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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Description
The YB1521 is a step-up DC-DC converter which operating as current source to drive up 45 white LEDs (3S15P) in parallel / series configuration from a 5V system rail. Series connecting of LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The light intensity of these LEDs is proportional to the current passing through them. The YB1521 is a fixed frequency of 1.3MHz, allowing the use of tiny low profile inductors and capacitors to minimize footprint and cost in space consideration applications for cellular phone backlighting or other hand held equipment. The wide input voltage range from 2.5V to 5.5V is idea for portable applications with higher conversion efficiency. To control LED brightness, the LED current can be pulsed by applying a PWM signal with a frequency which is typically 50 KHz to the EN pin.
Features
2.5V to 5.5V Input Voltage Range Built In Power MOSFET Drive Up to 15V @400mA at 5V Vin Less than 1A Shutdown Current Up to 94% efficiency Over Voltage Protection@18V UVLO, Thermal Shutdown Internal 1.6A Current Limit 200mv Low Reference Voltage Built In Soft Start PWM Dimming SOT23-5 Package Green Package (RoHS) Available
Applications
Cellular and Smart Phones Handheld Computers and PDAs Small LCD Displays GPS Terminals MP3 Players Digital Still and Video Cameras Electronic Books
Typical Application Circuit
100
D1 Schottk
Efficiency vs. Input Voltage
IOUT =100mA
L=10H
VI
= 4.5~5.5V
L1 6.8u
90 EFFICIENCY (%) IOUT =180mA 80
4.7u/2 V 4.7u/ V 1 I S
70
15 strings
YB1521
E O GN F
...
R1
60
50 4 4.5 5 INPUT VOLTAGE (V) 5.5 6
Figure 1: Typical Application Circuit
Figure 2: Efficiency
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Pin Configuration
Figure 3: Pin Configuration
Pin Assignment & Description
Table1
Pin 1 2 3 4 5
NAME SW GND FB EN IN
Description Switching Pin. This is the drain of internal power MOSFET switch. Connect the power inductor and output rectifier to SW. Ground pin. Connect directly to local ground plane. Feedback Pin. Reference voltage is 200mV. Connect LEDs and a resistor at this pin. LED current is determined by the resistance and FB voltage. Shutdown Pin and Dimming Control Pin. PWM duty cycle controls the LED current. Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible.
Ordering Information
Table2
Order Number YB1521ST25
Package Type SOT23-5
Supplied As 3000 Units Tape & Reel
Package Marking Y521
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Absolute Maximum Ratings (Note 1)
VIN to GND ............................ -0.3V to 6V VEN to GND ........................... -0.3V to 6V VSW to GND......................... -0.3V to 18V Junction Temperature ................... 150 Storage Temperature .......-55 to 150 Lead Temperature......................... 300 ESD HBM ........................................ 3KV ESD MM .........................................250V
Recommended Operating Conditions
(Note 2)
Supply Voltage VIN ............... 2.5V to 5.5V Operating Temperature ....... -40 to 85 Output Voltage ......................... 0V to 15V
Thermal Information
(Note 3)
JA ............................................ 220 / W
Note: 1. Exceeding these ratings may damage the device. 2. The device is not guaranteed to function outside of its operating conditions. 3. JA is measured in free air at TA = 25 on a low effective thermal conductivity board.
