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参数资料
| 型号： | SP6887EB |
| 厂商： | Exar Corporation |
| 文件页数： | 8/10页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR SP6887 |
| 标准包装： | 1 |
| 系列： | * |
THEORY OF OPERATION
Resistor Selection”. Method 2 is for setting
the LED current while allowing for
brightness control.
ILED (mA) RSET (k ? )
Standard Value %
Value (k ? ) Difference
30
20
15
10
9
8
14.7
21.8
29.3
44.1
49.4
55.7
14.7 0.0%
22.0 0.9%
29.4 0.3%
44.2 0.2%
49.9 1.0%
56.0 0.5%
7
6
5
4
64.2
75.0
90.0
114.2
63.4 0.3%
75.0 0.0%
88.7 -0.3%
115.0 0.7%
Figure 1: Analog Voltage for LED Current
Control
3
2
1
156.5 158.0 0.9%
238.0 237.0 -0.4%
540.0 536.0 -0.7%
Table 2: R SET Resistor Selection
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
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.
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:
I LEDAVG = (t ON *I LED_ON )/ (t ON + t OFF )
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.
VSET (V)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
ILED (mA)
30.0
27.7
25.3
22.9
20.5
18.1
15.7
VSET (V)
0.7
0.8
0.9
1.0
1.1
1.2
ILED (mA)
13.3
10.9
8.4
6.0
3.6
1.1
Table 3: Analog Voltage for LED Current
Control
Figure 2: PWM Signal at CTR0 Pin for LED
Current Control
Rev. C – 10/31/2007
SP6887 4 Channel WLED Driver
8/10
? 2007 Exar Corporation
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