LT3592EDDB-TR datasheet(9/24 Pages) LINER

部件名	LT3592EDDB-TR

功能描述	500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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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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