® RT8474A High Voltage Multiple-Topology LED Driver with Open Detection General Description Features The RT8474A is a current-mode LED driver supporting wide input voltage range from 4.5V to 50V in multiple topologies. With the internal 500kHz operating frequency, the size of the external PWM inductor and input/output capacitors can be minimized. High efficiency is achieved by a 100mV current sensing control. LED dimming control can be done from either analog or PWM signal. The RT8474A provides an internal soft-start function to avoid inrush current and thermal shutdown to prevent the device from overheat. The RT8474A is available in the SOP-8 (Exposed pad) package. Ordering Information High Voltage : VIN Up to 50V, VOUT Up to 50V Support Multiple-Topologies (Buck / Boost / BuckBoost) Built-In 2A Power Switch Current-Mode PWM Control 500kHz Fixed Switching Frequency Analog or PWM Control Signal for LED Dimming Internal Soft-Start to Avoid Inrush Current OVP Pin for Adjustable OVP Level Protection (for Buck Only) Under-Voltage Lockout Thermal Shutdown RoHS Compliant and Halogen Free Applications RT8474A Package Type SP : SOP-8 (Exposed-Option 2) Lead Plating System G : Green (Halogen Free and Pb Free) Desk Lights and Room Lighting Industrial Display Backlight Marking Information Note : RT8474AGSP : Product Number Richtek products are : RT8474A GSPYMDNN RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. YMDNN : Date Code Suitable for use in SnPb or Pb-free soldering processes. Simplified Application Circuit D1 VIN R5 C1 RT8474A VCC ISP C5 RSENSE ISN Analog Dimming or PWM Dimming C4 CTL L1 SW VC CREG C3 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 R3 OVP R1 R2 C2 R4 GND is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8474A Pin Configurations (TOP VIEW) 8 VCC ISP 2 ISN 3 OVP 4 GND CREG 7 SW 6 CTL 5 VC 9 SOP-8 (Exposed Pad) Functional Pin Description Pin No. Pin Name Pin Function 1 VCC Supply Voltage Input. For good bypass, connect a low ESR capacitor between this pin and GND. 2 ISP Positive Current Sense Input. 3 ISN Negative Current Sense Input. Voltage threshold between ISP and ISN is 100mV. 4 OVP Over-Voltage Protection Sense Input. OVP pin is used for OVP protection function in buck topology only. The OVP pin must be tied to ISP pin in boost and buck-boost topologies. 5 VC Compensation Node for Current Loop. 6 CTL Analog Dimming Control Input. Effective programming range is 0.2V to 1.2V. 7 SW Switch Node of the PWM Converter. 8 CREG Regulator Output for Internal Circuit. Place a 1F capacitor to stabilize the 5V output regulator. GND Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum 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. DS8474A-05 April 2015 RT8474A Function Block Diagram SW OSC - VCC 4.5V S + R OVP - ISP - 1.2V 5V LDO CREG R + + - VC Soft-Start GM + ISN ISP + - GND CTL Operation The RT8474A can be used in multiple topologies. In Buck converter applications, an OVP pin is specially designed to be used for an over voltage protection function. RT8474A uses a fixed frequency, current-mode control scheme to provide excellent line and load regulation. The control loop has a current sense amplifier which senses the voltage between the ISP and ISN pins and provides an output voltage at the VC pin. A PWM comparator then turns off the internal power switch when the sensed power switch current exceeds the compensated VC pin voltage. The power switch will not be reset by the oscillator clock in each cycle. If the comparator does not turn off the switch in a cycle, the power switch will be on for more than a full switching period until the comparator is tripped. In this manner, the programmed voltage across the sense resistor is regulated by the control loop. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 The current through the sense resistor is set by the programmed voltage and the sense resistance. The voltage across the sense resistor can be programmed by the analog or digital signal at the CTL pin. The RT8474A provides protection functions which include overtemperature, and switch current limit to prevent abnormal situations. is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8474A Absolute Maximum Ratings (Note 1) Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------SW Pin Voltage at Switching off, ISP, ISN, OVP -----------------------------------------------------------------CREG Voltage -------------------------------------------------------------------------------------------------------------CTL Voltage (Note 2) ---------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C −0.3V to 60V −0.3V to 60V −0.3V to 6V −0.3V to 20V SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 3) SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------------SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 4) 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 5) Supply Input Voltage ------------------------------------------------------------------------------------------------------ 4.5V to 50V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VCC = 5V, CIN = 1μF, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit 4.5 5 5.5 V mA Overall Regulator Output Voltage VCREG ICREG = 20mA Supply