LT3486
APPLICATIONS INFORMATION
The average forward current in normal operation is equal
to the output current, and the peak current is equal to the
peak inductor current. A Schottky diode rated at 1A is suf-
ficient for most LT3486 applications. Some recommended
Schottky diodes are listed in Table 3.
Table 3. Recommended Schottky Diodes
PART NUMBER
MBR0530
MBRM120E
ZLLS400
ZLLS1000
ZHCS400
ZHCS1000
V
R
(V)
30
20
40
40
40
40
I
AVG
(A)
0.5
1
0.4
1
0.4
1
MANUFACTURER
On Semiconductor
www.onsemi.com
Zetex
www.zetex.com
200mV
I
LED1
200mV
R
FB2
=
I
LED2
R
FB1
=
Table 4. R
FB
Value Selection
I
LED
(mA)
5
10
15
20
25
R
FB
(Ω)
40.2
20.0
13.3
10.0
8.06
When the LT3486 is set up for PWM dimming operation,
choose a Schottky diode with low reverse leakage current.
During PWM dimming operation, the output capacitor is
required to hold up the charge in the PWM “off” period.
A low reverse leakage Schottky helps in that mode of op-
eration. The Zetex ZLLS400 and ZLLS1000 are available
in a small surface mount package and are a good fit for
this application.
MOSFET Selection
The power MOSFET used in LT3486 applications with wide
dimming range requirements should be chosen based on
the maximum drain-source voltage. The maximum drain
current I
D(MAX)
and gate-to-source voltages should also
be considered when choosing the FET.
Choose a MOSFET with maximum V
DS
(drain source) volt-
age greater than the output clamp voltage i.e., 36V (typ).
Fairchild Semiconductor’s FDN5630 (60V, 1.7A N-channel
FET) is a good fit for most LT3486 applications. For dim-
ming low current LEDs (~25mA), Fairchild 2N7002 is a
good alternative.
Programming LED Current
The current in each LED string can be set independently
by the choice of resistors R
FB1
and R
FB2
respectively
(see front page application). The feedback reference is
200mV. In order to have accurate LED current, precision
resistors are preferred (1% is recommended).
Most low power white LEDs are driven at maximum cur-
rents of 15mA to 25mA. The LT3486 can be used to power
high power LEDs as well. Refer to the Typical Applications
for more detail.
Dimming Control
The dimming of the two LED strings can be controlled
independently by modulating the respective CTRL and
PWM pins. There are two ways to control the intensity
of the LEDs.
Adjusting the LED Current Value
Controlling the current flowing through the LEDs controls
the intensity of the LEDs.This is the easiest way to control
the intensity of the LEDs. The LED forward current can be
controlled by modulating the DC voltage at the respective
CRTL pin. The PWM pins are not in use when appying
this scheme. They must be connected to a 0.9V supply or
higher. The DC voltage at the CTRL pin can be modulated
in two ways.
(a) Using a DC Voltage Source
For some applications, the preferred method of brightness
control is a variable DC voltage fed to the CTRL pins. The
CTRL1, CTRL2 pin voltage can be modulated to set the
dimming of the respective LED string. As the voltage on
the CTRL1, CTRL2 pin increases from 0V to 1.8V, the LED
current increases from 0 to I
LED
. As the CTRL1, CTRL2
pin voltage increases beyond 1.8V, it has no effect on the
LED current.
3486fe
12
LINER [ LINEAR TECHNOLOGY ]
