® RT8458 High Efficiency PWM Buck LED Driver Controller General Description Features The RT8458 is a PWM controller with an integrated high side floating gate driver. It is used for step down converters by well controlling the external MOSFET and regulating a constant output current. The output duty cycle of the RT8458 can be up to 100% for wider input voltage application, such as E27 and PAR30 off-line LED lighting products. z Low Cost and Efficient Buck Converter Solution z Universal Input Voltage Range with Off-Line Topology Programmable Constant LED Current Dimmable LED Current by ACTL Output LED String Open Protection Output LED String Short Protection Output LED String Over Current Protection Built-in Thermal Protection TSOT-23-6 Package RoHS Compliant and Halogen Free The RT8458 also features a 47kHz fixed frequency oscillator, an internal −178mV precision reference, and a PWM comparator with latching logic. The accurate output LED current is achieved by an averaging current feedback loop and the LED current dimming can be easily controlled via the ACTL pin. The RT8458 also has multiple features to protect the controller from fault conditions, including Under Voltage Lockout (UVLO), Over Current Protection (OCP) and Over Voltage Protection (OVP). Additionally, to ensure the system reliability, the RT8458 is built with the thermal protection function. z z z z z z z z Applications z E27, PAR30, Offline LED Lights Pin Configurations (TOP VIEW) SENSE VC ACTL The RT8458 is housed in a TSOT-23-6 package. Thus, the components in the whole LED driver system can be made very compact. 6 5 4 2 3 VCC GND GATE Ordering Information TSOT-23-6 RT8458 Package Type J6 : TSOT-23-6 Lead Plating System G : Green (Halogen Free and Pb Free) Marking Information 01=DNN 01= : Product Code DNN : Date Code Note : Richtek 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. Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8458-03 October 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8458 Typical Application Circuit VIN CIN 10µF/ 400V RVCC1 1M RVCC2 511k RB 10 RT8458 1 VCC ACTL 4 CVCC 4.7µF CVC2 3.3nF D2 FR107 RACTL 1M 5 VC SENSE 6 RVC 2 RG 22 GND GATE 3 10k CVC1 Optional 1nF VIN : 90 to 264VAC ZD1 short Optional Q1 LED+ ZD2 39V Optional LED- COUT 47µF/50V D1 ES1J VOUT : 28V IOUT : 350mA L 680µH RS 0.51 Figure 1. Typical Application for LED Lamp Functional Pin Description Pin No. Pin Name Pin Function 1 VCC Power Supply Pin of the Chip. For good bypass, a ceramic capacitor near the VCC pin is required. 2 GND Ground of the Chip. 3 GATE 4 ACTL 5 VC Gate Driver for External MOSFET Switch. Analog Dimming Control. The typical effective dimming range is between 0V to 1.3V. PWM Loop Compensation Node. 6 SENSE LED Current Sense Input Pin. Typical sensing threshold is −178mV. Function Block Diagram + VCC + - Chip Enable 12V 17V/7.5V OVP + 35.5V - OSC GATE S 200k R R + - Control Circuit VC Dimming Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 ACTL - SENSE + GND is a registered trademark of Richtek Technology Corporation. DS8458-03 October 2013 RT8458 Absolute Maximum Ratings z z z z z z z z z z z (Note 1) Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------GATE Voltage -------------------------------------------------------------------------------------------------------------ACTL Voltage (Note 5) ------------------------------------------------------------------------------------------------VC Voltage -----------------------------------------------------------------------------------------------------------------SENSE Voltage -----------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C TSOT-23-6 ------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) TSOT-23-6, θJA ------------------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- Recommended Operating Conditions z z −0.3V to 40V −0.3V to 14V −0.3V to 8V −0.3V to 6V −1V