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EUP2538
10X2 Strings WLED Boost Converter
DESCRIPTION
The EUP2538 is a constant current step-up converter specifically designed to drive white LEDs. The Step-up converter topology allows series connection of the white LEDs so the LED currents are identical for uniform brightness. The EUP2538 switches at 1MHz, allowing the use of tiny external components. The output capacitor can be as small as 0.47F, saving space and cost versus alternative solutions. A low 0.3V feedback voltage minimizes power loss in the current setting resistor for better efficiency. The EUP2538 high-voltage output stage is perfect for driving mid-size and large panel displays containing up to ten white LEDs in series. The 1.2A current limit and 0.65[ RDSON allows the part using for up to 10x 2 string WLED lighting. LED dimming can be done by using a DC voltage, a logic signal, or a pulse width modulation(PWM) signal. The enable input pin allows the device to be placed in shutdown mode with "zero" quiescent current. The EUP2538 is available in low profile 8pin 3mm x 3mm TDFN package.
FEATURES
2.6V to 5.5V Input Range 38V Output with Over Voltage Protection Internal Soft-Start PWM Dimming Control Internal High Power 40V MOSFET Switch Fast 1MHz Switching Frequency Using Low-Profile Inductors and Capacitors Available in 3mmx 3mm TDFN-8 Package RoHS Compliant and 100% Lead (Pb)-Free
APPLICATIONS
GPS Navigation Systems Portable Media Players Handheld Devices, Digital Camera Portable Game Machines
Typical Application Circuit
Figure 1. White LED Application
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EUP2538
Pin Configurations
Package Type Pin Configurations
TDFN-8
Pin Description
PIN PGND IN EN GND FB LX TDFN-8 1,2 3 4 5 6 7,8 Input Supply Voltage Chip Enable Pin. Connect to 1.4V or higher to enable device, 0.3V or less to disable device. Common Ground. Connect the pin to the ground plane. Feedback Pin. Reference voltage is 0.3V. Connect cathode of lowest LED and resistor here. Calculate resistor value according to the formula: RFB=0.3/ILED Switch Pin. This is the drain of the internal power switch. Connect inductor/diode here. Minimize trace area at this pin to reduce EMI. DESCRIPTION Power Ground. Connect to GND and the exposed pad directly under the IC.
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Ordering Information
Order Number EUP2538JIR1 Package Type TDFN-8 Marking Operating Temperature Range -40 C to +85C
xxxxx P2538
EUP2538 1/4
1/4
1/4
1/4
Lead Free Code 1: Lead Free 0: Lead Packing R: Tape & Reel Operating temperature range I: Industry Standard Package Type J: TDFN
Block Diagram
Figure 2.
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Absolute Maximum Ratings (1)
Supply Voltage ,VIN ----------------------------------------------------------------SW ----------------------------------------------------------------------------------The Other Pins ---------------------------------------------------------------------Package Thermal Resistance TDFN-8,c JA ------------------------------------------------------------------------Maximum Junction Temperature --------------------------------------------------Lead Temperature (Soldering, 10sec.) --------------------------------------------Storage Temperature Range ---------------------------------------------------------0.3V to 6V -0.3V to 40V -0.3V to 6V 39.1C/W 125C 260C -65C to +150C
Recommend Operating Conditions (2)
Junction Temperature Range ------------------------------------------------------- -40C to +85C Supply Voltage , VIN----------------------------------------------------------------- 2.6V to 5.5V Note (1): Stress beyond those listed under "Absolute Maximum Ratings" may damage the device. Note (2): The device is not guaranteed to function outside the recommended operating conditions.
Electrical Characteristics
(VIN =3.6V, VOUT=34V, COUT=0.22F, CIN=2.2F, L1=22H, RFB=15[ Typical values are at TA= +25C) ,TA=-40C to +85C. Unless otherwise noted.
Symbol
UVLO ICC1 ICC2 ICC3 Oscillator Fosc Dmax VFB MOSFET
Parameter
Under Voltage Lock Out Maximum Output Voltage Supply Current Quiescent Current Shutdown current Operation Frequency Maximum Duty Cycle Feedback Voltage
Conditions
Rising No Switching VCC=5.5V, Continuous Switching VCC=5.5V, FB=1.3V, No Switching VCC=5.5V, VEN<0.4V
Min
2.2
EUP2538 Typ Max.
