lt3465a Linear Technology Corporation, lt3465a Datasheet - Page 10

• Manufacturer Part Number: lt3465a
• Description: 2.4mhz White Led Step-up Converters With Built-in Schottky In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
lower than the frequency of the PWM signal. R1 needs to
be much smaller than the internal impedance in the CTRL
pin, which is 50kΩ. A 5k resistor is suggested.
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
10
Figure 6. Dimming Control Using a Filtered PWM Signal
PWM
U
R1
5k
U
C1
100nF
CTRL
LT3465A
LT3465/
100mV/DIV
100mV/DIV
W
3465A F06
2V/DIV
2V/DIV
CTRL
CTRL
FB
FB
U
PWM
200µs/DIV (1kHz)
20µs/DIV (10kHz)
CTRL
Figure 7a.
Figure 7b.
LT3465A
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
3465A F07a
3465A F07b
3465afa