RT9285C
Tiny Package, High Performance, Diode Embedded White
LED Driver
General Description
The RT9285C is a
high frequency asynchronous boost
converter with internal diode, which can support 2 to 5
White LEDs for backlighting and OLED power supply. The
Internal soft start function can reduce the inrush current.
The device operates with 1MHz fixed switching frequency
to allow small external components and to simplify possible
EMI problems. The device comes with 20V over voltage
protection to allow inexpensive and small-output capacitors
with lower voltage rating. The LED current is initially set
with the external sense resistor R
SET
, and the feedback
voltage is 250mV. Tiny package type TSOT-23-6,
XDFN-8L 2x2 and WDFN-8L 2x2 packages provide the best
solution for PCB space saving and total BOM cost.
Features
V
IN
Operating Range : 2.7V to 5.5V
Up to 85% Efficiency
22V Internal Power NMOS
1MHz Switching Frequency
Built-in Diode
Digital Dimming with Zero-Inrush
Input UVLO Protection
Output Over Voltage Protection
Internal Soft Start and Compensation
TSOT-23-6, 8-Lead XDFN and WDFN Package
RoHS Compliant and 100% Lead (Pb)-Free
Applications
Cellular Phones
Digital Cameras
PDAs and Smart Phones
Porbable Instruments
MP3 Player
OLED Power
Ordering Information
RT9285C
Package Type
QW : WDFN-8L 2x2 (W-Type)
QX : XDFN-8L 2x2 (X-Type)
J6 : TSOT-23-6
Lead Plating System
P : Pb Free
G : Green (Halogen Free and Pb Free)
Z : ECO (Ecological Element with
Halogen Free and Pb free)
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Pin Configurations
(TOP VIEW)
GND
LX
NC
PGND
1
2
3
4
8
7
6
5
9
FB
EN
VOUT
VDD
XDFN/WDFN-8L 2x2
VDD VOUT EN
6
5
2
LX
GND
4
3
FB
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
TSOT-23-6
DS9285C-03
March 2011
GND
www.richtek.com
1
RT9285C
Typical Application Circuit
L1
10µH to 22µH
V
IN
2.7V to 5.5V
RT9285C
LX
EN
Chip Enable
GND
PGND
FB
VDD
VOUT
C1
1µF
C2
0.22µF to 1µF
R
SET
12.5
PGND pin for XDFN/WDFN-8L Packages
Function Pin Description
Pin No.
XDFN/WDFN-8L TSOT-23-6
1,
9 (Exposed Pad)
2
3
4
5
6
7
2
1
--
--
6
5
4
Pin Name
GND
LX
NC
PGND
VDD
VOUT
EN
Pin Function
Ground Pin.
The exposed pad must be soldered to a large PCB and
connected to GND for maximum power dissipation.
LX Pin. Connect this Pin to an inductor. Minimize the track area to
reduce EMI.
No Internal Connection.
Power Ground Pin.
Supply Input Voltage Pin. Bypass 1μF capacitor to GND to reduce the
input ripple.
Output Voltage pin. The pin internally connects to OVP diode to limit
output voltage while LEDs are disconnected.
Chip Enable (Active High). Note that this pin has an internal pull-down
resistance around 300kΩ.
Feedback Pin. Series connecting a resistor between WLED and
8
3
FB
ground as a current sense. Sense the current feedback voltage to set
the current rating.
0.5µs < t
HI
0.5µs < t
LO
< 300µs
EN
0
1
2
3
4
5
14
15
0
1
100%
15/16
Shutdown
Shutdown
I
WLED
100% 15/16
14/16 13/16
12/16
3/16
2/16
1/16
Figure 1. Operation of Digital Pulse Dimming Control
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2
DS9285C-03
March 2011
RT9285C
Function Block Diagram
1.0MHz OSC
Current
Sense
Slope
Compensation
OCP
LX
VOUT
OVP
PWM
Logic
VDD
UVLO/P
GOOD
Timer
Dimming
Controller
-
+
+
-
Soft Start/
Clamping
PGND
EN
FB
V
REF
GND
Operation
Soft-Start
The Soft-Start function is made by clamping the output
voltage of error amplifier with another voltage source that
is increased slowly from zero to near V
IN
in the Soft-Start
period. Therefore, the duty cycle of the PWM will be
increased from zero to maximum in this period. The soft-
start time is decided by a timer of 1.5ms. The charging
time of the inductor will be limited as the smaller duty so
that the inrush current can be reduced to an acceptable
value.
Over Voltage Protection
The Over Voltage Protection is detected by a junction
breakdown detecting circuit. Once V
OUT
goes over the
detecting voltage, LX pin stops switching and the power
NMOS is turned off. Then, the V
OUT
is clamped to be near
V
OVP
.
