View RT9271_184390.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

Preliminary
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 four 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.
Features
Inherently Matched LED Current High Efficiency: 85% Typical Drives Up to Four LEDs from 2.8V Supply 20V Internal Switch Fast 1.1 MHz Switching Frequency Uses Tiny 1 mm Tall 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
Ordering Information
RT9271 Package Type E : SOT-26
Pin Configurations
(TOP VIEW)
LX 1 2 3 6 5 4 VCC OVP CE
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.
GND FB
SOT-26
Functional Pin Description
Pin 1 2 3 4 5 6 Name Function LX FB CE Switch Pin. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI 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. the trip point is 15.5V. Leave it unconnected to disable this function. VCC Input Voltage Pin. Must be locally bypass with 1uF capacitor to GND. OVP Over Voltage Protection Pin. Voltage sensing input to trigger the function of over voltage protection, GND Ground Pin. Connect directly to local ground plane.
DS9271-02 October 2003
www.richtek.com 1
RT9271
Typical AppIication Circuit
VIN 2.4 to 3.2V C1 1uF
Preliminary
LX 10uH
D1 SS0520
VCC CE
Dimming Control
LX OVP FB
C3 1uF
D2
GND
R2 12
Figure 1. RT9271 Drivers 1 WLED Application Circuit
VIN 2.4 to 5V
C1 1uF
LX 10uH
D1 SS0520
VCC CE
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 CE
Dimming Control
LX OVP FB
C3 1uF
D3 D4
GND
R2 12
Figure 3. RT9271 Drivers 3 Series WLEDs Application Circuit
www.richtek.com 2
DS9271-02 October 2003
Preliminary
RT9271
VIN 2.8 to 6V
C1 1uF
LX 10uH
D1 SS0520 D2
VCC CE
Dimming Control
LX OVP FB
C3 1uF
D3 D4 D5
GND
R2 12
Figure 4. RT9271 Drivers 4 Series WLEDs Application Circuit
Note : 1. D1 is Schottky diode (SS0520). 2. D2 ~ D5 are the WLED (HT-S91CW-DT) of HARVATEK. 3. LX is the SH4018 series of ABC TAIWAN ELECTRONICS CORP.
Recommended Circuits for Driving LED' s Figure 1 to Figure 4 illustrates the recommended application circuits for driving white LED' s. The series connected LED' s 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 LED' s 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.
DS9271-02 October 2003
www.richtek.com 3
RT9271
Preliminary
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 Maximum Junction Temperature ---------------------------------------------------------------------------------- 125C Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------- 260C Storage Temperature Range --------------------------------------------------------------------------------------- -65C to 150C ESD Susceptibility (Note 2) HBM ---------------------------------------------------------------------------------------------------------------------- 2kV MM ------------------------------------------------------------------------------------------------------------------------ 200V
Recommended Operating Conditions (Note 3)
Supply Voltage, VCC -------------------------------------------------------------------------------------------------- 2.4V to 6V Ambient Temperature Range -------------------------------------------------------------------------------------- -40C to 85C
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 Limitation Current Limit Control and Protection Shut Down Voltage Enable Voltage CE Pin Pull Low Current OVP Threshold (Note 4)
Symbol UVLO ICC1 ICC2 ICC3 FOSC Dmax VFB Rds(on) Imax1 Imax2 VCE1 VCE2 ICE OVP
Test Condition
Min 1.8 --
Typ 2.2 --90 0.1 1.1 90 0.25 0.75 750 400 0.8 0.8 4 15.5
Max 2.3 20 2 120 1 1.3 -0.263 1.0 900 550 -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, VCE<0.4V
-50 -0.9 85 0.237 0.5
Normal Operation Start up Condition
600 250 0.4 --14.5
www.richtek.com 4
DS9271-02 October 2003
Preliminary
RT9271
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 1.5k 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.
