SP6887 datasheet(9/10 Pages) EXAR

9 / 10 page

THEORY OF OPERATION

Rev. C – 10/31/2007

SP6887 4 Channel WLED Driver

9/10

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

then

proportional

the

percentage of on-time when the EN pin is

logic

low.

Average

LED

current

approximately equal to:

ILEDAVG = (tON*ILED_ON)/ (tON + tOFF)

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

excellent

thermal

performance.

Figure 4: PCB Lay-out

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