Datasheet

LT3486
14
3486fe
APPLICATIONS INFORMATION
the figure. A PWM logic input is applied to the gate of the
NMOS and the PWM pin of the LT3486. When the PWM
input is taken high, the LEDs are connected to the R
FB
resistor and a current I
LED
= 200mV/R
FB
flows through
the LEDs. When the PWM input is taken low, the LEDs are
disconnected and turn off. The low PWM input applied to
the LT3486 ensures that the respective converter turns
off and its V
C
pin goes high impedance. This ensures that
the capacitor connected to the V
C
pin retains its voltage
which in turn allows the LEDs to turn on faster, as shown
in Figure 8(b). The CTRL pin is not used to modulate the
LED current in the scheme. It can be connected to a sup-
ply voltage greater than 1.8V.
The dimming control pins (PWM1, PWM2) can be used
to extend the dimming range for the individual switching
converters. The LED current can be controlled down to
µA levels by feeding a PWM signal with frequencies in the
range of 80Hz to 50kHz. The LED current can be controlled
by PWM frequencies above 50kHz but the controllable
current decreases with increasing frequency. Pulling the
PWM pins below 0.4V disables the respective switcher.
Taking it higher than 0.9V resumes normal operation.
Connect these pins to 0.9V or higher if not in use.
Figure 9 shows the LED current variation vs PWM duty
cycle. The LED current is controlled by applying a PWM of
frequency 100Hz, 1kHz and 25kHz to the circuit of Figure 8a.
As seen in the curves, the LED string is able to get a wide
(1000:1) dimming range with PWM frequency of 100Hz.
The dimming range decreases as PWM frequency goes up.
Board Layout Consideration
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essential.
Minimize the length and area of all traces connected to the
switching node pins (SW1 and SW2). Keep the feedback
pins (FB1 and FB2) away from the switching nodes.
The DFN and FE packages both have an exposed paddle
that must be connected to the system ground. The
ground connection for the feedback resistors should
be tied directly to the ground plane and not shared with
any other component, except the R
T
resistor, ensuring a
clean, noise-free connection. Recommended component
placement for the DFN package is shown in the Figure 10.
Figure 10. Recommended Layout for LT3486Figure 9. LED Current Variation vs PWM Duty Cycle
PWM DUTY CYCLE (%)
0.1
LED CURRENT (mA)
1
10
100
0.01 1 10 100
3486 F09
0.01
0.1
PWM FREQ = 100Hz
PWM FREQ = 1kHz
PWM FREQ = 25kHz
16
15
14
13
12
11
10
9
17
1
2
3
4
5
6
7
8
3486 F10
OVP1
V
OUT1
V
OUT2
FB1
FB2
LED1
LED2
CTRL1 CTRL2
V
IN
SW1 SW2
V
IN
V
IN
R
T
V
C1
PWM1
REF
OVP2
SHDN
V
C2
PWM2
VIAs TO V
IN
PLANE
VIAs TO GROUND PLANEVIAs TO V
IN
PLANE