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RT9271 Datasheet(PDF) 10 Page - Richtek Technology Corporation

Part # RT9271
Description  White LED Step-Up Converter in Tiny Package
PDF  15 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT9271 Datasheet(HTML) 10 Page - Richtek Technology Corporation

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RT9271
10
DS9271-13 March 2007
www.richtek.com
Figure 6. Recommended Soft-Start Circuit
Application Information
LED Current Control
The RT9271 regulates the LED current by setting the current
sense resistor (R2) connecting to feedback and ground.
The internal feedback reference voltage is 0.25V. The LED
current can be set from following equation easily.
In order to have an accurate LEDcurrent, precision resistors
are preferred (1% is recommended). The table for R2
selection is shown below.
LED
I
V
25
.
0
2
R
=
Dimming Control
a. Using a PWM Signal to EN Pin
For controlling the LEDbrightness, the RT9271 can perform
the dimming control by applying a PWM signal to EN pin.
The average LED current is proportional to the PWM signal
duty cycle. The magnitude of the PWM signal should be
higher than the maximum enable voltage of EN pin, in order
to let the dimming control perform correctly.
b. Using a DC Voltage
Using a variable DC voltage to adjust the brightness is a
popular method in some applications. The dimming control
using a DC voltage circuit is shown in Figure 5. According
to the Superposition Theorem, as theDC voltage increases,
the voltage contributed to VFB increases and the voltage
drop on R2 decreases, i.e. the LED current decreases.
For example, if the VDC range is from 0V to 2.8V, the
selection of resistors in Figure 5 sets dimming control of
LED current from 20mA to 0mA.
Figure 5. Dimming Control Using a DC Voltage
ILED (mA)
R2 (Ω)
5
49.9
10
24.9
12
21
15
16.5
20
12.4
R2 Resistor Value Selection
Figure 4. PWM Dimming Control Using the EN Pin
Condition
Inductance (H) Schottky Diode
2 WLEDs
4.7u~10u
SS0520
3 WLEDs
4.7u~10u
SS0520
Recommended Inductance and Rectifier (for Li-Ion cell)
VCC
EN
GND
LX
FB
RT9271
PWM signal
C1
1uF
C3
1uF
D1
SS0520
LX
10uH
R2
12
OVP
D2
D4
D3
V
IN
2.4 to 6V
VCC
EN
GND
LX
FB
RT9271
C1
1uF
C3
1uF
LX
10uH
R2
12
OVP
D2
D4
D3
VDC Dimmimg
0 to 2.8V
R4
82k
R3
6.8k
D1
SS0520
V
IN
2.4 to 6V
VCC
EN
GND
LX
FB
RT9271
PWM signal
C1
1uF
C3
1uF
D1
SS0520
LX
10uH
R2
12
OVP
D2
D4
D3
C6
10nF
R6
10k
R5
1k
V
IN
2.4 to 6V



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