Datasheet

LT3592
10
3592fc
Oscillator
The frequency of operation is programmed by an external
resistor from R
T
to ground. Table 1 shows R
T
values for
commonly used oscillator frequencies, and refer to the Typi-
cal Performance Characteristics curve for other values.
APPLICATIONS INFORMATION
The BRIGHT mode current is given by:
I
BRIGHT
= 200mV/R
SENSE
The DIM mode current is 10% of the BRIGHT mode value.
The maximum allowed DC value of the BRIGHT mode cur-
rent is 500mA. When the recommended component values
are used in a 900kHz 2 LED application, the transient from
switching between BRIGHT and DIM currents will be less
than 50µs in duration.
The sense resistor used should exhibit a low TC to keep
the LED current from drifting as the operating temperature
changes.
The BRIGHT pin can tolerate voltages as high as 36V and
can be safely tied to V
IN
even in high voltage applications,
but it also has a low threshold voltage (~0.7V) that allows
it to interface to logic level control signals.
Input Voltage Range
The maximum allowed input voltage for the LT3592 is
36V. The minimum input voltage is determined by either
the LT3592’s minimum operating voltage of 3.6V or by
its maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC =
V
OUT
+ V
D
V
IN
–V
SW
+ V
D
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
Table 2. Inductor Vendor Information
SUPPLIER PHONE FAX WEBSITE
Panasonic [phone] www.panasonic.com/industrial/components/components.html
Vishay [phone] [phone] www.vishay.com/resistors
Coilcraft [phone] [phone] www.coilcraft.com
CoEv Magnetics [phone] [phone] www.circuitprotection.com/magnetics.asp
Murata [phone]
[phone]
[phone] www.murata.com
Sumida USA: [phone]
Japan: 81(3) 3607-5111
USA: [phone]
Japan: 81(3) 3607-5144
www.sumida.com
TDK [phone] [phone] www.component.tdk.com
TOKO [phone] [phone] www.tokoam.com
FB Resistor Network
The output voltage limit is programmed with a resistor
divider between the output and the V
FB
pin. This is the
voltage that the output will be clamped to in case the LED
goes open circuit. Choose the resistors according to
R1 = R2([V
OUT
/1.21V] – 1)
Be sure to choose V
OUT
such that it does not interfere with
the operation of the current control loop; it should be set
at least 10% above the maximum expected LED voltage
for the selected BRIGHT output current. R2 should be 20k
or less to avoid bias current errors. An optional phase-
lead capacitor of 22pF between V
OUT
and V
FB
reduces
light-load ripple.
Output Current Selection
The output current levels are programmed by the value of
the external current sense resistor between CAP and OUT.
Table 1. R
T
Values for Selector Oscillator Frequencies
f
OSC
R
T
400kHz 357k
900kHz 140k
2.2MHz 48.7k