DC1205A Datasheet, PDF (10/24 Pages) Linear Integrated Systems – 500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming

LT3592
APPLICATIONS INFORMATION
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.
Table 1. R_T Values for Selector Oscillator Frequencies
f_OSC
R_T
400kHz
357k
900kHz
140k
2.2MHz
48.7k
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.
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_IN – V_SW
V_D
+ 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] USA: [phone] www.sumida.com
Japan: 81(3) 3607-5111 Japan: 81(3) 3607-5144
TDK
[phone]
[phone]
www.component.tdk.com
TOKO
[phone]
[phone]
www.tokoam.com
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