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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 @ VOUT < 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 Commercial 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 requirements 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.
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RT9271
Typical AppIication Circuit
VIN 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
VIN 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
VIN 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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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: RFB = 0.25/ILED 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
+
VREF 0.25V
A1
+
COMPARATOR A2 R S Q
DRIVER M1
VCC
EN
CHIP ENABLE 4A
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.
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+ 0.75
GND
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RT9271
Absolute Maximum Ratings
(Note 1) Supply Voltage, VCC -------------------------------------------------------------------------------------------------- -0.3V to 7V LX, OVP ----------------------------------------------------------------------------------------------------------------- -0.3V to 21V The Other Pins --------------------------------------------------------------------------------------------------------- -0.3V to 7V Power Dissipation, PD @ TA = 25C SOT-23-6 ---------------------------------------------------------------------------------------------------------------- 0.4W Package Thermal Resistance (Note 4) SOT-23-6, JA ----------------------------------------------------------------------------------------------------------- 250C/W Maximum Junction Temperature ----------------------------------------------------------------------------------- 125C Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------- 260C Storage Temperature Range ---------------------------------------------------------------------------------------- -65C to 150C ESD Susceptibility (Note 2) HBM (Human Body Mode) ------------------------------------------------------------------------------------------ 2kV MM (Machine Mode) -------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 3)
Supply Voltage, VCC -------------------------------------------------------------------------------------------------- 2.4V to 6V Junction Temperature Range ---------------------------------------------------------------------------------------- -40C to 125C
Electrical Characteristics
(VCC = 3.6V, TA = 25C, 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 ICC1 ICC2 ICC3 FOSC Dmax VFB Rds(on) Imax1 Imax2 VEN1 VEN2 IEN 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 A A MHz % V mA mA V V A V
VCC=6V, Continuously Switching VCC=6V, FB=1.3V, No Switching VCC=6V, VEN<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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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. JA is measured in the natural convection at T A = 25C 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.
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RT9271
Typical Operating Characteristics
Efficiency vs. VIN (Driving 1 WLED)
Refer to Application Circuit Figure 1
TA = 25C
90 89 88
91 90
TA = 25C
Efficiency (%)
Efficiency (%)
89 88 87 86 85 84 83 82 2 2.5 3 3.5 4
87 86 85 84 83 82 81 80 2 3 4 5 6
IO = 20mA
IO = 20mA IO = 15mA
IO = 15mA
VIN (V)
VIN (V)
89 88
TA = 25C
Refer to Application Circuit Figure 3
90
Efficiency vs. VIN (Driving 3 WLEDs)
1.3 1.2
Frequency vs. VIN
Driving 3 WLEDs TA = 25C
Efficiency (%)
87 86 85 84 83 82 81 80 2 3 4 5 6
Frequency (MHz) 1
1.1 1 0.9 0.8 0.7 2 3 4 5 6
IO = 20mA
IO = 15mA
VIN (V)
VIN (V)
VFB vs. Temperature
260 255 250
Driving 3 WLEDs VIN = 3.6V
3
VIN vs. Temperature
2.8
VFB (mV)
VIN (V)
2.6
245 240 235 230 0 10 20 30 40 50 60 70
3 WLEDs
2.4
2.2
2 20
Minimum Input Voltage vs. Temperature for Delivering Full Brightness
