® RT8511B 43V Asynchronous Boost WLED Driver General Description Features The RT8511B is an LED driver IC that can support up to 10 WLED in series. It is composed of a current mode boost converter integrated with a 43V/2.2A power switch running at a fixed 500kHz frequency and covering a wide VIN range from 2.7V to 24V. Wide Input Voltage Range : 2.7V to 24V High Output Voltage : up to 43V Direct PWM Dimming Control and Frequency from 100Hz to 8kHz Internal Soft-Start and Compensation 200mV Reference Voltage PWM Dimming with Internal Filter Programmable Over Voltage Protection Over Temperature Protection Current Limit Protection Thin 8-Lead WDFN Package RoHS Compliant and Halogen Free For brightness dimming, the RT8511B is able to maintain steady control of the LED current. Therefore, no audible noises are generated on the output capacitor. The RT8511B also has programmable over voltage pin to prevent the output from exceeding absolute maximum ratings during open LED conditions. The RT8511B is available in WDFN-8L 2x2 package. Ordering Information Applications UMPC and Notebook Computer Backlight GPS, Portable DVD Backlight Pin Configurations (TOP VIEW) RT8511B Package Type QW : WDFN-8L 2x2 (W-Type) OVP FB DIMC GND Lead Plating System G : Green (Halogen Free and Pb Free) Note : 8 1 3 GND The white LED current is set with an external resistor, and the feedback voltage is regulated to 200mV (typ.). During operation, the LED current can be controlled by the PWM input signal in which the duty cycle determines the feedback reference voltage. 6 4 9 5 2 7 EN PWM VIN LX WDFN-8L 2x2 Richtek products are : RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Marking Information 0F : Product Code Suitable for use in SnPb or Pb-free soldering processes. 0FW Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8511B-05 February 2015 W : Date Code is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8511B Typical Application Circuit VOUT L 10µH VIN 4.2V to 24V CIN 1µF x 2 Chip Enable 6 VIN D RT8511B R2 3.3M LX 5 OVP COUT 1µF x 2 1 8 EN : : WLEDs : : R1 100k FB 2 7 PWM 3 DIMC PWM 100Hz to 8kHz : : : : : : : : GND RSET 3.3 4, 9 (Exposed Pad) CDIMC 1µF Figure 1. Typical Application Circuit of Normal Operation VOUT L 10µH VLED 2.7V to 24V CLED 1µF x 2 VIN 2.7V to 4.2V D RT8511B CIN 1µF Chip Enable R2 3.3M LX 5 6 VIN OVP COUT 1µF x 2 1 8 EN 7 PWM 3 DIMC PWM 100Hz to 8kHz : : : : : : : : : : WLEDs : : R1 100k FB 2 GND 4, 9 (Exposed Pad) RSET 3.3 CDIMC 1µF Figure 2. Typical Application Circuit of Low Voltage Operation Functional Pin Description Pin No. Pin Name Pin Function 1 OVP Over Voltage Protection for Boost Converter. The detecting threshold is 1.2V. 2 FB Feedback. Connect a resistor between this pin and GND to set the LED current. 3 DIMC PWM Filter. Filter the PWM signal to a DC voltage. 4 GND Ground. 5 LX Switch Node for Boost Converter. 6 VIN Power Supply Input. 7 PWM Dimming Control Input. 8 EN Chip Enable (Active High) for Boost Converter. GND The exposed pad must be soldered to a large PCB and connected to AGND for m aximum power dissipation. 9 (Exposed Pad) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8511B-05 February 2015 RT8511B Function Block Diagram OVP LX + VIN 1.2V - EN OTP OSC S Q R Q OCP - PWM Controller PWM DIMC D/A Dimming + + GND - FB Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8511B-05 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8511B Absolute Maximum Ratings (Note 1) VIN, EN, PWM, DIMC to GND -----------------------------------------------------------------------------------------FB, OVP to GND ---------------------------------------------------------------------------------------------------------LX to GND -----------------------------------------------------------------------------------------------------------------< 500ns ---------------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C WDFN-8L 2x2 -------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WDFN-8L 2x2, θJA --------------------------------------------------------------------------------------------------------WDFN-8L 2x2, θJC --------------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- Recommended