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Electrical Characteristics
Table4 (VIN=5V, VEN=5V, TA=25 unless otherwise noted.) C, Symbol VIN IQ ISD VFB VFB=0.3V VEN=0V, IOUT = 0mA IOUT = 260mA, VOUT = 10.5V VFB = 0.2V RDSON DTMX FSW VENH VENL VEN Rising, VIN = 5V VEN Rising, VIN = 2.5V VFB = 0V 85 1 1 0.8 20 IILM UVLO VIN Rising 1 50 1.6 2.25 100 OVP TSD The Hysteresis = 20 16 18 160 22 2.45 0.4 92 1.3 1.5 1.6 2 180 Test Conditions MIN 2.5 TYP 5 0.2 0.1 200 MAX 5.5 0.8 1 220 0.1 Units V mA A mV A % MHz V V KHz A V mV V Parameters Input Voltage Range Quiescent Current Shutdown Current Feedback Voltage FB Input Bias Current Switch On-Resistance Maximum Duty Cycle Switching Frequency EN Threshold EN Threshold EN Dimming Frequency Switch Current Limit Under Voltage Lock Out UVLO Hysteresis Over Voltage Protection Thermal Shutdown
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Typical Performance Characteristics
Steady State Operation
VIN =5V, 39 LEDS, 260mA
Startup Waveforms
VIN =5V, 39 LEDS, 260mA
Startup Waveforms into an Open Load
VIN =5V, 39 LEDS, 260mA
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Functional Block
Figure 4: Function Block
Operation
The YB1521 features a constant frequency, current mode control scheme to regulate the output LED current. Its operation can be understood by referring to the block diagram in Figure 3. At the start of each oscillator cycle, a voltage proportional to the switch current is added to a ramp output and the resulting sum is fed into the positive terminal of the PWM comparator. When this voltage exceeds the level of the comparator negative input, the peak current has been reached, and the SR latch is reset and turns off the power switch. The voltage at the negative input of the comparator comes from the output of the error amplifier. The error amplifier sets the correct peak current level to keep the output in regulation. If the output of error
YB1521 MRev.1.1
amplifier increases, more current is delivered to the output; if it decreases, less current is delivered.
Application Information
Inrush Current The YB1521 has internal soft-start to limit the amount of current through the IN pin at startup and to also limit the amount of overshoot on the output. The current limit is increased by 4 steps to give a total soft-start time of 160s. LED Current and Dimming Control The LED's current is set by a resistor connected at FB pin to GND: ILED = 200mV / R1. The maximum LED current set initially can
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
be reduced by pulse width modulating the EN. A better approach is to adjust the internal reference voltage for dimming control by a logic signal illustrated in Figure 5. YB1521 contains circuitry to support this automatically. The typical dimming frequency is 50KHz and recommended not to lower than 20KHz. Besides the EN dimming control scheme, either a DC level signal or a filtered PWM signal can be used to control the LED current as illustrated in Figure 6 and Figure 7 respectively. The average current through the LEDs will increase linearly to the duty cycle of the controlling PWM signal.
YB1521
FB
R1 20K
R2 80K
R3 20K
0.1uF
Figure 7: Dimming Control Using a Filtered PWM Signal
Over Voltage Protection The YB1521 has an internal over voltage protection circuit which also acts as an open-circuit protection. In the case of open circuit or the LEDs failure, the LEDs are disconnected from the circuit, and the feedback voltage will be zero. The YB1521 will then switch to a high duty cycle resulting in a high output voltage, which may cause SW voltage to exceed its maximum 18V rating. The YB1521 will shutdown automatically until the power is recycled. Inductor Selection A 6.8H is recommended to drive 39 LEDs in series/parallel. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1.3MHz and low DCR (copper wire resistance). A higher 10H inductor is recommended to improve efficiency. Diode Selection
YB1521
EN FB
Figure 5: PWM Dimming Control Using a Logic Signal
YB1521
FB
R1 80K
VDC
R2 20K
Figure 6: Dimming Control Using a DC Voltage
To maintain high efficiency, the average current rating of the Schottky diode should be larger than the peak inductor current. Schottky diode with a low forward drop and fast switching speeds are idea for increasing efficiency in portable application. Choose a reverse breakdown of the
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Schottky diode large than the output voltage. Capacitor Selection Choose low ESR capacitors for the output to minimize output voltage ripple. The ceramic capacitor is a good choice for this as well. A 4.7F output capacitor is sufficient for most applications. For applications desire a smaller output capacitance, a 2.2F may be used. A 4.7F ceramic capacitor is recommended for the input capacitance in the real system. PCB Layout Consideration Proper layout of the high frequency switching path is critical to prevent from noise and electromagnetic interference problems. Due to high frequency switching, the length and area of all the traces connected to the switch node should be minimized.
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YB1521
High Power Step-Up DC-DC Converter & WLED Driver
Package Information (SOT23-5)
The information described herein is subject to change without notice. Yobon does not assume any responsibility for use of any circuitry or applications described herein, nor does it convey any patent license.
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