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THEORY OF OPERATION
Method 4: LED Current Setting Using
a PWM Signal to EN Pin
The 4 LEDs turn on and off by applying a
PWM signal to the EN pin as shown in
figure 3. The circuit is the same as to the
method 3, using a resistor RSET to set the
on state current and the average LED
current is then proportional to the
percentage of on-time when the EN pin is
logic low. Average LED current is
approximately equal to:
I LEDAVG = (t ON *I LED_ON )/ (t ON + t OFF )
Similarly, the recommended PWM
frequency is between 100Hz and 1KHz.
Due to start up delay and ramp up time,
frequency >1KHz will result in error in the
average value of ILED. Frequency <100Hz
can naturally cause the LEDs to blink
visibly.
Figure 3: PWM Signal at EN Pin for LED
Current Control
PCB Layout
When the driver is in the 1.5X charge
pump mode, the 1MHz switching
frequency operation require to minimize
the trace length and impedance to ground
on all 4 capacitors. A ground plane should
cover the area on the bottom side of the
PCB opposite to the IC and the bypass
capacitors. Capacitors CIN and Co require
to short connection to ground which can
be done with multiple vias as shown on
Figure4.
Square copper area matches the QFN 16
exposed pad (GND) which is connected by
a trace to the pin 12 pad (GND). A large
via (metalized hole) centered in the
square pad provides a low impedance
connection to the ground plane on the
opposite side of the PCB and allows the
heat dissipated by the driver IC to spread
out resulting in excellent thermal
performance.
Figure 4: PCB Lay-out
Rev. C – 10/31/2007
SP6887 4 Channel WLED Driver
9/10
? 2007 Exar Corporation
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