Function Block Diagram
OVP
15V
LX
FB
+
VREF 0.25V
A1
+
COMPARATOR A2 R S Q
VCC
DRIVER M1
CE
CHIP ENABLE
RAMP GENERATOR 1.1MHz OSCILLATOR
Operation The RT9271 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the block diagram. At the start of each oscillator cycle, the SR latch is set, which turns on the power switch M1. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the SR latch is reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 250mV. In this manner, the error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier' s output increases, more current is delivered to the output; if it decreases, less current is delivered. A 15V Zener diode connects OVP pin to FB pin internally to provide an optional protection function preventing LX pin from over-voltage damage, in case the feedback loop broken due to component wear-out or improper connection on the application boards. This function is especially suitable for applications driving white LED' s less than 4 in series
DS9271-02 October 2003
+ -
0.75
GND
www.richtek.com 5
RT9271
Preliminary
Typical Operating Characteristics
Efficiency vs. VIN (Driving 1 WLED)
Refer to Application Circuit Figure 1
TA = 25C
90 89 88 87 86 85 84 83 82 81 80 2 2.5 3 3.5 4 2 3 4 5 6
91 90 89 88 87 86 85 84 83 82
TA = 25C
IO = 20mA
IO = 20mA IO = 15mA
IO = 15mA
VIN (V)
VIN (V)
Refer to Application Circuit Figure 3
89 88 87 86 85 84 83 82 81 80 2
TA = 25C
85 84 83 82 81 80 79 78 77 76
TA = 25C IO = 20mA
IO = 20mA
IO = 15mA
IO = 15mA
3
4
5
6
2
3
4
5
6
VIN (V)
VIN (V)
1.3 1.2 1.1 1 0.9 0.8 0.7 2
Frequency vs. VIN
Driving 3 WLEDs TA = 25C
260 255 250 245
FB
VFB vs. Temperature
Driving 3 WLEDs VIN = 3.6V
240 235 230 0 10 20 30 40 50 60 70
3
4
5
6
VIN (V)
Temperature (C)
www.richtek.com 6
DS9271-02 October 2003
Refer to Application Circuit Figure 4
90
Efficiency vs. VIN (Driving 3 WLEDs)
86
Efficiency vs. VIN (Driving 4 WLEDs)
Refer to Application Circuit Figure 2
92
Efficiency vs. VIN (Driving 2 WLEDs)
Preliminary
RT9271
VIN = 2.4V
2.8
4 WLEDs
VOUT (AC)
2.6
IN
2.4
3 WLEDs
VLX
2.2
IIN
Minimum Input Voltage vs. Temperature for Delivering Full Brightness
20 30 40 50 60 70
100mA/Div Time (500 ns/Div)
2
Temperature (C)
Stability for Driving 1 WLED
Refer to Application Circuit Figure 1
Stability for Driving 2 WLEDs
VIN = 2.4V VOUT (AC)
Refer to Application Circuit Figure 2 Refer to Application Circuit Figure 2
VIN = 3.2V VOUT (AC)
VLX
VLX
IIN 100mA/Div Time (500 ns/Div)
IIN 100mA/Div Time (500 ns/Div)
Stability for Driving 2 WLEDs
VIN = 3.6V VOUT (AC)
Refer to Application Circuit Figure 2
Stability for Driving 2 WLEDs
VIN = 5.2V VOUT (AC)
VLX
VLX
IIN 100mA/Div Time (500 ns/Div)
IIN 100mA/Div Time (500 ns/Div)
DS9271-02 October 2003
www.richtek.com 7
Refer to Application Circuit Figure 1
3
VIN vs. Temperature
Stability for Driving 1 WLED
RT9271
Stability for Driving 3 WLEDs
Preliminary
Stability for Driving 3 WLEDs
Refer to Application Circuit Figure 3
VIN = 2.4V VOUT (AC)
VIN = 3.6V VOUT (AC)
VLX IIN 100mA/Div
VLX
IIN 100mA/Div Time (500 ns/Div)
Time (500 ns/Div)
Stability for Driving 3 WLEDs
Refer to Application Circuit Figure 3
Stability for Driving 4 WLEDs
VIN = 2.4V VOUT (AC)
Refer to Application Circuit Figure 4 Refer to Application Circuit Figure 4
VIN = 6.0V VOUT (AC)
VLX
VLX IIN 100mA/Div
IIN 100mA/Div Time (500 ns/Div)
Time (500 ns/Div)
Stability for Driving 4 WLEDs
Refer to Application Circuit Figure 4
Stability for Driving 4 WLEDs
VIN = 6.0V VOUT (AC)
VIN = 3.6V VOUT (AC)
VLX IIN 100mA/Div
VLX
IIN 100mA/Div Time (500 ns/Div)
Time (500 ns/Div)
www.richtek.com 8
DS9271-02 October 2003
Refer to Application Circuit Figure 3
Preliminary
RT9271
Inrush Current for Driving 2 WLEDs
Frequency = 200Hz
Inrush Current for Driving 1 WLED
Frequency = 200Hz VCE = 2V VIN = 3V IIN(max) = 730mA
Refer to Application Circuit Figure 1
VCE = 2V VIN = 3.6V
IIN(max) = 730mA IIN VOUT
IIN VOUT
Time (20 us/Div)
Time (20 us/Div)
Inrush Current for Driving 3 WLEDs
Refer to Application Circuit Figure 3
Inrush Current for Driving 4 WLEDs
Frequency = 200Hz VCE = 2V VIN = 3.6V
Refer to Application Circuit Figure 4 Refer to Application Circuit Figure 4
Frequency = 200Hz VCE = 2V VIN = 3.6V IIN(max) = 730mA IIN VOUT
IIN(max) = 750mA IIN VOUT
Time (20 us/Div)
Time (20 us/Div)
Inrush Current for Driving 3 WLEDs with soft-start
Frequency = 200Hz VCE = 2V
Refer to Application Circuit Figure 3
Dimming Control for Driving 4 WLEDs
VCE
VIN = 3.6V IIN(max) = 730mA IIN VOUT
VLX
VOUT VIN = 3.6V Time (20 us/Div)
Time (20 us/Div)
DS9271-02 October 2003
www.richtek.com 9
Refer to Application Circuit Figure 2
RT9271
Application Information
LED Current Control
Preliminary
The LED current is controlled by the feedback resistor (R2 in Application Circuit). The feedback reference is 0.25V. The LED current is 0.25V/R2. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula and table for R2 selection are shown below.