2.4 1.7 130 0.1 2.6 40 2.6 170 1 1.3
Unit
V V mA A A MHz %
0.8
1 92
Reference Voltage 285 300 0.65 0.85 0.4 1.2 0.7 0.7 0.1 38 1.2 1 315 1.3 1.6 mV [ A V V A V
Rds (on) On resistance of MOSFET ILX VEN1 VEN2 IEN OVP Current Limit Shutdown Voltage Enable Voltage EN Pin Pull Low Current OVP Threshold Control and Protection
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Typical Operating Characteristic Quiescent Current vs. VIN (Not Switching)
150
Quiescent Current vs. VIN (Switching)
2
Quiescent Current (mA)
Quiescent Current (A)
125
1.5
VFB=1.3V
100
1
75
0.5
50 3 3.5
0
Input Voltage (V)
4
4.5
5
5.5
3
3.5
4
4.5
5
5.5
Input Voltage (V)
310 308
FB PIN Voltage vs. Temperature
310
FB PIN Voltage vs. Output Current
FB PIN Voltage (mV)
306 304 302 300 298 -50
FB PIN Voltage (mV)
305
300
295
10WLEDs
290
0
50
100
150
Temperature (ae )
5
10
15
20
25
30
Output Current (mA)
40 35
Switching Frequency vs. Supply Voltage
1.1
LED Current vs. Input Voltage
RFB=10 RFB=15
Switching Frequency (MHz)
1
LED Current (mA)
30 25 20 15 10 5
0.9
RFB=20
0.8 3 3.5 4 4.5 5 5.5
0 3 3.5
Input Voltage (V)
DS2538 Ver1.3 Nov. 2008
Input Voltage (V)
4
4.5
5
5.5
5
EUP2538
Typical Operating Characteristics (continued) LED Current Regulation (10mA)
1
Efficiency vs. Load Current (10 WLEDs)
100
LED Current Variation (%)
0.5
90
0
Efficiency (%)
80
-0.5
70 vin=3.6v vin=5v
-1 3 3.5 4 4.5 5 5.5
60 5 10 15 20 25 30
Input Voltage (V)
LED Current (mA)
Efficiency vs. Input Voltage (10WLEDs)
100 95 90
Efficiency vs. Input Voltage (20WLEDs)
83 82 81
Efficiency (%)
85 80 75 70 65 60 3 3.5 15mA 20mA
Efficiency(%)
80 79 78 77 76
Input Voltage (V)
4
4.5
5
5.5
3.4
3.9
4.4
4.9
5.4
VIN(V)
Switch On Resistance vs. Input Voltage
2
120 100
Maximum Output Current vs. Input Voltage (10WLEDs)
Switch Resistance ()
1.5
Output Current (mA)
3 3.5 4 4.5 5 5.5
80 60 40 20
1
0.5
0
0 3 3.5
Input Voltage (V)
Input Voltage (V)
4
4.5
5
5.5
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Typical Operating Characteristics (continued) Shutdown Voltage vs. Input Voltage
1
Light Load Switching Waveform VIN=5V, ILED=1.5mA, L=22H
SW 20V/div
Shutdown Voltage (V)
0.8
Inductor
0.6
Current 100mA/d
-40J -25J 85J
0.4
VOUT AC Coupled 20mV/div
5
0.2 3 3.5 4 4.5
125J
Input Voltage (V)
500ns/div
Typical Switching Waveforms
SW 20V/div Inductor Current 100mA/d VOUT AC Coupled 100mV/div 500ns/div Input Current 100mA/div VEN 5V/div VOUT 20V/div
Power-up With 10 LEDs at 20mA
50.0s/div
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Application Information
LED Current Control The EUP2538 regulates the LED current by setting the current sense resistor (RFB) connecting to feedback and ground. The internal feedback reference voltage is 0.3V. The LED current can be set from following equation easily.
