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LT3592IDDB-TR Datasheet(PDF) 9 Page - Linear Technology |
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LT3592IDDB-TR Datasheet(HTML) 9 Page - Linear Technology |
9 / 24 page LT3592 9 3592fa OPERATION The LT3592 is a constant frequency, current mode step- down LED driver. An internal oscillator that is programmed by a resistor from the RT pin to ground enables an RS flip-flop, turning on the internal 1A power switch Q1. 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 that servos the VC node has two inputs, one from a voltage measurement and one from a current measurement. An instrumentation amplifier measures the drop across an external current sense resistor between the CAP and OUT pins and applies a gain of 60 (BRIGHT low for dim mode) or 6 (BRIGHT high for bright mode) to this signal and presents it to one negative error amp input. The output of a external resistor divider between OUT and ground is tied to the VFB pin and presented to a second negative error amp input. Whichever input is higher in voltage will end up controlling the loop, so a circuit in which current control is desired (as for driving a LED) will be set up such that the output of the instrumentation amp will be higher than the VFB pin at the current level that is desired. The voltage feedback loop will act to limit the output voltage and prevent circuit damage if an LED should go open circuit. The positive input to the error amp is a 1.21V reference, so the voltage loop forces the VFB pin to 1.21V and the current loop forces the voltage difference between CAP and OUT to be 200mV for BRIGHT mode and 20mV for DIM mode. A rise in the output of the error amplifier results in a increase in output current, and a fall in the error amplifier output means less output current. Current limit is provided by an active clamp on the VC node, and this node is also clamped to the SHDN pin. Soft-start is implemented by ramping the SHDN pin using an external resistor and capacitor. An internal regulator provides power to the control circuitry and also includes an undervoltage lockout to prevent switching when VIN is less than 3.25V. If SHDN is low, the output is disconnected and the input current is less than 2μA. The switch driver operates from the input of the BOOST pin. An external capacitor and internal diode are used to generate a voltage at the BOOST pin that is higher than the input supply, which allows the driver to fully saturate the internal bipolar NPN power switch for efficient operation. An external diode can be used to make the BOOST drive more effective at low output voltage. The oscillator reduces the LT3592’s operating frequency when the voltage at the OUT pin is low. This frequency foldback helps to control the output current during startup and overload. The anode of the catch diode for the step-down circuit is connected to the DA pin to provide a direct sense of the current in this device. If this diode’s current goes above a level set by an internal catch diode current limit circuit, the oscillator frequency is slowed down. This prevents current runaway due to minimum on time limitations at high VIN voltages. This function can easily be disabled by tying the DA pin and the catch diode anode to ground. |
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