to 0.3V 0.392W 255°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------- 17V to 32V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Electrical Characteristics (VCC = 24VDC, TA = 25°C, unless otherwise specified) Parameter Input Start-Up Voltage Symbol Test Conditions Min Typ Max Unit VST 15 17 19 V VIN(MIN) 6 7.5 9 V Minimum Operation Voltage After Start-Up Maximum Startup Current in VCC Hiccup Operation Input Quiescent Current IST(MAX) Maximum ICC at low end of VCC -- 250 300 μA IQC After Start-Up, VCC = 24V -- 1.65 5 mA Input Shutdown Current ISHDN Before Start-Up, VCC = 15V -- 0.1 5 μA Over Voltage Protection VOVP VCC Pin 32.5 35.5 36.5 V Current Sense Voltage VSENSE −169 −178 −187 mV Switching Frequency f SW 38 47 55 kHz Oscillator Maximum Duty Cycle DMAX -- -- 100 % Minimum Turn-On Time tON(MIN) 300 -- -- ns GATE Pin Maxim um Voltage VGATE No Load at GATE Pin 11.5 12.5 13.5 V GATE Voltage High VGATE_H IGATE = −20mA 11.4 12.4 13.4 IGATE = −100μA VGATE_L IGATE = 20mA 12.5 0.75 13.5 GATE Voltage Low 11.5 0.55 0.95 IGATE = 100μA 0.3 0.5 0.7 Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8458-03 October 2013 VC = 3V V V is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8458 Parameter Min Typ Max Unit 1nF Load at GATE -- 40 60 ns 1nF Load at GATE -- 0.5 -- A -- 1 5 μA 0 -- 1.3 V High Level -- 1.2 1.3 Low Level 0 0.1 -- 1.1 1.25 1.4 V 150 -- -- °C GATE Drive Rise and Fall Time GATE Drive Source and Sink Peak Current ACTL LED Dimming Analog Dimming ACTL Pin Input Current Analog Dimming Range Analog Dimming Threshold Voltage Symbol Test Conditions IACTL VC Threshold for PWM Switch Off VVC V Thermal Protection Thermal Shutdown Temperature TSD 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 low effective thermal conductivity single-layer test board per JEDEC 51-3. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Note 5. If the ACTL pin is connected with a serial 1MΩ resistor, the maximum voltage can go up to 36V. Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8458-03 October 2013 RT8458 Typical Operating Characteristics Efficiency vs. Input Voltage 100% 100 VIN = 85V to 264VAC, IOUT = 350mA, LED 3 to 10 pcs 10LEDs 9LEDs 8LEDs 7LEDs 95% 95 Efficiency vs. Number of LEDs 85 85% 6LEDs 5LEDs 4LEDs 3LEDs 75 75% VIN = 85V to 264VAC, IOUT = 350mA 95% 95 Efficiency (%) Efficiency (%) 90 90% 80 80% 100% 100 90 90% 85 85% 85VAC 110VAC 180VAC 150VAC 220VAC 264VAC 80% 80 75 75% 70 70% 70 70% 85 105 125 145 165 185 205 225 245 3 265 4 Input Voltage (V) Switching Frequency vs. Supply Voltage 8 9 10 Switching Frequency vs. Temperature Switching Frequency (kHz)1 Switching Frequency (kHz)1 7 50 51 47 43 39 48 46 44 42 40 35 0 5 10 15 20 25 30 -50 35 -25 25 50 75 100 125 LED Current vs. Input Voltage LED Current vs. VACTL 500 0 Temperature (°C) Supply Voltage (V) 380 VIN = 110VAC, IOUT = 350mA, LED 10 pcs, L = 0.68mH, VACTL = 0 to 1.8V LED Current (mA) 400 LED Current (mA) 6 Number of LED (pcs) 55 300 200 100 0 VIN = 85V to 264VAC, IOUT = 350mA, LED 3 to 10 pcs 6LEDs 5LEDs 4LEDs 3LEDs 370 10LEDs 9LEDs 8LEDs 7LEDs 360 350 340 0 0.3 0.6 0.9 1.2 1.5 VACTL (V) Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8458-03 5 October 2013 1.8 85 115 145 175 205 235 265 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8458 LED Current vs. Output Voltage Input Voltage and Input Current 400 VIN (400V/Div) LED Current (mA) 380 360 I IN (500mA/Div) 340 320 VIN = 110VAC IOUT = 350mA, LED 3 to 10 pcs, L = 0.68mH VIN = 264VAC, IOUT = 350mA, LED 10 pcs, L = 0.68mH 300 10 13 16 19 22 25 28 31 34 Time (25ms/Div) 37 Output Voltage (V) Output Current and Output Voltage Ripple VGATE Voltage and Inductor Current VOUT (1V/Div) VGATE (10V/Div) IOUT (10mA/Div) IL (500mA/Div) VIN = 264VAC, IOUT = 350mA, LED10 pcs, L = 0.68mH VIN = 264VAC, IOUT = 350mA, LED 10 pcs, L = 0.68mH Time (10μs/Div) Time (10μs/Div) Power On Power Off VIN (400V/Div) VIN (400V/Div) VOUT (20V/Div) VOUT (20V/Div) IOUT (200mA/Div) VIN = 264VAC, IOUT = 350mA, LED 10 pcs, L = 0.68mH Time (25ms/Div) Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 IOUT (200mA/Div) VIN = 264VAC IOUT = 350mA, LED 10 pcs, L = 0.68mH Time (25ms/Div) is a registered trademark of Richtek Technology Corporation. DS8458-03 October 2013 RT8458 Application Information The RT8458 is a high efficiency PWM buck LED driver controller for high brightness LED application. Its high side floating gate driver is used to control the buck converter via an external MOSFET and regulate the constant output current. The RT8458 can achieve high accuracy LED output current via the average current feedback loop control. The internal sense voltage (−178mV typ.) is used to set the average output current. The oscillator’s frequency is fixed at 47kHz to get better switching performance. Once the average current is set by the external resistor, RS, the output LED current can be dimmed by varying the ACTL voltage. Under Voltage Lockout (UVLO) The RT8458 includes a UVLO feature with 9.5V hysteresis. The GATE terminal turns on when VCC rises over 17V (typ.). The GATE terminal turns off when VCC falls below 7.5V (typ.) Setting Average Output Current The output current that flows through the LED string is set by an external resistor, RS, which is connected between the GND and SENSE terminal. The relationship between output current, IOUT, and RS is shown below : 0.178 IOUT = (A) RS Analog Dimming Control The ACTL terminal is driven by an external voltage, VACTL, to adjust the output current to an average value set by RS. The voltage range for VACTL to adjust the output current is from 0V to 1.3V. If VACTL becomes larger than 1.3V, the output current value will just be determined by the external resistor, RS. V IOUTavg = (0.178V/RS ) × ACTL 1.3 Component Selection For component selection, an example is shown below for a typical RT8458 application, where VIN = 110 to 90VAC/ 60Hz, LED output voltage = 30V, and output current = 200mA. The user can follow this procedure to design applications with wider AC voltage input and DC output voltage as well. Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8458-03 October 2013 Start-up Resistor Start-up resistor should be chosen not to exceed the maximum start-up current. Otherwise, the RT8458 may latch low and will never start. Start-up current = 130V/R1 for 110VAC regions, 260V/R1 for 220VAC regions. The typical start-up current is 250μA. Input Diode Bridge Rectifier Selection The current rating of the input bridge rectifier is dependent on the VOUT /VIN transformation ratio. The voltage rating of the input bridge rectifier, VBR, on the other hand, is only dependent on the input voltage. Thus, the VBR rating is calculated as below : VBR = 1.2 × ( 2 × VAC(MAX) ) where VAC,Max is the maximum input voltage (RMS) and the parameter 1.2 is used for safety margin. For this example : VBR = 1.2 × ( 2 × VAC(MAX) ) = (1.2 × 2 × 110) = 187V If the input source was universal, VBR will reach 466V. In this case, a 600V, 0.5A bridge rectifier can be chosen. Input Capacitor Selection The input capacitor supplies the peak current to the inductor and flattens the current ripple on the input. The low ESR condition is required to avoid increasing power loss. The ceramic capacitor is recommended due to its excellent high frequency characteristic and low ESR. For maximum stability over the entire operating temperature range, capacitors with better dielectric are suggested. The minimum capacitor is given by : VOUT(MAX) × IOUT(MAX) CIN ≥ ⎡( 2 × VAC(MIN) )2 − V 2DC(MIN) ⎤ ×η × fAC ⎣ ⎦ where fAC is the AC input source frequency and η is the efficiency of whole system. Notice that VDC(MIN) is the minimum voltage at bridge rectifier, output and VDC(MIN) should be larger than 2 x VOUT(MAX). For a 90 to 264VAC universal input range, the