30 40 50 60 70
Temperature (C)
Temperature (C)
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Refer to Application Circuit Figure 2
92
Efficiency vs. VIN (Driving 2 WLEDs)
RT9271
Stability for Driving 1 WLED
VIN = 2.4V VOUT (AC)
Refer to Application Circuit Figure 1
Stability for Driving 1 WLED
VIN = 3.2V VOUT (AC)
Refer to Application Circuit Figure 1 Refer to Application Circuit Figure 3 Refer to Application Circuit Figure 2
VLX
VLX
IIN
(100mA/Div)
IIN
(100mA/Div)
Time (500ns/Div)
Time (500ns/Div)
Stability for Driving 2 WLEDs
VIN = 2.4V VOUT (AC)
Refer to Application Circuit Figure 2
Stability for Driving 2 WLEDs
VIN = 3.6V VOUT (AC)
VLX
VLX
IIN
(100mA/Div)
IIN
(100mA/Div)
Time (500ns/Div)
Time (500ns/Div)
Stability for Driving 2 WLEDs
Refer to Application Circuit Figure 2
Stability for Driving 3 WLEDs
VIN = 2.4V VOUT (AC)
VIN = 5.2V VOUT (AC)
VLX
VLX IIN
(100mA/Div)
IIN
(100mA/Div)
Time (500ns/Div)
Time (500ns/Div)
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RT9271
Stability for Driving 3 WLEDs
Refer to Application Circuit Figure 3
Stability for Driving 3 WLEDs
VIN = 6.0V VOUT (AC)
Refer to Application Circuit Figure 3 Refer to Application Circuit Figure 3 Refer to Application Circuit Figure 2
VIN = 3.6V VOUT (AC)
VLX
VLX
IIN
(100mA/Div)
IIN
(100mA/Div)
Time (500ns/Div)
Time (500ns/Div)
Inrush Current for Driving 1 WLED
f = 200Hz
Inrush Current for Driving 2 WLEDs
Refer to Application Circuit Figure 1
f = 200Hz
VEN = 2V VIN = 3V
IIN(MAX) = 730mA
VEN = 2V VIN = 3.6V
IIN VOUT
IIN VOUT
IIN(MAX) = 730mA
Time (20us/Div)
Time (20us/Div)
Inrush Current for Driving 3 WLEDs
f = 200Hz
Inrush Current for Driving 3 WLEDs with soft-start
Refer to Application Circuit Figure 3
f = 200Hz
VEN = 2V VIN = 3.6V
IIN(MAX) = 730mA
VEN = 2V
VIN = 3.6V
IIN(MAX) = 730mA
IIN VOUT
IIN VOUT
Time (20us/Div)
Time (20us/Div)
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RT9271
Dimming Control for Driving 3
f = 200Hz
VEN
VLX
VOUT
VIN = 3.6V
Time (20us/Div)
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Refer to Application Circuit Figure 3
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RT9271
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.
R2 = 0.25 V ILED
VIN 2.4 to 6V C1 1uF LX 10uH D1 SS0520 D2 VCC EN PWM signal GND RT9271 LX OVP FB R2 12 C3 1uF D3 D4
In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for R2 selection is shown below. R2 Resistor Value Selection
ILED (mA) 5 10 12 15 20 R2 () 49.9 24.9 21 16.5 12.4
Figure 4. PWM Dimming Control Using the EN Pin 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 the DC 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.
C1 1uF LX 10uH D1 SS0520 D2 D3 D4
Recommended Inductance and Rectifier (for Li-Ion cell)
Condition 2 WLEDs 3 WLEDs Inductance (H) Schottky Diode 4.7u~10u 4.7u~10u SS0520 SS0520
VCC EN LX OVP FB R4 82k VDC Dimmimg 0 to 2.8V VIN 2.4 to 6V
C3 1uF R3 6.8k R2 12
Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, 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.
GND RT9271
Figure 5. Dimming Control Using a DC Voltage
C1 1uF LX 10uH D1 SS0520 C6 10nF D2 D3 D4 R5 1k R6 10k R2 12
VIN 2.4 to 6V
VCC EN PWM signal GND RT9271
LX OVP FB
C3 1uF
Figure 6. Recommended Soft-Start Circuit
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RT9271
c. Using a Filtered PWM signal: Another common application is using a filtered PWM signal as an adjustable DC voltage for LED dimming control. A filtered PWM signal acts as the DC voltage to regulate the output current. The recommended application circuit is shown in the Figure 7. In this circuit, the output ripple depends on the frequency of PWM signal. For smaller output voltage ripple (<100mV), the recommended frequency of 2.8V PWM signal should be above 2kHz. To fix the frequency of PWM signal and change the duty cycle of PWM signal can get different output current as Figure 8. According to the application circuit of Figure 7, output current is from 20.5mA to 5.5mA by adjusting the PWM duty cycle from 10% to 90%.
