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SP6887 Datasheet(PDF) 8 Page - Exar Corporation

Part # SP6887
Description  4 CHANNEL REGULATED CHARGE PUMP WLED DRIVER
PDF  10 Pages
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Manufacturer  EXAR [Exar Corporation]
Direct Link  http://www.exar.com
Logo EXAR - Exar Corporation

SP6887 Datasheet(HTML) 8 Page - Exar Corporation

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THEORY OF OPERATION
Rev. C – 10/31/2007
SP6887 4 Channel WLED Driver
© 2007 Exar Corporation
8/10
Resistor Selection”. Method 2 is for setting
the
LED
current
while
allowing
for
brightness control.
ILED (mA) RSET (kΩ)
Standard
Value (kΩ)
Value %
Difference
30
14.7
14.7
0.0%
20
21.8
22.0
0.9%
15
29.3
29.4
0.3%
10
44.1
44.2
0.2%
9
49.4
49.9
1.0%
8
55.7
56.0
0.5%
7
64.2
63.4
0.3%
6
75.0
75.0
0.0%
5
90.0
88.7
-0.3%
4
114.2
115.0
0.7%
3
156.5
158.0
0.9%
2
238.0
237.0
-0.4%
1
540.0
536.0
-0.7%
Table 2: RSET Resistor Selection
Method 2: LED Current Setting Using
a DC Voltage to RSET Pin
The example circuit in Figure 1 uses a
14.7k resistor and an analog input DC
voltage, VSET, which varies from 1.2V to
0V to control LED current from 1mA to
30mA. Table 3 shows the resulting output.
If necessary, the analog VSET voltage can
be sourced from a voltage higher than
1.20V, but the source must be divided
down so that the V mode will not exceed
1.20V. For lower current applications and
for higher resolution, a larger resistor may
be
substituted
in
this
circuit.
PWM
applications are also possible with this
circuit by application of RC filtering.
VSET (V)
ILED (mA)
VSET (V)
ILED (mA)
0.0
30.0
0.7
13.3
0.1
27.7
0.8
10.9
0.2
25.3
0.9
8.4
0.3
22.9
1.0
6.0
0.4
20.5
1.1
3.6
0.5
18.1
1.2
1.1
0.6
15.7
Table 3: Analog Voltage for LED Current
Control
Figure 1: Analog Voltage for LED Current
Control
Method 3: LED Current Setting Using
a PWM Signal to CTRx Pin
The circuit in Figure 2 turns 4 LEDs on and
off by a PWM signal to the CTR0. This
circuit uses resistor RSET to set the on
state current and the average LED current
is, then proportional to the percentage of
on-time when the CTR0 pin is logic low.
Average LED current is approximately
equal to:
ILEDAVG = (tON*ILED_ON)/ (tON + tOFF)
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 2: PWM Signal at CTR0 Pin for LED
Current Control



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