Operating Conditions −0.3V to 26.5V −0.3V to 48V −0.3V to 48V −1V to 48V 0.833W 120°C/W 8.2°C/W 260°C 150°C –65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VIN ------------------------------------------------------------------------------------------------ 2.7V to 24V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 4.5V, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit IQ VFB = 1.5V, No Switching -- 725 -- A IQ_SW VFB = 0V, Switching -- -- 2.2 mA ISHDN VIN = 4.5V, VEN = 0V -- 1 4 A Logic-High EN, PWM Threshold Voltage Logic-Low VIH VIN = 2.7V to 24V 1.6 -- -- VIL VIN = 2.7V to 24V -- -- 0.8 EN Sink Current IIH VEN = 3V 1 -- 10 A Shutdown Delay tSHDN EN high to low 52 64 80 ms 0.1 -- 8 kHz 0.4 0.5 0.6 MHz -- 0.4 0.6 -- 60 -- ns -- 92 -- % VIN Quiescent Current VIN Shutdown Current Control Input PWM Dimming Frequency V Boost Converter Switching Frequency LX On Resistance (N-MOSFET) fOSC VIN = 2.7V to 24V RDS(ON) VIN > 5V Minimum ON Time Maximum Duty Cycle DMAX VFB = 0V, Switching Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8511B-05 February 2015 RT8511B Parameter Symbol Test Conditions Min Typ Max Unit 1 -- -- % LED Current Minimum PWM Dimming Duty Cycle DMIN Feedback Voltage VFB 195 200 205 mV ILIM 1.66 2.2 2.74 A VOVP 1.14 1.2 1.26 V TSD -- 160 -- °C TSD -- 30 -- °C Dimming Freq. = 100Hz to 8kHz Fault Protection LX Current Limit Over Voltage Protection Threshold Thermal Shutdown Temperature Thermal Shutdown Hysteresis Note 1. Stresses beyond those listed “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 may affect device reliability. Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is measured at the exposed pad of the package. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8511B-05 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8511B Typical Operating Characteristics FB Reference Voltage vs. Input Voltage Efficiency vs. Input Voltage 100 199.5 FB Reference Voltage (mV) 95 Efficiency (%) 90 85 80 75 70 65 199.2 198.9 198.6 198.3 VOUT = 29.5V 198.0 60 4 7 9 12 14 17 19 22 4 24 8 16 20 24 Input Voltage (V) Input Voltage (V) FB Reference Voltage vs. Temperature Frequency vs. Input Voltage 200 600 198 550 Frequency (kHz)1 FB Reference Voltage (mV) 12 196 194 192 500 450 400 VIN = 4.5V 190 350 -20 5 30 55 80 105 4 8 Current Limit vs. Input Voltage 16 20 24 LED Current vs. PWM Duty Cycle 60 3.0 50 LED Current (mA) 2.6 Current Limit (A) 12 Input Voltage (V) Temperature (°C) 2.2 1.8 1.4 40 PWM = 100Hz PWM = 2kHz PWM = 8kHz 30 20 10 0 1.0 2.5 5.25 8 10.75 13.5 16.25 19 21.75 24.5 Input Voltage (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 0 10 20 30 40 50 60 70 80 90 100 PWM Duty Cycle (%) is a registered trademark of Richtek Technology Corporation. DS8511B-05 February 2015 RT8511B Application Information The RT8511B is a current mode boost converter which operates at a fixed frequency of 500kHz. It is capable of driving up to 10 white LEDs in series and integrates functions such as soft-start, compensation, and internal analog dimming control. The protection block also provides over-voltage, over-temperature, and current- limit protection features. LED Current Setting The loop structure of the boost converter keeps the FB pin voltage equal to the reference voltage VFB. Therefore, by connecting the resistor, RSET between the FB pin and GND, the LED current will be determined by the current through RSET. The LED current can be calculated by the following equation : V ILED = FB RSET Brightness Control For the brightness dimming control of the RT8511B, the IC provides typically 200mV reference voltage when the PWM pin is constantly pulled high. However, the PWM pin allows a PWM signal to adjust the reference voltage by changing the PWM duty cycle to achieve LED brightness dimming control. The relationship between the duty cycle and the FB voltage can be calculated according to the following equation : VFB = 200mV x Duty where 200mV is the typical internal reference voltage and Duty is the duty cycle of the PWM signal. As shown in Figure 3, the duty cycle of the PWM signal is used to modify the internal 200mV reference voltage. With an on-chip output clamping amplifier and a serial resistor, the PWM dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor, CDIMC, for noise-free PWM dimming. Dimming frequency can be sufficiently adjusted from 100Hz to 8kHz. However, the LED current cannot be 100% proportional to the duty cycle. Referring to Table 1, the minimum dimming duty can be as low as 1% for the frequency range from 100Hz to 8kHz. It should be noted that the accuracy of 1% duty is not guaranteed. