PAM2801 Datasheet, PDF (7/9 Pages) Power Analog Micoelectronics – 1W High Power White LED Driver

Application Information(continued)
Dimming Control Using a PWM Signal to FB
Pin
By using the PWM signal to FB pin as shown in
the Figure 2, the LED turns on or off and its
current operates at either 0mA or the set
maximum current. The PWM signal can be
considered as an adjustable DC voltage. As the
PWM duty cycle increases, the LED current
decreases. Typically, the PWM frequency ranges
from 5KHz to 40KHz.
PAM2801
FB
10KΩ
R1
PWM
22kΩ
0.1uF
R2
1KΩ
1W
WLED
R_S
0.27Ω
Figure 2. Dimming Control Using a PWM Signal
to FB Pin
Dimming Control Using a PWM Signal to
SHDN Pin
With the PWM signal applied to the SHDN pin of
the PAM2801, the LED turns on or off. When the
SHDN pin is high, the LED turn on and its current
is at the set maximum current; when the SHDN
pin is low, the LED turn off and its current turns to
0mA. The average LED current increase
proportionally with the PWM duty cycle. A 0%
duty cycle produces 0mA of LED current; a 100%
duty cycle corresponds to the set maximum
current. The magnitude of the PWM signal
should be higher than the SHDN input high (V ) _SH .
The typical frequency of the PWM signal ranges
from 100Hz to 1KHz. Figure 3 shows the LED
current with the PWM duty cycle set to 50% and
the PWM frequency set respectively to 100Hz
and 1KHz. Figure 4 shows the LED current
versus the PWM duty cycle, setting the PWM
frequency to 100Hz.
PAM2801
1W High Power White LED Driver
SHDN
FB
I_LED
（3a）100Hz
SHDN
FB
I_LED
（3b）1 KHz
Figure 3. Dimming Control Using a PWM Signal
to SHDN Pin
400
350
300
250
200
150
100
50
0
0
LED Current vs Duty Cycle
20
40
60
80
100
Duty Cycle(%)
Figure 4. LED Current vs Duty Cycle
Power Analog Microelectronics,Inc
www.poweranalog.com
7
08/2007