sp6887 Exar Corporation, sp6887 Datasheet - Page 8

• Manufacturer Part Number: sp6887
• Description: 4 Channel Regulated Charge Pump Wled Driver
• Manufacturer: Exar Corporation

Resistor Selection”. Method 2 is for setting
the
brightness control.
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
applications are also possible with this
circuit by application of RC filtering.
Rev. C – 10/31/2007
ILED (mA) RSET (kΩ)
VSET (V)
Table 3: Analog Voltage for LED Current
0.0
0.1
0.2
0.3
0.4
0.5
0.6
30
20
15
10
9
8
7
6
5
4
3
2
1
substituted
LED
Table 2: R
ILED (mA)
current
114.2
156.5
238.0
540.0
14.7
21.8
29.3
44.1
49.4
55.7
64.2
75.0
90.0
30.0
27.7
25.3
22.9
20.5
18.1
15.7
SET
Control
in
Resistor Selection
while
Value (kΩ)
Standard
this
VSET (V)
115.0
158.0
237.0
536.0
14.7
22.0
29.4
44.2
49.9
56.0
63.4
75.0
88.7
0.7
0.8
0.9
1.0
1.1
1.2
circuit.
allowing
SP6887 4 Channel WLED Driver
ILED (mA)
Difference
Value %
-0.3%
-0.4%
-0.7%
0.0%
0.9%
0.3%
0.2%
1.0%
0.5%
0.3%
0.0%
0.7%
0.9%
13.3
10.9
8.4
6.0
3.6
1.1
PWM
for
8/10
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:
The
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
Figure 1: Analog Voltage for LED Current
I
LEDAVG
recommended
= (t
THEORY OF OPERATION
Current Control
ON
*I
Control
LED_ON
© 2007 Exar Corporation
PWM
)/ (t
ON
frequency
+ t
OFF
)
is