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8511B-05 February 2015 Because the voltage of DIMC and FB is small to 2mV and easily affected by LX switching noise. 200mV DIMC PWM R + EA - CDIMC 1µF To Controller FB Figure 3. Block Diagram of Programmable FB Voltage Table 1. Minimum Duty for Dimming Frequency Dimming Frequency Minimum Duty Cycle 100Hz to 8kHz 1% The FB pin voltage will be decreased by lower PWM duty ratio . That will achieve LED current diming function for different brightness. But LED current is more accurate when higher PWM duty. The Table 2. shows typical variation value comparison between different PWM duty and condition is VIN = 3.7V, LED array = 6S2P, RSET = 5Ω. Table 2. LED Current Variation vs PWM Duty PWM Duty (%) Variation (%) PWM Duty (%) Variation (%) 1 ±60 8 ±7 2 ±25 9 ±6 3 ±17 10 ±5 4 ±13 20 ±4 5 ±10 50 ±3 6 ±9 100 ±2.5 7 ±8 It also should be noted that when the input voltage is too close to the output voltage [(VOUT −VIN) < 6V] , excessive audible noise may occur. Additionally, for accurate brightness dimming control, the input voltage should be kept lower than the LEDs' turn on voltage. When operating in the light load, excessive output ripple may occur. is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8511B Soft-Start The RT8511B provides a built-in soft-start function to limit the inrush current, while allowing for an increased PWM frequency for dimming. Current Limiting Protection The RT8511B can limit the peak current to achieve over current protection. The IC senses the inductor current through the LX pin in the charging period. When the value exceeds the current limiting threshold, the internal N- MOSFET will be turned off. In the off period, the inductor current will descend. The internal MOSFET is turned on by the oscillator during the beginning of the next cycle. Power Sequence In order to assure that the normal soft-start function is in place for suppressing the inrush current, the input voltage and enable voltage should be ready before PWM pulls high. Figure 4 and Figure 5 show the power on and power off sequences. VIN VIN EN EN PWM PWM soft-start VOUT VOUT Mode1 Mode1 VIN VIN EN EN PWM VOUT VOUT soft-start Mode2 Mode2 VIN VIN EN EN PWM PWM VOUT soft-start VOUT Mode3 Figure 4. Power On Sequence Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 Shutdown Delay Mode3 Figure 5. Power Off Sequence is a registered trademark of Richtek Technology Corporation. DS8511B-05 February 2015 RT8511B Over Voltage Protection The RT8511B equips Over Voltage Protection (OVP) function. When the voltage at the OVP pin reaches a threshold of approximately 1.2V, the MOSFET drive output will turn off. The MOSFET drive output will turn on again once the voltage at the OVP pin drops below the threshold. Thus, the output voltage can be clamped at a certain voltage level, as shown in the following equation : VOUT, OVP = VOVP 1+ R2 R1 where R1 and R2 make up the voltage divider connected to the OVP pin. Over Temperature Protection The RT8511B has an Over Temperature Protection (OTP) function to prevent overheating caused by excessive power dissipation from overheating the device. The OTP will shut down switching operation if the junction temperature exceeds 160°C. The boost converter will start switching again when the junction temperature is cooled down by approximately 30°C. Inductor Selection The inductance depends on the maximum input current. As a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. If 40% is selected as an example, the inductor ripple current can be calculated according to the following equation : VOUT IOUT IIN(MAX) = (MIN) VIN(MIN) IRIPPLE = 0.4 IIN(MAX) where η is the efficiency of the boost converter, IIN(MAX) is the maximum input current, IOUT is the total current from all LED strings, and IRIPPLE is the inductor ripple current. The input peak current can be calculated by maximum input current plus half of inductor ripple current shown as following equation : IPEAK = 1.2 x IIN(MAX) Note that the saturated current of the inductor must be greater than IPEAK. The inductance can eventually be determined according to the following equation : VIN (VOUT VIN ) 2 L= where fOSC is the switching frequency. For better efficiency, it is suggested to choose an inductor with small series resistance. Diode Selection The Schottky diode is a good choice for an asynchronous boost converter due to its small forward voltage. However, when selecting a Schottky diode, important parameters such as power dissipation, reverse voltage rating, and pulsating peak current must all be taken into consideration. A suitable Schottky diode's reverse voltage rating must be greater than the maximum output voltage, and its average current rating must exceed the average output current. Capacitor Selection Two 1μF ceramic input capacitors and two 1μF ceramic output capacitors are recommended for driving 10 WLEDs in series. For better voltage filtering, ceramic capacitors with low ESR are recommended. Note that the X5R and X7R types are suitable because of their wide voltage and temperature ranges. Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For WDFN-8L 2x2 package, the thermal resistance, θJA, is 120°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formulas : 0.4 VOUT IOUT fOSC 2 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8511B-05 February 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8511B PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for WDFN-8L 2X2 package Layout Consideration For high frequency switching power supplies, the PCB layout is important to obtain good regulation, high efficiency and stability. The following descriptions are the suggestions for better PCB layout. Maximum Power Dissipation (W)1 The maximum power dissipation depends on operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curves in Figure 6 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation. Input and output capacitors should be placed close to the IC and connected to the ground plane to reduce noise coupling. The GND and Exposed Pad should be connected to a strong ground plane for heat sinking and noise protection. The components L, D, CIN and COUT must be placed as close as possible to reduce current loop. Keep the main current traces as possible as short and wide. The LX node of the DC/DC converter experiences is with high frequency voltage swings. It should be kept in a small area. The component RSET should be placed as close as possible to the IC and kept away from noisy devices. 8 EN PWM VIN LX 1.0 Four-Layer PCB 0.8 0.6 0.4 0.2 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 6. Derating Curve of Maximum Power Dissipation Locate RSET close to FB as possible R1 OVP FB DIMC CDIMC GND RSET : : WLEDs : : : : : : : : : : 1 3 GND R2 6 4 9 5 2 7 D L The inductor should be placed as close as possible to the switch pin to minimize the noise coupling into other circuits. LX node copper area should be minimized for reducing EMI VIN VOUT The COUT should be connected directly from the output schottky diode to ground rather than across the WLEDs. COUT CIN CIN should be placed as closed as possible to V I N pin for good filtering. Figure 7. PCB Layout Guide Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8511B-05 February 2015 RT8511B Outline Dimension D2 D L E E2 1 e SEE DETAIL A b 2 1 2 1 A A1 A3 DETAIL A Pin #1 ID and Tie Bar Mark Options Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.200 0.300 0.008 0.012 D 1.950 2.050 0.077 0.081 D2 1.000 1.250 0.039 0.049 E 1.950 2.050 0.077 0.081 E2 0.400 0.650 0.016 0.026 e L 0.500 0.300 0.020 0.400 0.012 0.016 W-Type 8L DFN 2x2 Package Richtek Technology Corporation 14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. DS8511B-05 February 2015 www.richtek.com 11