedback loop will act to limit the output voltage and prevent The anode of the catch diode for the step-down circuit is circuit damage if an LED should go open circuit. connected to the DA pin to provide a direct sense of the The positive input to the error amp is a 1.21V reference, current in this device. If this diode’s current goes above a so the voltage loop forces the V level set by an internal catch diode current limit circuit, the FB pin to 1.21V and the current loop forces the voltage difference between CAP oscillator frequency is slowed down. This prevents current and OUT to be 200mV for BRIGHT mode and 20mV for runaway due to minimum on time limitations at high VIN DIM mode. A rise in the output of the error amplifi er voltages. This function can easily be disabled by tying the results in a increase in output current, and a fall in the DA pin and the catch diode anode to ground. 3592fc 9