LT3465/LT3465A

8

3465af

APPLICATIO S I FOR ATIO

Figure 8. Recommended Component Placement

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.

1

L

2

3

6

5

4

GND

3465A F08

CTRL

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

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.

Figure 7a.

Figure 7b.

FB

100mV/DIV

CTRL

2V/DIV

200

µs/DIV (1kHz)

3465A F07a

FB

100mV/DIV

CTRL

2V/DIV

20

µs/DIV (10kHz)

3465A F07b

LT3465A

CTRL

PWM

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.

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

LT3465A will then switch at a very low frequency to

Figure 9.

FB

200mV/DIV

CTRL

2V/DIV

50

µs/DIV

3465A F09

200mV/DIV