RT9271
White LED Step-Up Converter in Tiny Package
General Description
The RT9271 is a step-up DC/DC converter specifically
designed to drive white LEDs with a constant current. The
device can drive one
to three LEDs in series from a Li-Ion
cell. Series connection of the LEDs provides
identical LED
currents resulting in uniform brightness and eliminating
the need for ballast resistors. The RT9271 switches at 1.1
MHz, allowing the use of tiny external components. The
input and output capacitor can be as small as 1uF, saving
space and cost versus alternative solutions. A low 0.25V
feedback voltage minimizes power loss in the current
setting resistor for better efficiency.
The RT9271 is available in low profile SOT-23-6 package.
Features
Inherently Matched LED Current
Up to 80mA Output Current @ V
OUT
< 12V
High Efficiency : 85% Typical
Drives Up to Three LEDs from 2.8V Supply
20V Internal Switch
Fast 1.1 MHz Switching Frequency
Uses Tiny 1 mm Height Inductors
Requires Only 1uF Output Capacitor
Low Profile
SOT-23-6
Package
Optional 15V Over Voltage Protection
RoHS Compliant and 100% Lead (Pb)-Free
Applications
Mobile Phone
Digital Still Camera
PDAs, Handheld Computers
MP3 Players
GPS Receivers
Ordering Information
RT9271
Package Type
B : SOT-23-5
E : SOT-23-6
Operating Temperature Range
P : Pb Free with Commercial Standard
G : Green (Halogen Free with Commer-
cial Standard)
Pin Configurations
(TOP VIEW)
VCC
5
1
LX
2
GND
EN
4
3
FB
VCC OVP
6
1
LX
5
2
GND
EN
4
3
FB
Note :
RichTek Pb-free and Green 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.
100% matte tin (Sn) plating.
SOT-23-5
SOT-23-6
Marking Information
For marking information, contact our sales representative
directly or through a RichTek distributor located in your
area, otherwise visit our website for detail.
Note
: There is no pin1 indicator on top mark for
SOT-23-6
type, and pin1 will be lower left pin when reading top mark
from left to right.
DS9271-13 March 2007
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1
RT9271
Typical AppIication Circuit
V
IN
2.4 to 3.2V
C1
1uF
LX
10uH
D1
SS0520
VCC
EN
Dimming
Control
LX
OVP
FB
C3
1uF
D2
GND
R2
12
Figure 1. RT9271 Drivers 1 WLED Application Circuit
C1
1uF
LX
10uH
D1
SS0520
V
IN
2.4 to 5V
VCC
EN
Dimming
Control
LX
OVP
FB
C3
1uF
D2
D3
GND
R2
12
Figure 2. RT9271 Drivers 2 Series WLEDs Application Circuit
V
IN
2.4 to 6V
C1
1uF
LX
10uH
D1
SS0520
D2
VCC
EN
Dimming
Control
LX
OVP
FB
C3
1uF
D3
D4
GND
R2
12
Figure 3. RT9271 Drivers 3 Series WLEDs Application Circuit
Note : 1. D1 is Schottky diode (SS0520).
2. D2 ~ D4 are the WLED (HT-S91CW-DT) of HARVATEK.
3. LX is the SH4018 series of ABC TAIWAN ELECTRONICS CORP.
Recommended Circuits for Driving LEDs
Figure 1 to Figure 3 illustrates the recommended application circuits for driving white LEDs. The series connected LEDs
are driven with identical current to emit uniform luminescence, and the 250mV low reference voltage can minimize the
efficiency loss across the current-sensing resistor. The recommended current setting for driving white LEDs is 10mA to
20mA, and the dimming control can be implemented by toggling EN pin with 60Hz to 1kHz PWM clock. Please refer to
application notes for guidance of component selection and board layout.
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DS9271-13 March 2007
RT9271
Functional Pin Description
Pin No.
−XB
1
2
3
4
−
5
−XE
1
2
3
4
5
6
Pin Name Pin Function
LX
GND
FB
EN
OVP
VCC
Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI.
Ground Pin. Connect directly to local ground plane.
Feedback Pin. Reference voltage is 0.25V. Connect cathode of lowest LED and resistor
here. Calculate resistor value according to the formula: R
FB
= 0.25/I
LED
Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.4V or less to disable
device.
Over Voltage Protection Pin. Voltage sensing input to trigger the function of over voltage
protection, the trip point is 15.5V. Leave it unconnected to disable this function.
