LT3465 LINER [Linear Technology], LT3465 Datasheet - Page 8

• Manufacturer Part Number: LT3465
• Description: 1.2MHz/2.4MHz White LED Step-Up Converters with Built-In Schottky in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Dimming Using Direct PWM (LT3465A)
Unlike the LT3465, the LT3465A does not have internal
soft-start. Although the input current is higher during start-
up, the absence of soft-start allows the CTRL pin to be
directly driven with a PWM signal for dimming. A zero
percent duty cycle sets the LED current to zero, while 100%
duty cycle sets it to full current. Average LED current
increases proportionally with the duty cycle of the PWM
signal. PWM frequency should be between 1kHz and 10kHz
for best performance. The PWM signal should be at least
1.8V in magnitude; lower voltage will lower the feedback
voltage as shown in Equation 1. Waveforms are shown for
a 1kHz PWM and 10kHz PWM signal in Figures 7a and 7b
respectively.
LT3465/LT3465A
Open-Circuit Protection
The LT3465 and LT3465A have an internal open-circuit
protection circuit. In the cases of output open circuit,
when the LEDs are disconnected from the circuit or the
LEDs fail, the V
LT3465A will then switch at a very low frequency to
8
100mV/DIV
100mV/DIV
2V/DIV
2V/DIV
CTRL
CTRL
FB
FB
OUT
U
PWM
is clamped at 30V. The LT3465 and
200 s/DIV (1kHz)
20 s/DIV (10kHz)
Figure 7a.
Figure 7b.
U
CTRL
LT3465A
W
3465A F07a
3465A F07b
U
minimize the input current. V
output open circuit are shown in the Typical Performance
Characteristics.
Board Layout Consideration
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent electromagnetic interfer-
ence (EMI) problems, proper layout of the high frequency
switching path is essential. Place C
pin. Always use a ground plane under the switching
regulator to minimize interplane coupling. In addition, the
ground connection for the feedback resistor R1 should be
tied directly to the GND pin and not shared with any other
component, ensuring a clean, noise-free connection. Rec-
ommended component placement is shown in Figure 8.
Start-Up Input Current (LT3465A)
As previously mentioned, the LT3465A does not have an
internal soft-start circuit. Inrush current can therefore rise
to approximately 400mA as shown in Figure 9 when
driving 4 LEDs. The LT3465 has an internal soft-start
circuit and is recommended if inrush current must be
minimized.
200mV/DIV
200mV/DIV
2V/DIV
Figure 8. Recommended Component Placement
CTRL
I
FB
IN
R
FB
C
OUT
1
2
3
Figure 9.
50 s/DIV
OUT
6
5
4
and input current during
OUT
L
next to the V
3465A F08
C
IN
3465A F09
GND
V
CTRL
IN
3465af
OUT