LT3592
9
3592fc
OPERATION
The LT3592 is a constant frequency, current mode step-
down LED driver. An internal oscillator that is programmed
by a resistor from the R
T
pin to ground enables an RS
fl ip-fl op, turning on the internal 1.25A power switch Q1.
An amplifi er and comparator monitor the current fl owing
between the V
IN
and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
V
C
. An error amplifi er that servos the V
C
node has two
inputs, one from a voltage measurement and one from a
current measurement.
An instrumentation amplifi er 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 V
FB
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 V
FB
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 V
FB
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 amplifi er
results in a increase in output current, and a fall in the
error amplifi er output means less output current. Current
limit is provided by an active clamp on the V
C
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 V
IN
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 effi cient 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 V
IN
voltages. This function can easily be disabled by tying the
DA pin and the catch diode anode to ground.