R2 = 0.25 V
VIN 2.4 to 6V
C1 1uF
LX 10uH
D1 SS0520 D2
VCC EN PWM signal GND RT9271
LX OVP FB
C3 1uF
D3 D4
R2 12
ILED
R2 Resistor Value Selection
ILED (mA) 5 10 12 15 20 R2 () 49.9 24.9 21 16.5 12.4
Figure 5. PWM Dimming Control Using the CE Pin
b. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The dimming control using a DC voltage is shown in Figure 6. As the DC voltage increases, the voltage drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases. For VDC range from 0V to 2.6V, the selection of resistors in Figure 6 gives dimming control of LED current from 20mA to 0mA.
Recommended Inductance and Rectifier (for Li-Ion cell)
Condition 2 WLEDs 3 WLEDs 4 WLEDs Inductance (H) Schottky Diode 4.7u~10u 4.7u~10u 4.7u~10u SS0520 SS0520 SS0520
VIN 2.4 to 6V
C1 1uF
LX 10uH
D1 SS0520 D2
Dimming Control a. Using a PWM Signal to CE Pin With the PWM signal applied to the CE pin, the RT9271 is turned on or off by the PWM signal. The average LED current increases proportionally with the duty cycle of the PWM signal, A 0% duty cycle will turn off the RT9271 and corresponds to zero LED current. A 100% duty cycle corresponds to full current. The magnitude of the PWM signal should be higher than minimum CE voltage high.
VCC EN GND RT9271
LX OVP
C3 1uF 6.8K
D3 D4
FB 82K VDC Dimmimg 0 to 2.6V R2 12
Figure 6. Dimming Control Using a DC Voltage
www.richtek.com 10
DS9271-02 October 2003
Preliminary
VIN 2.4 to 6V C1 1uF LX 10uH D1 SS0520 D2 D3 D4 R3 1K R4 10K R2 12
RT9271
VCC EN PWM signal GND RT9271
LX OVP FB
C3 1uF
C6 10nF
Figure 7. Recommended Soft-Start Circuit 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.
VIN S1
+
C5 C1 C2 CE S3
R1 L1 VCC D1 LX OVP FB R2 S2 C3 D2 D3 D4 D5 C4
RT9271
GND
EVB Circuit Board Layout Example (2-Layer EVB Board) (Refer to EVB Circuit)
- Top Layer -
- Bottom Layer -
DS9271-02 October 2003
www.richtek.com 11
RT9271
Outline Dimension
Preliminary
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 -1.397 0.356 2.591 2.692 0.838 0.102 0.356 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 -0.055 0.014 0.102 0.106 0.033 0.004 0.014 Max 0.051 0.006 0.071 0.022 0.118 0.122 0.041 0.010 0.024
SOT- 26 Surface Mount Package
RICHTEK TECHNOLOGY CORP.
Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611
RICHTEK TECHNOLOGY CORP.
Taipei Office (Marketing) 8F-1, No. 137, Lane 235, Paochiao Road, Hsintien City Taipei County, Taiwan, R.O.C. Tel: (8862)89191466 Fax: (8862)89191465 Email: marketing@richtek.com
www.richtek.com 12
DS9271-02 October 2003

▲Up To Search▲

Price & Availability of RT9271

	To Download RT9271 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .