® RT8479B Two-Stage Hysteretic LED Driver with Internal MOSFETs General Description Features The RT8479B is a two-stage controller with dual MOSFETs and consists of a Boost converter (first stage) and a Buck converter (second stage). The advantage of two-stage topology is highly compatible with ET (Electronic Transformer) and extremely high Power Factor performance in MR16 / AR111 lighting market fields applications. Two-Stage Topology (Boost + Buck) Dual MOSFETs Inside Wide Input Voltage Range : 4.5V to 36V Excellent Power Factor Programmable Boost Output Voltage Independent Dual Stage Function Programmable LED current with ±5% LED Current Accuracy Input Under-Voltage Lockout Detection Thermal Shutdown Protection The first stage is a Boost converter for constant voltage output with inductor peak current over-current protection. The second stage is a Buck converter for constant output current by typical constant peak current regulation. The RT8479B is equipped with dual output gate drivers for internal power MOSFETs. Ordering Information RT8479B Package Type SP : SOP-8 (Exposed Pad-Option 2) The RT8479B is available in the SOP-8 (Exposed Pad) package. Applications Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : MR16 Lighting Signage and Decorative LED Lighting Architectural Lighting High Power LED Lighting Low Voltage Industrial Lighting Indicator and Emergency Lighting Automotive LED Lighting RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Simplified Application Circuit L1 D1 VCC R1 D2 VL AC 12V D5 RT8479B OVP VCC ISN CIN C1 Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 LED- C2 VCOMP D4 D6 C3 CREG LX1 D3 RSENSE LED+ R2 VN COUT L2 LX2 GND is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8479B Marking Information Pin Configurations (TOP VIEW) RT8479BGSP : Product Number RT8479B GSPYMDNN YMDNN : Date Code 8 LX1 OVP 2 GND 3 VCOMP 4 GND LX2 7 CREG 6 VCC 5 ISN 9 SOP-8 (Exposed Pad) Functional Pin Description Pin No. Pin Name Pin Function 1 LX1 Switch Node. The first stage internal MOSFET Drain. 2 OVP Over-Voltage Protection Sense Input. 3, 9 (Exposed Pad) GND Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. 4 VCOMP Compensation Node. A compensation network between VCOMP and GND is needed. 5 ISN LED Negative Current Sense Input. 6 VCC Supply Voltage Input. For good bypass, place a ceramic capacitor near the VCC pin. 7 CREG Internal Regulator Output. Place an 1F capacitor between the CREG and GND pins. 8 LX2 Switch Node. The second stage internal MOSFET Drain. Function Block Diagram ISN VCC -130mV Regulator V VCC + - UV/OV CREG LX2 OVP Core Logic EN2 EN2 EN1 LX1 EN1 VCOMP + - Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 GND is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B Operation The RT8479B VCC is supplied from the first stage Boost output. The first stage is a constant output voltage Boost topology and senses the peak inductor current for over-current protection with excellent Power Factor. The second stage is a constant output current Buck topology. The current sense voltage threshold between the VCC and ISN pins is only 130mV to reduce power loss. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8479B Absolute Maximum Ratings (Note 1) Supply Voltage, VCC to GND -----------------------------------------------------------------------------------------CREG, OVP, VCOMP to GND ----------------------------------------------------------------------------------------LX1, LX2 to GND ----------------------------------------------------------------------------------------------------------VCC to ISN ----------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C −0.3V to 45V −0.3V to 6V −0.3V to 40V −0.3V to 3V SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------------SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- 3.44W Recommended Operating Conditions 29°C/W 2°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------- 4.5V to 40V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VCC = 10VDC, No Load, CLOAD = 1nF, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Supply Voltage CREG UVLO_ ON VUVLO_ ON OVP = 0V -- 4.2 -- V CREG UVLO_ OFF VUVLO_ OFF OVP = 0V -- 3.9 -- V VCC Shutdown Current ISHDN VCC = 3.5V -- 10 -- A VCC Quiescent Current IQ VCC = 10V -- 1.5 -- mA Internal Reference Voltage VCREG -- 5 -- V -- 4.9 -- V -- 5 -- s Supply Current Internal Reference Voltage (ICREG = 20mA) I CREG = 20mA Stage 1 Max On-Time Stage 1 OVP High-Lev el VOVP_H 1.85 1.94 2.04 Low-Level VOVP_L 1.52 1.6 1.68 -- 1 -- OVP Pin Leakage Current IOVP Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 V A is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B Parameter ISN Threshold Symbol Test Conditions Min Typ Max Unit 123.5 130 136.5 mV (dV1 + dV2) / 2 -- 15 -- % VISN Stage 2 Peak to Peak Sense Voltage LX1 Internal Switch RDS(ON) RDS(ON)_1 Sink = 100mA -- 0.2 -- LX2 Internal Switch RDS(ON) RDS(ON) _2 Sink = 100mA -- 0.3 -- 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 © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8479B Typical Application Circuit L1 10µH D1 CIN 1µF VN D3 VCC R1 130k D2 VL AC 12V D5 RT8479B 2 OVP VCC 6 R2 10k 1 LX1 4 VCOMP D4 COUT 4.7µF C1 0.47µF ISN 5 CREG 7 COUT_EC 220µF RSENSE 250m LED+ C5 C2 4.7µF C3 4.7µF 4LED D6 LED- L2 68µH LX2 8 GND 3, 9 (Exposed Pad) D1,D2, D3, D4, D5, D6 = PMEG4020 C5 depends on PCB layout and noise immunity. Figure 1. Typical MR16 LED Lamp for 5W Application Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B Typical Operating Characteristics Quiescent Current vs. Temperature 3.5 1.6 3.0 Quiescent Current (mA) Quiescent Current (mA) Quiescent Current vs.VCC 1.7 1.5 1.4 1.3 1.2 2.5 2.0 1.5 1.0 0.5 OVP = 5V VCC = 4.5V to 30V, OVP = 5V 0.0 1.1 4 9.2 14.4 19.6 24.8 -50 30 -25 0 Operating Current vs. VCC 75 100 125 Operating Current vs. Temperature 4.0 Operating Current (mA) 3.6 Operating Current (mA) 50 Temperature (°C) VCC (V) 3.2 2.8 2.4 2.0 VCC = 4.5V to 30V, LX1/LX2 Capacitor = 1nF, OVP = 0V 3.5 3.0 2.5 2.0 1.5 VCC = 10V, LX1/LX2 Capacitor = 1nF, OVP = 0V 1.0 1.6 4 9.2 14.4 19.6 24.8 -50 30 -25 0 VCC (V) 25 50 75 100 125 Temperature (°C) CREG Voltage vs. VCC CREG Voltage vs. Temperature 7 5.4 5.3 CREG Voltage (V) 6 CREG Voltage (V) 25 ICREG = 0mA 5 ICREG = −20mA 4 3 5.2 ICREG = 0mA 5.1 ICREG = −20mA 5.0 4.9 VCC = 4.5V to 30V 2 VCC = 10V 4.8 4.5 9.6 14.7 19.8 24.9 VCC (V) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 30 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8479B ISN Threshold vs. Temperature ISN Threshold vs. VCC 150 150 140 ISN Threshold (V) ISN Threshold (mV) 140 130 120 130 120 110 110 100 VCC = 10V VCC = 4.5V to 30V 90 100 4 9.2 14.4 19.6 24.8 -50 30 -25 0 OVP Hi/Low Level Voltage vs. VCC 75 100 125 OVP Hi/Low Level Voltage vs. Temperature 2.2 2.0 Hi 1.9 1.8 1.7 Low 1.6 1.5 VCC = 4.5V to 30V OVP Hi/Low Level Voltage (V) 2.1 OVP Hi/Low Level Voltage (V) 50 Temperature (°C) VCC (V) 1.4 2.1 2.0 Hi 1.9 1.8 1.7 Low 1.6 1.5 1.4 VCC = 10V 1.3 4.5 9.6 14.7 19.8 24.9 30 -50 -25 0 VCC (V) 25 50 75 100 125 Temperature (°C) LX1_RDS(ON) vs. Temperature LX2_RDS(ON) vs. Temperature 0.25 0.30 0.25 LX2 RDS(ON) (Ω) 0.20 LX1 RDS(ON) (Ω) 25 0.15 0.10 0.05 0.20 0.15 0.10 0.05 VCC = 10V 0.00 VCC = 10V 0.00 -50 -25 0 25 50 75 100 Temperature (°C) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 125 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B LED Current vs. Output Voltage 440 440 435 LED Current (mA) LED Current (mA) LED Current vs. Input Voltage 450 430 420 410 400 430 425 420 415 390 Load = 1LED to 6LED VCC = 7V to 20V, IOUT = 420mA, Load = 4LED 410 380 6 8 10 12 14 16 18 4.5 20 7.6 10.7 Input Voltage (V) PK-Current vs. Temperature 16.9 20 CREG UVLO vs. Temperature 5.0 2500 4.5 2000 UVLO-H VC = 5V 4.0 UVLO (V) PK-Current (mA) 13.8 Output Voltage (V) 1500 1000 VC = 0V UVLO-L 3.5 3.0 500 2.5 VCC = 10V 0 2.0 -50 -25 0 25 50 75 100 125 -50 -25 0 25 50 75 Temperature (°C) Temperature (°C) Power On from VIN Power Off from VIN IOUT (500mA/Div) IOUT (500mA/Div) LX2 (50V/Div) LX2 (50V/Div) VOUT (10V/Div) VOUT (10V/Div) VIN (10V/Div) VIN (10V/Div) VIN = 10V, 4LEDs Time (25ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 100 125 VIN = 10V, 4LEDs Time (25ms/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8479B Power On from AC-IN Power Off from AC-IN IOUT (200mA/Div) IOUT (200mA/Div) VOUT (10V/Div) VOUT (10V/Div) V CC (20V/Div) AC-IN (50V/Div) V CC (20V/Div) AC-IN (50V/Div) Time (10ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 Time (10ms/Div) is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B Application Information The RT8479B consists of a constant output current Buck controller and a fixed off-time controlled Boost controller. The Boost controller is based on a peak current, fixed offtime control architecture and designed to operate up to 1MHz to use a very small inductor for space constrained applications. A high-side current sense resistor is used to set the output current of the Buck controller. A 1% sense resistor performs a ±5% LED current accuracy for the best performance. Under-Voltage Lockout (UVLO) The RT8479B includes an under-voltage lockout function with 100mV hysteresis. The internal MOSFET turns off when VCC falls below 4.2V (typ.). CREG Regulator The CREG pin requires a capacitor for stable operation and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R or X5R, ceramic capacitor for best performance. A 4.7μF capacitor will be adequate for many applications. Place the capacitor close to the IC to minimize the trace length to the CREG pin and to the IC ground. An internal current limit on the CREG output protects the RT8479B from excessive on-chip power dissipation. The CREG pin has set the output to 4.3V (typ.) to protect the internal FETs from excessive power dissipation caused by not being fully enhanced. If the CREG pin is used to drive extra circuits beside RT8479B, the extra loads should be limited to less than 10mA. Internal MOSFET There are two drivers, LX1 and LX2, in the RT8479B. The driver consists of a CMOS buffer designed to drive the internal power MOSFET. It features great sink capabilities to optimize switch on and off performance without additional external components. Whenever the IC supply voltage is lower than the under voltage threshold, the internal MOSFET is turned off. Average Output Current Setting The output current that flows through the LED string is set by an external resistor, RSENSE, which is connected between the VCC and ISN terminal. The relationship between output current, IOUT, and RSENSE is shown below: IOUT = 130mV / RSENSE LED Current Ripple Reduction Higher LED current ripple will shorten the LED life time and increase heat accumulation of LED. To reduce the LED current ripple, an output capacitor in parallel with the LED should be added. The typical value of output capacitor is 4.7μF. VCC Voltage Setting The VCC voltage setting is equipped with an Over-Voltage Protection (OVP) function. When the voltage at the OVP pin exceeds threshold approximately 1.9V, the power switch is turned off. The power switch can be turned on again once the voltage at the OVP pin drops below 1.6V. For Boost applications, the output voltage can be set by the following equation : VCC(MAX) = 1.9 x (1 + R4 / R5) R4 and R5 are the voltage divider resistors from VOUT to GND with the divider center node connected to the OVP pin. For MR16 LED lamp application, the minimum voltage of VCC should maintain above 25V for stable operation. Step-Down Converter Inductor Selection The RT8479B implemented a simple high efficiency, continuous mode inductive step-down converter. The inductance L2 in Buck converter is determined by the following factors : inductor ripple current, switching frequency, VOUT/VIN ratio, internal MOSFET, topology specifications, and component parameter. The inductance L2 is calculated according to the following equation : L2 ≥ [VCC(MAX) − VOUT − VISN − (RDS2(ON) x IOUT)] x D / [fSW x ΔIOUT] where fsw is switching frequency (Hz). RDS2(ON) is the low-side switch on-resistance of external Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8479B MOSFET (M2). The typical value is 0.35Ω. D is the duty cycle = VOUT / VIN IOUT is the required LED current (A) ΔIOUT is the inductor peak-peak ripple current (internally set to 0.3 x IOUT) VCC is the supply input voltage (V) VOUT is the total LED forward voltage (V) VISN is the voltage cross current sense resistor (V) L2 is the inductance (H) The selected inductor must have saturation current