VDC(MIN) is 90V, therefore the LED string voltage VOUT(MAX) should be less than 45V. is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8458 For this particular example : 30 × 0.2 CIN ≥ = 13.7μF 2 ⎡( 2 × 90) − 902 ⎤ × 0.9 × 60 ⎣ ⎦ In addition, the voltage rating of the input filter capacitor, VCIN, should be large enough to handle the input voltage. VCIN ≥ (1.2 × 2 × VAC(MAX) ) = (1.2 × 2 × 110) = 187V Thus, a 22μF / 250V electrolytic capacitor can be chosen in this case. Due to its large ESR, the electrolytic capacitor is not suggested for high current ripple applications. For DC applications, an input capacitor, CIN, is needed to filter out the trapezoid current on the high side MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used. Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the design. Generally, one 10μF low ESR ceramic capacitor is recommended for the input capacitor. Ceramic capacitors have high ripple current, high voltage rating and low ESR, which makes them ideal for switching regulator applications. Forward Diode Selection When the power switch turns off, the path for the current is through the diode connected between the switch output and ground. This forward biased diode must have minimum voltage drop and recovery time. The reverse voltage rating of the diode should be greater than the maximum input voltage and the current rating should be greater than the maximum load current. In reality, the peak current through the diode is more than the maximum output current. This component current rating should be greater than 1.2 times the maximum load current and the diode reverse voltage rating should be greater than 1.2 times the maximum input voltage, assuming a ± 20% output current ripple. The peak voltage stress of diode is : VD = 1.2 × ( 2 × VAC(MAX) ) = 1.2 × ( 2 × 110) = 187V The current rating of diode is : ID = 1.2 × IOUT,PK = 1.2 × 1.2 × 0.2 = 0.288A If the input source is universal (VIN = 90V to 264V), VD will reach 466V. A 600V, 2A ultra-fast diode can be used in this example. MOSFET Selection Inductor Selection The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current, ΔIL, increases with higher VIN and decreases with higher inductance, as shown in equation below : ⎤ ⎡V ⎤ ⎡ V ΔIL = ⎢ OUT ⎥ × ⎢1− OUT ⎥ VIN ⎦ ⎣ fxL ⎦ ⎣ To optimize the ripple current, the RT8458 operates the buck converter in BCM (Boundary-Condition Mode). The largest ripple current will occur at the highest VIN. To guarantee that the ripple current stays below the specified value, the inductor value should be chosen according to the following equation : L= VOUT × TS × (1− D) 2 × IOUT 30 × 20.83μs × (1− 0.333) = = 1.04mH 2 × 0.2 where D is the duty cycle and TS is the switching period. Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 The peak current through this MOSFET will be over the maximum output current. This component current rating should be greater than 1.2 times the maximum load current and the reverse voltage rating of the MOSFET should be greater than 1.2 times the maximum input voltage, assuming a ± 20% output current ripple. The peak voltage rating of the MOSFET is : VQ = 1.2 × ( 2 × VAC(MAX) ) = 1.2 × ( 2 × 110) = 187V The current rating of MOSFET is : IQ = 1.2 × IOUT,PK = 1.2 × 1.2 × 0.2 = 0.288A If the input source was universal (VIN = 90V to 264V), VQ will reach 466V. A 600V, 2A N-MOSFET can be chosen for this example. Output Capacitor Selection The selection of COUT is determined by the required ESR to minimize output voltage ripple. Moreover, the amount of bulk capacitance is also a key for COUT selection to is a registered trademark of Richtek Technology Corporation. DS8458-03 October 2013 RT8458 where fOSC is the switching frequency and ΔIL is the inductor ripple current. The output voltage ripple will be the highest at the maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirement. