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RT9285BGJ6

P1-P3P4-P6P7-P9P10-P12P13-P13
RT9285A/B
10
DS9285A/B-06 August 2012www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
LED Current Control
The RT9285A/B regulates the LED current by setting the
current sense resistor (R
SET
) connecting to feedback and
ground. The RT9284A/B feedback voltage (V
FB
) is 0.25V.
The LED current (I
LED
) can be set by a resistor R
SET
.
I
LED
= 0.25/R
SET
In order to have an accurate LED current, a precision
resistor is preferred (1% is recommended).
Inductor Selection
The recommended value of inductor for 4 to 5WLEDs
applications are 10μH to 22μH. For 3WLEDs, the
recommended value of inductor is 4.7μH to 22μH. Small
size and better efficiency are the major concerns for
portable device, such as RT9285A/B used for mobile
phone. The inductor should have low core loss at 1MHz
and low DCR for better efficiency.
Figure 3. Application for Driving 4 Series WLEDs
Figure 4. Application for Driving 5 Series WLEDs
The inductor saturation current rating should be considered
to cover the inductor peak current.
Capacitor Selection
Input and output ceramic capacitors of 1μF are recommend-
ed for RT9285A/B applications. For better voltage filtering,
ceramic capacitors with low ESR are recommended. X5R
and X7R types are suitable because of their wider voltage
and temperature ranges.
Output Voltage Control
The output voltage of R9285 can be adjusted by the divider
circuit on FB pin. Figure 6 shows a 2-level voltage control
circuit for OLED application. The output voltage can be
calculated by the following equations in Figure 6.
Figure 5. Application for Constant Output Voltage
OUT
R1 R2
V 0.25 ; R2 10k
R2
+
=× >
Figure 6. Application Circuit for Output Voltage Control
and Related Equations
LX
GND
VDD
FB
VOUT
RT9285A/B
EN
C1
1µF
R
SET
L1
C2
Chip Enable
V
IN
10µH to 22µH
2.7V to 5.5V
0.22µF to 1µF
12.5
LX
GND
VDD
FB
VOUT
RT9285A/B
EN
C1
1µF
R
SET
L1
C2
Chip Enable
V
IN
10µH to 22µH
2.7V to 5.5V
0.22µF to 1µF
12.5
LX
GND
VDD
FB
VOUT
RT9285A/B
EN
C1
1µF
R2
L1
C2
Chip Enable
V
IN
10µH to 22µH
2.7V to 5.5V
0.22µF to 1µF
10k
R1
590k
V
OUT
15V
VDD
EN
VOUT
GND
FB
RT9285A/B
LX
R
GPIO
V
IN
GPIO
OLED
R
A
R
B
V
EN
RT9285A/B
11
DS9285A/B-06 August 2012 www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Figure 7. TOP
Figure 8. Bottom
LX
GND
FB
EN
VOUT
VDD
4
2
3
5
61
EN
C1
L1
WLEDs
GND
C2
VIN
*1
*2
*3
*4
R
SET
` LX node copper area should be minimized for reducing
EMI. (*1)
` The input capacitor C1 should be placed as closed as
possible to Pin 6. (*2)
` The output capacitor C2 should be connected directly
from the Pin 5 to ground rather than across the LEDs.
(*3)
` FB node copper area should be minimized and keep
far away from noise sources (Pin 1, Pin 5, Pin 6). (*4)
` The Inductor is far away receiver and microphone.
` The voice trace is far away RT9285.
` The embedded antenna is far away and different side
RT9285.
` R1 should be placed as close as RT9285.
` The through hole of RT9285's GND pin is recommended
as large and many as possible.
Considering the output voltage deviation from the GPIO
voltage tolerance, as GPIO voltage vibrated by 0 ± 50mV
and 1.8(2.8) ±5% ,the output voltage could be kept within
±2.5%.
Layout guide
` A full GND plane without gap break.
` Traces in bold need to be routed first and should be
kept as short as possible.
` VDD to GND noise bypass : Short and wide connection
for the 1μF MLCC capacitor between Pin 6 and Pin 2.
V
OUT
= R
A
x {(FB/R
B
) + (FB-GPIO)/R
GPIO
} + FB (1)
As GPIO = 0V,
V
OUT
= R
A
x {(0.25/R
B
) + (0.25/R
GPIO
)} + 0.25 (2)
As GPIO = 2.8V,
V
OUT
= R
A
x {(0.25/R
B
) + (0.25-2.8)/R
GPIO
)} + 0.25
(3)
As GPIO = 1.8V, V
OUT
= R
A
x {(0.25/R
B
) + (0.25-1.8)/
R
GPIO
)} + 0.25 (4)
For Efficiency Consideration :
Set R
A
= 990kΩ,
If 2 levels are 16V (GPIO = 0V) and 14V (GPIO = 1.8V)
Get R
B
= 16kΩ, R
GPIO
= 890kΩ
Table 3. Suggested Resistance for Output Voltage
Control
Conditions
R
A
(kΩ)
R
B
(kΩ)
R
GPIO
(kΩ)
Case A :
Normal Voltage = 16V
(GPIO = 0V)
Dimming Voltage = 12V
(GPIO = 1.8V)
1100 18 495
Case B :
Normal Voltage = 16V
(GPIO = 0V)
Dimming Voltage = 12V
(GPIO = 2.8V)
1200 19.5 840
RT9285A/B
12
DS9285A/B-06 August 2012www.richtek.com
©
Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Outline Dimension
TSOT-23-6 Surface Mount Package
Dimensions In Millimeters Dimensions In Inches
Symbol
Min Max Min Max
A 0.700 1.000 0.028 0.039
A1 0.000 0.100 0.000 0.004
B 1.397 1.803 0.055 0.071
b 0.300 0.559 0.012 0.022
C 2.591 3.000 0.102 0.118
D 2.692 3.099 0.106 0.122
e 0.838 1.041 0.033 0.041
H 0.080 0.254 0.003 0.010
L 0.300 0.610 0.012 0.024
A
A1
e
b
B
D
C
H
L
P1-P3P4-P6P7-P9P10-P12P13-P13

RT9285BGJ6

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IC LED DRIVER RGLTR DIM TSOT23-6
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