I LED = 0.3 R FB
Switching Waveform with 1kHz PWM on EN
VEN 5V/div VOUT 20V/div LED Current 20mA/div 250s/div
--------------------------------------(1)
In order to have an accurate LED current, precision resistors are preferred (1% is recommended). The table for RFB selection is shown below. RFB Resistor Value selection LED Current (mA) 5 10 15 20 25 30 Dimming Control a. Using a PWM Signal to EN Pin For controlling the LED brightness, the EUP2538 can perform the dimming control by applying a PWM signal to EN pin, and the PWM signal frequency range is from 100Hz to 100KHz. 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. RFB ([ 60 30 20 15 12 10 )
Figure 4. 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.
Figure 5. DC Voltage Dimming Control 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 6. 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. According to the application circuit of Figure 6, output current is from 20.5mA to 5.5mA by adjusting the PWM duty cycle from 10% to 90%.
Figure 3. Direct PWM Dimming Control
DS2538
Ver1.3
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EUP2538
1
Shutdown Voltage vs. Input Voltage
Shutdown Voltage (V)
0.8
0.6
-40J 0.4 -25J 85J 125J 0.2 3 3.5 4 4.5 5
Figure 6. Filtered PWM Dimming Control
Input Voltage (V)
Figure 9.
25
Filtered PWM Dimming (0V to 2.5V) Enable to Open LED Waveforms
VEN 5V/div
20
LED Current (mA)
15
VOUT
10
20V/div SW
5
0 0 10 20 30 40 50 60 70 80 90 100
20V/div 100s/div
PWM Duty Cycle (%)
Figure 7. Open Load Shutdown In the event of an "Open LED" fault condition, the EUP2538 will continue to boost the output voltage with maximum power until the output voltage reaches approximately 38V. Once the output exceeds this level, the device will cease operation until the EN pin is cycled off and on.
Figure 10. Thermal Shutdown Thermal overload protection circuitry has been included to prevent the device from operating at unsafe junction temperatures above 150C. In the event of a thermal overload condition the device will automatically shutdown and wait till the junction temperatures cools to 130C before normal operation is resumed. Capacitors Selection A 4.7F to 10F ceramic input capacitor (CIN) and a 0.22F to 4.7F ceramic output capacitor (COUT) are sufficient for most applications. During Direct PWM Dimming control, a larger output capacitor will significantly reduce audio noise induced by output capacitor, and a smaller will enlarge the audio noise, a 2.2uF COUT is recommended. Under normal condition, a 4.7F input capacitor is sufficient. For applications with higher output power, a larger input capacitor of 10F may be appropriate. X5R and X7R capacitor types are ideal due to their stability across temperature range.
Figure 8. Open LED Protection
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Ver1.3
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EUP2538
Inductor Selection The recommended value of inductor for 2 to 10 WLEDs applications are 4.7H to 47H. Small size and better efficiency are the major concerns for portable device, such as EUP2538 used for mobile phone. The inductor should have low core loss at 1MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered. Schottky Diode Selection The current rating of the Schottky diode must exceed the peak current flowing through it. The Schottky diode performance is rated in terms of its forward voltage at a given current. In order to achieve the best efficiency, this forward voltage should be as low as possible. The response time is also critical since the driver is operating at 1MHz. Board Layout Careful PC board layout is required due to fast switching. All components must be placed as close to the device as possible. Keep the path between the inductor L1, diode D1, and output capacitor COUT extremely short for minimal noise and ringing. The feedback components such as the sense resistor RFB must be kept close to the FB pin to prevent noise injection on the FB pin trace. The ground return of CIN and COUT should be tied close to the GND pin. See the EUP2538 demo board layout for reference.
DS2538
Ver1.3
Nov. 2008
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EUP2538
Packaging Information
TDFN-8
DETAIL A
SYMBOLS A A1 b D D1 E E1 e L
MILLIMETERS MIN. MAX. 0.70 0.80 0.00 0.20 2.90 2.30 2.90 1.50 0.65 0.25 0.45 3.10 0.05 0.40 3.10
INCHES MIN. 0.028 0.000 0.008 0.114 0.090 0.114 0.059 0.026 0.010 0.018 0.122 MAX. 0.031 0.002 0.016 0.122
DS2538
Ver1.3
Nov. 2008
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