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all common selections and available in surface mount packages. Tantalum capacitors have the highest capacitance density, but it is important to only use ones that pass the surge test for use in switching power supplies. Special polymer capacitors offer very low ESR value, but with the trade-off of lower capacitance density. Aluminum electrolytic capacitors have significantly higher ESR, but still can be used in cost-sensitive applications for ripple current rating and long term reliability considerations. Thermal Protection A thermal protection feature is included to protect the RT8458 from excessive heat damage. When the junction temperature exceeds a threshold of 150°C, the thermal protection will turn off the GATE terminal. Soldering Process of Pb-free Package Plating To meet the current RoHS requirements, pure tin is selected to provide forward and backward compatibility with both the current industry standard SnPb-based soldering processes and higher temperature Pb-free processes. In the whole Pb-free soldering processes pure tin is required with a maximum 260°C (<10s) for proper soldering on board, referring to J-STD-020 for more information. 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 TSOT-23-6 package, the thermal resistance, θJA, is 255°C/ W on a standard JEDEC 51-3 single-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by the following formula : PD(MAX) = (125°C − 25°C) / (255°C/W) = 0.392W for TSOT-23-6 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 2 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Maximum Power Dissipation (W)1 ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response. The output voltage ripple, ΔVOUT, is determined by : ⎡ ⎤ 1 ΔVOUT ≤ ΔIL ⎢ESR + ⎥ 8fOSCCOUT ⎦ ⎣ 0.45 Single-Layer PCB 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0 25 50 75 100 125 Ambient Temperature (°C) Figure 2. Derating Curve of Maximum Power Dissipation 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 Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8458-03 October 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8458 Layout Considerations ` Keep the main current traces as short and wide (2 to 3mm) as possible. Lay out the traces straight without any via. For best performance of the RT8458, the following layout guidelines should be strictly followed. ` The hold up capacitor, CVCC, must be placed as close as possible to the VCC pin. ` Place L, Q1, RS, and D1 as close to each other as possible. ` The output capacitor, COUT, must be placed as close as possible to the LED terminal. ` ` The power GND should be connected to a strong ground plane. The components Q1, D1, D2, AC Line L / N terminal and CIN could take very high voltage. Please keep the gaps between them to be larger than 3mm to meet the requirements of safety standards. ` RS should be connected between the GND pin and SENSE pin. ` The trace from the GATE pin to Q1 should be short and has no vias. ` AC Line L / N layout traces should not cross and overlap LED+ and LED− traces to prevent the noise interference between each other. RVCC1 RACTL RT8458 L VIN CIN N RVC CVCC D2 RB 1 VCC ACTL 4 5 VC SENSE 6 2 GND GATE 3 RG Q1 CVC RS CS Analog GND L LED+ COUT D1 Power GND LED- Place the capacitor CVCC as close as possible to the VCC. Place the output capacitor COUT as close as possible to LED terminal. Figure 3. PCB Layout Guide Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8458-03 October 2013 RT8458 Outline Dimension H D L C B b A A1 e Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 1.000 0.028 0.039 A1 0.000 0.100 0.000 0.004 B 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 C 2.591 3.000 0.102 0.118 D 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 TSOT-23-6 Surface Mount 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. DS8458-03 October 2013 www.richtek.com 11