C1 1uF LX 10uH D1 SS0520 D2 VCC EN GND LX OVP FB VFB R4 23.7k Rdc 100k
2.8V 0V
Constant Output Voltage for Backlight of Main Panel and Flashlight: Figure 9 is an application of RT9271 for backlight of main panel and flashlight. Setting the divider-resistors (R1 & R2) is to get a constant output voltage that depends on the forward voltage and the numbers of series-LEDs. There are three kinds of mode controlled by the switches backlight mode /flashlight mode /backlight + flashlight mode. It can turn on backlight or flashlight at one time or both at the same time. Applying different duty cycle of PWM signal above 22kHz to backlight's switch can also control the brightness. The following formula (1)(2) can determine R3 and R4. R3 = R4 = VOUT - 3VFb - VDS Ib VOUT - 3VFf - VDS If (1) (2) (3)
D1 SS0520 C3 1uF VOUT =12V IOUT MAX=80mA backlight flashlight + + VFb VFf R1 239k R3 GPIO1 PWM R2 5.1k Ib R4 GPIO2 If
VIN
C3 1uF R3 10k
VDS = Ib x RDS(ON)
VIN 2.4 to 6V C1 2.2uF LX 10uH
D3 D4 VLED R2 12
RT9271
VCC EN GND RT9271
LX OVP FB
Cdc 0.1uF
PWM signal
Figure 7. Filtered PWM Signal for LED Dimming Control
PWM >22kHz
Figure 9. Constant output voltage for backlight and flashlight Constant output voltage for backlight of main panel and keypad: Figure 10 is another application of RT9271 for backlight and keypad. Setting the divider-resistors (R1 & R2) is to get a constant output voltage that depends on the forward voltage and the numbers of series-LEDs. It can turn on backlight of main panel and keypad at the same time. Applying different duty cycle of PWM signal above 22kHz to the backlight's switch can also control the brightness of main panel's backlight. The keypad's backlight will keep the same brightness during the dimming control of main panel. Otherwise the brightness of keypad's s backlight can also change during the dimming control of main panel by using the application circuit as figure 5. The following formula (4)(5) can determine the resistors of Figure 10.
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PWM Duty Cycle vs. IOUT
25
20
I OUT (mA)
15
10
5
0 10 20 30 40 50 60 70 80 90
PWM Duty Cycle (%)
Figure 8
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RT9271
R3 = VOUT - 3VFb - VDS Ib VOUT - 3VFk Ik (4) (5) (6)
R4 = R5 = R6 =
VDS = Ib x RDS(ON)
VIN
C1 2.2uF
LX 10uH
D1 C3 SS0520 1uF
VOUT =12V backlight
+ VFb + VFk Ik
keypad
VCC EN GND RT9271
LX OVP FB R2 5.1k R1 239k R3
PWM
Ib
Ik
Ik
R4
R5
R6
PWM >22kHz
Figure 10. Constant output voltage for backlight and keypad
VIN
C1 1uF
LX 10uH
D1 SS0520 D2
+ VFk Ik
VOUT keypad
VCC EN GND
LX OVP FB VFB R4 23.7k Rdc 100k
2.8V 0V
C3 1uF R3 10k
D3 D4 VLED R2 12
Ik
Ik
R5
R6
R7
RT9271
Cdc 0.1uF
PWM signal
Figure 11. Constant output current for backlight and keypad
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RT9271
Layout Guide A full GND plane without gap break. VCC to GND noise bypass - Short and wide connection for the 1F MLCC capacitor between Pin6 and Pin2. Minimized LX node copper area to reduce EMI. Minimized FB node copper area and keep far away from noise sources.
S1
+
VIN C5
R1 C1 C2 VCC EN S3 GND LX OVP FB R2 S2 L1 D1 C3 D2 D3 D4 C4
RT9271
EVB Circuit Board Layout Example (2-Layer EVB Board) (Refer to EVB Circuit)
- Top Layer -
- Bottom Layer -
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RT9271
Outline Dimension
H D L C B
b A A1 e
Symbol A A1 B b C D e H L
Dimensions In Millimeters Min 0.889 0.000 1.397 0.356 2.591 2.692 0.838 0.080 0.300 Max 1.295 0.152 1.803 0.559 2.997 3.099 1.041 0.254 0.610
Dimensions In Inches Min 0.035 0.000 0.055 0.014 0.102 0.106 0.033 0.003 0.012 Max 0.051 0.006 0.071 0.022 0.118 0.122 0.041 0.010 0.024
SOT-23-5 Surface Mount Package
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RT9271
H D L C B
b A A1 e
Symbol A A1 B b C D e H L
Dimensions In Millimeters Min 0.889 0.000 1.397 0.250 2.591 2.692 0.838 0.080 0.300 Max 1.295 0.152 1.803 0.560 2.997 3.099 1.041 0.254 0.610
Dimensions In Inches Min 0.031 0.000 0.055 0.010 0.102 0.106 0.033 0.003 0.012 Max 0.051 0.006 0.071 0.022 0.118 0.122 0.041 0.010 0.024
SOT-23-6 Surface Mount Package
Richtek Technology Corporation
Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611
Richtek Technology Corporation
Taipei Office (Marketing) 8F, No. 137, Lane 235, Paochiao Road, Hsintien City Taipei County, Taiwan, R.O.C. Tel: (8862)89191466 Fax: (8862)89191465 Email: marketing@richtek.com
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