LED Current Setting
The RT9285C regulates
the LED current by setting the
current sense resistor (R
SET
) connecting to feedback and
ground. The internal feedback reference voltage is 0.25V.
The LED current can be set from following equation easily.
I
LED
(mA) = 0.25/R
SET
In order to have an accurate LED current, precision resistors
are preferred (1% is recommended). The table for R
SET
selection is shown below.
Table 1. R
SET
Value Selection
I
LED
(mA)
5
10
12
15
20
R
SET
(Ω)
49.9
24.9
21
16.5
12.4
Digital Pulse Dimming Control
RT9285C implements the pulse dimming method being
used to control the brightness of white LEDs. There are
16 steps to set the current of white LEDs. The maximum
LED current is up to 20mA that is sufficient for most
application in backlight. The detail operation of brightness
dimming is showed in the Figure 1.
Current Limiting
The current flow through the inductor as charging period is
detected by a current sensing circuit. As the value over
the current limiting, the NMOS will be turned-off so that
the inductor will be forced to leave charging stage and
enter discharging stage. Therefore, the inductor current
will not increase over the current limiting.
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3
DS9285C-03
March 2011
RT9285C
Absolute Maximum Ratings
(Note 1)
Supply Voltage, V
IN
---------------------------------------------------------------------------------------------------
−0.3
to 6V
LX Input Voltage -------------------------------------------------------------------------------------------------------
−
0.3V to 22V
Output Voltage ---------------------------------------------------------------------------------------------------------
−
0.3V to 21V
The other pins ----------------------------------------------------------------------------------------------------------
−
0.3V to 6V
Power Dissipation, P
D
@ T
A
= 25°C
TSOT23-6 --------------------------------------------------------------------------------------------------------------- 0.392W
XDFN/WDFN-8L 2x2 -------------------------------------------------------------------------------------------------- 0.606W
Package Thermal Resistance (Note 2)
TSOT23-6,
θ
JA
---------------------------------------------------------------------------------------------------------- 255°C/W
XDFN/WDFN-8L 2x2,
θ
JA
--------------------------------------------------------------------------------------------- 165°C/W
XDFN/WDFN-8L 2x2,
θ
JC
-------------------------------------------------------------------------------------------- 20°C/W
Junction Temperature ------------------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260°C
Storage Temperature Range ----------------------------------------------------------------------------------------
−
65°C to 150°C
Recommended Operating Conditions
(Note 3)
Junction Temperature Range ----------------------------------------------------------------------------------------
−
40°C to 125°C
Ambient Temperature Range ----------------------------------------------------------------------------------------
−
40°C to 85°C
Electrical Characteristics
(V
IN
= 3.7V, FREQ left floating, T
A
= 25°C, Unless Otherwise specification)
Parameter
System Supply Input
Operation voltage Range
Under Voltage Lock Out
Quiescent Current
Supply Current
Shut Down Current
Output
Line Regulation
Oscillator
Operation Frequency
Maximum Duty Cycle
Reference Voltage
Feedback Reference Voltage
Diode
Forward Voltage
MOSFET
On Resistance of MOSFET
Protection
OVP Threshold
OCP
Symbol
V
IN
V
UVLO
I
Q
I
IN
I
SHDN
Test Conditions
Min
2.7
1.7
Typ
--
2
300
--
2
--
1
90
0.25
0.9
0.75
20
400
Max
5.5
2.3
450
2
5
3
--
--
0.263
--
1
--
--
Unit
V
V
μA
mA
μA
%
MHz
%
V
V
Ω
V
mA
FB = 1.5V, No switch
FB = 0V, Switch
V
EN
< 0.4V
V
IN
= 3V to 4.3V
--
--
--
--
--
85
f
OSC
V
REF
V
FW
R
DS(ON)
V
OVP
I
FW
= 100mA
0.237
--
0.5
--
--
To be continued
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4
DS9285C-03
March 2011
RT9285C
Parameter
Control Interface
EN Threshold
Logic-Low Voltage
V
IL
T
LO
T
HI
T
SHDN
Refer to Figure 1
Refer to Figure 1
Refer to Figure 1
--
1.4
0.5
0.5
1
--
--
--
--
--
0.4
--
300
--
--
Logic-High Voltage V
IH
V
μs
μs
ms
Symbol
Test Conditions
Min
Typ
Max
Unit
EN Low Time for Dimming
Delay Between Steps Time
EN Low Time for Shut Down
Note 1.Stresses
listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for
stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended
periods may remain possibility to affect device reliability.
Note 2.
θ
JA
is measured in the natural convection at T
A
= 25°C on a low effective thermal conductivity test board of
JEDEC 51-3 thermal measurement standard.
Note 3.
The device is not guaranteed to function outside its operating conditions.
DS9285C-03
March 2011
www.richtek.com
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