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LT3465/LT3465A

3465afa

lower than the frequency of the PWM signal. R1 needs to

be much smaller than the internal impedance in the CTRL

pin, which is 50k

Ω. A 5k resistor is suggested.

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. With the PWM signal at the CTRL pin to turn

the LT3465A on and off, the output capacitor is charged

and discharged accordingly. This capacitor charging/

discharging affects the waveform at the FB pin. For low

PWM frequencies the output capacitor charging/discharg-

ing time is a very small portion in a PWM period. The

average FB voltage increases linearly with the PWM duty

cycle. As the PWM frequency increases, the capacitor

charging/discharging has a larger effect on the linearity of

the PWM control. Waveforms for a 1kHz and 10kHz PWM

CTRL signals are shown in Figures 7a and 7b respectively.

The capacitor charging/discharging has a larger effect on

the FB waveform in the 10kHz case than that in the 1kHz

APPLICATIO S I FOR ATIO

Figure 7a.

Figure 7b.

100mV/DIV

CTRL

2V/DIV

200

µs/DIV (1kHz)

3465A F07a

100mV/DIV

CTRL

2V/DIV

µs/DIV (10kHz)

3465A F07b

LT3465A

CTRL

PWM

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

LT3465/

LT3465A

CTRL

3465A F06

PWM

100nF

Figure 6. Dimming Control Using a Filtered PWM Signal

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