Richtek RT9271GB, RT9271GE, RT9271PB, RT9271PE Schematic [ru]

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-26 package.

Ordering Information

RT9271

Package Type B : SOT-25 E : SOT-26

Operating Temperature Range C : Commercial Standard P : Pb Free with Commercial Standard

Features

 Inherently Matched LED Current

 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-26 Package

 Optional 15V Over Voltage Protection

Applications

 Mobile Phone

 Digital Still Camera

 PDAs, Handheld Computers

 MP3 Players

 GPS Receivers

Pin Configurations

(TOP VIEW)

VCC

Marking Information

For marking information, contact our sales representative

GND

FB CE

354

GND

235

FB CE

directly or through a RichTek distributor located in your

area, otherwise visit our website for detail.

SOT-25

SOT-26

Functional Pin Description

Pin No.

−−−−XB −−−−XE

1 1 LX Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI.

2 2 GND Ground Pin. Connect directly to local ground plane.

3 3 FB Feedback Pin. Reference voltage is 0.25V. Connect cathode of lowest LED and resistor

4 4 CE Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.4V or less to disable

− 5 OVP Over Voltage Protection Pin. Voltage sensing input to trigger the function of over voltage

5 6 VCC Input Voltage Pin. Must be locally bypass with 1uF capacitor to GND.

Pin Name Pin Function

here. Calculate resistor value according to the formula: R

device.

protection, the trip point is 15.5V. Leave it unconnected to disable this function.

= 0.25/I

LED

OVP

DS9271-07 April 2004 www.richtek.com

RT9271

Typical AppIication Circuit

SS0520

2.4 to 3.2V

1uF

Dimming

Control

VCC

GND

10uH

OVP

Figure 1. RT9271 Drivers 1 WLED Application Circuit

2.4 to 5V

1uF

Dimming

Control

VCC

GND

10uH

OVP

SS0520

1uF

12Ω

Figure 2. RT9271 Drivers 2 Series WLEDs Application Circuit

10uH

VCC

GNDLXFB

2.4 to 6V

1uF

Dimming

Control

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.

OVP

SS0520

1uF

12Ω

DS9271-07 April 2004www.richtek.com

RT9271

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 CE pin with 60Hz to 1KHz PWM clock. Please

refer to application notes for guidance of component selection and board layout.

Function Block Diagram

OVP

REF

0.25V

15.5V

CHIP

ENABLE

4µA

RAMP

GENERATOR

1.1MHz

OSCILLATOR

COMPARATOR

R Q

DRIVER

VCC

0.75Ω

GND

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.

DS9271-07 April 2004 www.richtek.com

RT9271

Absolute Maximum Ratings (Note 1)

 Supply Voltage, V

 LX, OVP ----------------------------------------------------------------------------------------------------------------- −0.3V to 21V



The Other Pins -------------------------------------------------------------------------------------------------------- −0.3V to 7V

 Maximum Junction Temperature ---------------------------------------------------------------------------------- 125°C  Operating Junction Temperature Range ------------------------------------------------------------------------ −40°C to 125°C

 Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------- 260° C

 Storage Temperature Range --------------------------------------------------------------------------------------- −65° C to 150° C

 ESD Susceptibility (Note 2)

HBM ---------------------------------------------------------------------------------------------------------------------- 2kV

MM ------------------------------------------------------------------------------------------------------------------------ 200V

Recommended Operating Conditions(Note 3)

 Supply Voltage, V

Electrical Characteristics

= 3.6V, T

System Supply Input

Under Voltage Lock Out UVLO 1.8 2.2 2.3 V

Maximum Output Voltage -- -- 20 V

Supply Current I

Quiescent Current I

Shut Down Current I

Oscillator

Operation Frequency F

Maximum Duty Cycle Dmax 85 90 -- %

Reference Voltage

Feedback Voltage VFB 0.237 0.25 0.263 V

MOSFET

On Resistance of MOSFET Rds(on) 0.5 0.75 1.0 Ω

Current Limit I

Control and Protection

Shut Down Voltage V

Enable Voltage V

CE Pin Pull Low Current ICE -- 4 6 µA

OVP Threshold (Note 4) OVP 14.5 15.5 20.0 V

Note 1. Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These

are stress ratings only, and 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 affect device reliability.

Note 2. Devices are ESD sensitive. Handling precaution recommended. The human body model is a 100pF capacitor discharged

through a 1.5kΩ resistor into each pin.

Note 3. The device is not guaranteed to function outside its operating conditions.

Note 4. Floating the OVP pin to disable OVP function.

= 25°C, unless otherwise specified.)

Parameter Symbol Test Condition Min Typ Max Units

-------------------------------------------------------------------------------------------------- −0.3V to 7V

-------------------------------------------------------------------------------------------------- 2.4V to 6V

CC1

CC2

CC3

0.9 1.1 1.3 MHz

OSC

Normal Operation 800 900 1000 mA

max1

Start up Condition 500 625 750 mA

max2

0.4 0.8 -- V

CE1

-- 0.8 1.4 V

CE2

=6V, Continuously Switching -- -- 2 mA

=6V, FB=1.3V, No Switching 50 90 120 µA

=6V, VCE<0.4V -- 0.1 1 µA

DS9271-07 April 2004www.richtek.com

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