LT3592
OPERATION
error amplifier output means less output current. Current
The LT3592 is a constant frequency, current mode step-
limit is provided by an active clamp on the V node, and
downLEDdriver.Aninternaloscillatorthatisprogrammed
C
this node is also clamped to the SHDN pin. Soft-start is
implemented by ramping the SHDN pin using an external
resistor and capacitor.
by a resistor from the R pin to ground enables an RS
T
flip-flop, turning on the internal 1.25A power switch Q1.
An amplifier and comparator monitor the current flowing
between the V and SW pins, turning the switch off when
IN
An internal regulator provides power to the control
circuitry and also includes an undervoltage lockout to
this current reaches a level determined by the voltage at
V . An error amplifier that servos the V node has two
C
C
prevent switching when V is less than 3.25V. If SHDN
IN
inputs, one from a voltage measurement and one from a
is low, the output is disconnected and the input current
current measurement.
is less than 2μA.
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
andpresentsittoonenegativeerrorampinput.Theoutput
of a external resistor divider between OUT and ground is
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.
tiedtotheV pinandpresentedtoasecondnegativeerror
FB
ampinput. Whicheverinputishigherinvoltagewillendup
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 oscillator reduces the LT3592’s operating frequency
when the voltage at the OUT pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
the V pin at the current level that is desired. The voltage
FB
feedbackloopwillacttolimittheoutputvoltageandprevent
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
oscillatorfrequencyissloweddown.Thispreventscurrent
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 V pin to 1.21V and the
FB
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
runaway due to minimum on time limitations at high V
IN
voltages. This function can easily be disabled by tying the
DA pin and the catch diode anode to ground.
3592fc
9
Linear Systems [ Linear Systems ]