higher than the peak output LED current and continuous current rating above the required average output LED current. In general, the inductor saturation current should be 1.5 times the LED current. In order to minimize output current ripple, higher values of inductance are recommended at higher supply voltages. Because high values of inductance has high line resistance, it will cause lower efficiency. Step-Up Converter Inductor Selection The RT8479B uses a constant off-time control to provide high efficiency step-up converter. Following the constant off-time mechanism, the inductance L1 is calculated according to the following equation : L1 > tOFF x (VCC(MAX) − VIN(MIN) + VF) / ILIM where tOFF is Off-Time. The typical value is 1.5μs. ILIM is the input current. The typical value is 2A for MR16 application. VCC is the supply input voltage (V) VIN is the input voltage after bridge diodes (V) VF is the forward voltage (V) L1 is the inductance (H) D = 1 − (VIN / VOUT) fsw = (1 − D) / tOFF where D is the operation duty Check the ILIM setting satisfied the output LED current request by the following equation : (IOUT + ΔIOUT) < [2 x L1 x ILIM + tOFF x (VIN − VOUT − VF)] x VIN / [2 x L1 x (VCC)] Diode Selection To obtain better efficiency, the Schottky diode is recommended for its low reverse leakage current, low recovery time and low forward voltage. With its low power dissipation, the Schottky diode outperforms other silicon diodes and increases overall efficiency. Input Capacitor selection Input capacitor has to supply peak current to the inductor and flatten 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, which is suitable for the RT8479B. For maximum stability over the entire operating temperature range, capacitors with better dielectric are suggested. Thermal Protection A thermal protection feature is to protect the RT8479B from excessive heat damage. When the junction temperature exceeds 150°C, the thermal protection will turn off the LX terminal. When the junction temperature drops below 125°C, the RT8479B will turn on the LX terminal and return to normal operation. 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. fsw is the switching frequency of Boost controller. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP-8 (Exposed Pad) package, the thermal resistance, θJA, is 29°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 formula : P D(MAX) = (125°C − 25°C) / (29°C/W) = 3.44W for SOP-8 (Exposed Pad) 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 3.6 Four-Layer PCB 3.0 2.4 1.8 1.2 0.6 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 2. Derating Curve of Maximum Power Dissipation For 5W MR16 LED Lamp application in Figure 1, the typical PCB size is 2x2 mm2 with two-layer layout plane. Under 25°C room temperature, the case temperature of RT8479B is around 65°C. If RT8479B is operated in higher output power or smaller PCB size, the thermal plane for heat dissipation should be concerned seriously. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8479B-02 August 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8479B Layout Consideration Locate input capacitor as close to the VCC as possible. D5 L1 VCC R1 OVP R2 RSENSE COUT C15 D6 C3 GND D1 D2 CIN L2 VN D3 8 LX1 VL D4 OVP 2 GND 3 VCOMP 4 GND LX2 7 CREG 6 VCC 5 ISN 9 ISN LED+ C8 C5 LED- C2 C5: VCC-ISN bypass capacitor; noise interference like inductive and magnetic pick up will be rejected by C5. C1 GND Figure 3. PCB Layout Guide Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8479B-02 August 2014 RT8479B Outline Dimension H A M EXPOSED THERMAL PAD (Bottom of Package) Y J X B F C I D Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 4.801 5.004 0.189 0.197 B 3.810 4.000 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.510 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.170 0.254 0.007 0.010 I 0.000 0.152 0.000 0.006 J 5.791 6.200 0.228 0.244 M 0.406 1.270 0.016 0.050 X 2.000 2.300 0.079 0.091 Y 2.000 2.300 0.079 0.091 X 2.100 2.500 0.083 0.098 Y 3.000 3.500 0.118 0.138 Option 1 Option 2 8-Lead SOP (Exposed Pad) Plastic 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. DS8479B-02 August 2014 www.richtek.com 15