Input Voltage Pin. Must be locally bypass with 1uF capacitor to GND.
Function Block Diagram
OVP
15.5V
LX
FB
-
+
V
REF
0.25V
A1
-
+
COMPARATOR
A2
R
S
Q
DRIVER
M1
VCC
EN
CHIP
ENABLE
4μA
RAMP
GENERATOR
1.1MHz
OSCILLATOR
Operation
The RT9271 is a constant frequency step-up converter with an internal switch. For excellent line and load regulation, the
current mode control is adopted. The operations of RT9271 can be understood from block diagram clearly. The oscillator
triggers the SET input of SR latch to turn on the power switch M1 at the start of each cycle. A current sense voltage sum
with a stabilizing ramp is connected to the positive terminal of the PWM comparator A2. When this voltage exceeds the
output voltage of the error amplifier A1, the SR latch is reset to turn off the power switch till next cycle starts. The output
voltage of the error amplifier A1 is amplified from the difference between the reference voltage 0.25V and the feedback
voltage. In this manner, if the error amplifiers voltage increases, more current is delivered to the output; if it decreases,
less current is delivered. A 15.5V Zener diode connects from OVP pin to FB pin internally to provide an optional protection
function which prevents LX pin from over-voltage damage. Especially when the case of the feedback loop broken due to
component wear-out or improper connection occurs. The behavior of OVP is to clamp the output voltage to 15.5V
typically. This function is suitable for the applications while driving white LEDs less than 4 in series.
DS9271-13 March 2007
+
-
0.75Ω
GND
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RT9271
Absolute Maximum Ratings
(Note 1)
Supply Voltage, V
CC
--------------------------------------------------------------------------------------------------
−0.3V
to 7V
LX, OVP -----------------------------------------------------------------------------------------------------------------
−0.3V
to 21V
The Other Pins ---------------------------------------------------------------------------------------------------------
−0.3V
to 7V
Power Dissipation, P
D
@ T
A
= 25°C
SOT-23-6 ---------------------------------------------------------------------------------------------------------------- 0.4W
Package Thermal Resistance (Note 4)
SOT-23-6,
θ
JA
-----------------------------------------------------------------------------------------------------------
250°C/W
Maximum Junction Temperature ----------------------------------------------------------------------------------- 125°C
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260°C
Storage Temperature Range ----------------------------------------------------------------------------------------
−
65°C to 150°C
ESD Susceptibility (Note 2)
HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV
MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 3)
Supply Voltage, V
CC
-------------------------------------------------------------------------------------------------- 2.4V to 6V
Junction Temperature Range ----------------------------------------------------------------------------------------
−40°C
to 125°C
Electrical Characteristics
(V
CC
= 3.6V, T
A
= 25°C, unless otherwise specified.)
Parameter
System Supply Input
Under Voltage Lock Out
Maximum Output Voltage
Supply Current
Quiescent Current
Shut Down Current
Oscillator
Operation Frequency
Maximum Duty Cycle
Reference Voltage
Feedback Voltage
MOSFET
On Resistance of MOSFET
Current Limit
Current Limit
Control and Protection
Shut Down Voltage
Enable Voltage
EN Pin Pull Low Current
OVP Threshold (Note 5)
Symbol
UVLO
I
CC1
I
CC2
I
CC3
F
OSC
Dmax
V
FB
Rds(on)
I
max1
I
max2
V
EN1
V
EN2
I
EN
OVP
Test Condition
Min
1.8
--
Typ
2.2
--
--
90
0.1
1.1
90
0.25
0.75
900
625
0.8
0.8
4
15.5
Max
2.3
20
2
120
1
1.3
--
0.263
1.0
1000
750
--
1.4
6
20.0
Units
V
V
mA
μA
μA
MHz
%
V
Ω
mA
mA
V
V
μA
V
V
CC
=6V, Continuously Switching
V
CC
=6V, FB=1.3V, No Switching
V
CC
=6V, V
EN
<0.4V
--
50
--
0.9
85
0.237
0.5
Normal Operation
Start up Condition
800
500
0.4
--
--
14.5
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DS9271-13 March 2007
RT9271
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.
Devices are ESD sensitive. Handling precaution recommended.
Note 3.
The device is not guaranteed to function outside its operating conditions.
Note 4.
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 5.
Floating the OVP pin to disable OVP function.
DS9271-13 March 2007
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