SP6887ER4-L Exar Corporation, SP6887ER4-L Datasheet - Page 9

• Manufacturer Part Number: SP6887ER4-L
• Description: IC, LED DRIVER, CHARGE PUMP, TQFN-16
• Manufacturer: Exar Corporation

Specifications of SP6887ER4-L

• Led Driver Application: LCD Backlighting, LCD Modules, Digital Cameras
• No. Of Outputs: 4
• Output Current: 30mA
• Input Voltage: 2.7V To 5.5V
• Dimming Control Type: PWM / DC
• Topology: Charge Pump
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
percentage of on-time when the EN pin is
logic
approximately equal to:
Similarly,
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.
Rev. C – 10/31/2007
Figure 3: PWM Signal at EN Pin for LED
I
LEDAVG
low.
is
= (t
the
then
Current Control
Average
ON
*I
LED_ON
recommended
proportional
LED
)/ (t
ON
current
+ t
SP6887 4 Channel WLED Driver
OFF
to
)
PWM
the
is
9/10
PCB Layout
When the driver is in the 1.5X charge
pump
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
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
performance.
(metalized
resulting
mode,
Figure 4: PCB Lay-out
THEORY OF OPERATION
hole)
in
the
© 2007 Exar Corporation
excellent
1MHz
centered
switching
thermal
in
the