FAN5607 Fairchild Semiconductor, FAN5607 Datasheet - Page 10

• Manufacturer Part Number: FAN5607
• Description: LED Driver with Adaptive Charge Pump DC/DC Converter
• Manufacturer: Fairchild Semiconductor

PWM Control
Unless otherwise noted, R
Enable is Controlled by PWM Signal
FAN5607
PWM Control
Once R
(I
trol. By turning the ENABLE pin ON and OFF, the current
can be modulated between 0 to I
I
to be set sufficiently high in order to avoid a flickering effect
(50Hz to 100Hz).
The best LED to LED matching and the purest white light
are achieved over the entire range of average current settings,
when the PWM brightness control is used to modulate the
LED current between zero and the maximum value.
Application Information
Brightness Control
1. Dimming Using PWM at EN Pin
A PWM signal applied to EN can control the LED brightness
in direct dependence on the duty cycle. The recommended
PWM signal frequency is 100Hz to ensure a good match
between the input signal duty cycle and the LED average
current. If this ripple frequency is too low for a particular
noise sensitive application, then DC-based dimming control
circuits or higher-frequency-filtered PWM signals may be
used.
2. Dimming with DC Voltage
The brightness control using a variable DC voltage is shown
in Figure 5. If R1=125kΩ, R2=13.9kΩ, adjusting V
(0V to 0.5V) range results in dimming the LED current from
20mA to 2mA.
10
Average
LED_MAX
value. In PWM mode, the modulating frequency has
SET
), PWM modes can be used for brightness con-
is chosen to set maximum LED current
ENABLE Input (PWM)
I LED
SET
= 12.5KΩ, I
LED_MAX
0mA
Duty Cycle
LED_MAX
30%
1KHz
to achieve any
I
LED
EXT
(Average) = 0.3 x I
= 20mA
in the
LED-MAX
The FAN5607 internal circuit maintains a constant I
age = 0.5V. Adjusting V
accordingly.
Selecting different values for R1, R2 and V
I
tion:
Where 0V < V
3. Dimming Using a Filtered PWM Signal
The external PWM signal is filtered by an R3C network
resulting in a DC component dependent on the PWM signal
duty cycle as shown in Figure 6. The resistor R3 needs to be
much smaller than R2 and the corner frequency of R3C
group is much smaller than the PWM signal frequency.
I
LED
LED
variation range can be changed according to the rela-
=
250
--------- -
R1
I
LED
Duty Cycle
+
1KHz
70%
(Average) = 0.7 x I
Figure 6. Filtered PWM Control
(
-------------------------------------------------- -
EXT
Figure 5. DC Voltage Control
250 500 V
V
EXT
< 0.5V and R1 and R2 are in KΩ.
–
R2
R2
R3
FAN5607
×
EXT
LED-MAX
OFF
EXT
I
C
SET
changes the I
)
R1
PRODUCT SPECIFICATION
mA
FAN5607
R2
REV. 1.0.0 6/22/04
I
SET
EXT
SET
R1
range, the
and I
SET
volt-
LED