Current IVCC VC 0.4V -- -- 3 VIN Under-Voltage Lockout Threshold VUVLO VIN Rising -- 4.2 4.5 VIN Falling 3.9 4.1 -- VCTL 1.25V 97 100 103 mV V Current Sense Amplifier Input Threshold (VISP VISN) Input Current IISP VISP = 24V -- 200 -- A Input Current IISN VISN = 24V -- 20 -- A Output Current IVC 2.4V > VC > 0.3V -- ±10 -- A -- 0.4 -- V -- 1 2 A VC Threshold for CTL Switch Off LED Dimming 0.2V VCTL 1.2V Input Current of CTL Pin ICTL LED Current Off Threshold at CTL VCTL_OFF 0.15 0.2 0.25 V LED Current On Threshold at CTL VCTL_ON -- 1.2 1.5 V Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8474A-05 April 2015 RT8474A Parameter Symbol Test Conditions Min Typ Max Unit 440 500 540 kHz -- -- 100 % Minimum On-Time -- 100 200 ns SW RDS(ON) -- 0.15 -- 2 2.5 -- A VISP VOVP 1.15 1.2 1.25 V VOVP = 1.2V -- 1 -- V VOVP 1.5V -- 30 -- A (Note 6) -- 5.7 -- ms PWM Converter Switch Frequency fSW Maximum Duty Cycle DMAX SW Current Limit ILIM_SW Over-Voltage Protection and Soft-Start OVP Threshold VOVP OVP Recovery OVP Input Current IOVP Soft-Start Time Over-Temperature Protection Thermal Shutdown Threshold TSD -- 150 -- C Thermal Shutdown Hysteresis TSD -- 20 -- C 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. If connected with a 20kΩ serial resistor, PWM can go up to 40V. Note 3. 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 4. Devices are ESD sensitive. Handling precaution is recommended. Note 5. The device is not guaranteed to function outside its operating conditions. Note 6. Guarantee by design, not subject to production testing. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8474A Typical Application Circuit Buck Configuration D1 VIN 4.5V to 50V R5 10 C1 RT8474A 1 VCC C5 1µF Analog Dimming or PWM Dimming ISP 6 CTL ISN 5 VC R1 10k C2 3.3nF R2 5.1M C3 1µF 2 3 R4 (Short Option) R5 (Short Option) L1 SW 7 8 CREG RSENSE 100mV C4 R4 R3 OVP 4 GND 9 (Exposed Pad) Note : VIN, VSW, VISP, VISN < 50V Boost Configuration L1 22µH VIN R2 10 C1 10µF Analog Dimming or PWM Dimming SW C3 1µF R3 10k C5 3.3nF C4 1µF RT8474A 1 VCC R1 0.1 D1 VLED 50V (MAX) 7 OVP 4 C2 1µF 6 CTL ISP 5 VC ISN 2 20R 51R 3 8 CREG VZ VZ > VLED GND 9 (Exposed Pad) Note : 1. VIN, VSW, VISP, VISN < 50V 2. VLED : the voltage across the LED string 3. Vz : Zener diode breakdown voltage Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8474A-05 April 2015 RT8474A Buck-Boost Configuration L1 22µH VIN R2 10 C1 10µF SW C3 1µF R3 10k C5 3.3nF R1 0.1 RT8474A 1 VCC Analog Dimming or PWM Dimming D1 7 VLED C4 1µF OVP 4 C2 1µF 6 CTL ISP 5 VC ISN 2 3 20R 51R VZ VZ > VLED 8 CREG GND 9 (Exposed Pad) Note : 1. VSW < 50V, VIN + VLED < 50V 2. VLED : the voltage across the LED string 3. Vz : Zener diode breakdown voltage Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8474A Typical Operating Characteristics LED Current vs. VCTL 450 95 400 90 350 LED Current (mA) Efficiency (%) Efficiency vs. Input Voltage 100 85 80 LED = 6pcs LED = 5pcs LED = 4pcs LED = 3pcs LED = 2pcs LED = 1pcs 75 70 65 60 300 250 200 150 100 55 50 IOUT = 340mA, L = 47μH RSENS = 300mΩ, LED = 6pcs 0 50 5 10 15 20 25 30 35 40 45 0 50 0.5 1 1.5 2 2.5 3 VCTL (V) Input Voltage (V) Supply Current vs. VCC ISP-ISN Threshold vs. Temperature 120 2.30 ISP-ISN Threshold (mV) Supply Current (mA) 2.25 2.20 2.15 2.10 2.05 2.00 110 100 90 80 1.95 ICC VCC = 24V 70 1.90 0 5 10 15 20 25 30 35 40 45 -50 50 -25 0 VCC (V) 50 75 100 125 Temperature (°C) SW RDS(ON) vs. Temperature SW RDS(ON) vs. VCC 0.30 0.180 0.178 0.26 RDS(ON) ( Ω) RDS(ON) (Ω) 25 0.176 0.174 0.22 0.18 0.172 VCC = 24V VCC = 24V 0.14 0.170 0 5 10 15 20 25 30 35 40 45 VCC (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 50 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS8474A-05 April 2015 RT8474A OVP Voltage vs. Temperature Frequency vs. VCC 510 1.3 505 Frequency (kHz) OVP Voltage (V) VOVP 1.2 1.1 1.0 OVP Recovery 500 495 490 485 VCC = 24V 0.9 480 -50 -25 0 25 50 75 100 125 0 5 10 15 20 25 30 35 Temperature (°C) VCC (V) Power On from VCC Power Off from VCC VIN (20V/Div) VIN (20V/Div) VOUT (20V/Div) VOUT (20V/Div) IOUT (200mA/Div) IOUT (200mA/Div) VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs 40 45 50 = 24V, 24V, IIOUT = 340mA, 340mA, LL == 47μH, 47μH, LED LED == 6pcs 6pcs VVIN IN= OUT = Time (25ms/Div) Time (50ms/Div) OVP Turn On when LED Take Off and Restore Switching SW (50V/Div) VIN (20V/Div) VOUT (20V/Div) VOVP (1V/Div) VIN (20V/Div) VOUT (20V/Div) IOUT (500mA/Div) IOUT (200mA/Div) VIN = 30V, IOUT = 340mA, L = 47μH, LED = 7pcs Time (250ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs Time (2.5μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8474A Application Information The RT8474A can be used in multiple topologies. In Buck converter applications, an OVP pin is specially designed to be used for an over voltage protection function. RT8474A uses a fixed frequency, current-mode control scheme to provide excellent line and load regulation. The control loop has a current sense amplifier which senses the voltage between the ISP and ISN pins and provides an output voltage at the VC pin. A PWM comparator then turns off the internal power switch when the sensed power switch current exceeds the compensated VC pin voltage. The power switch will not be reset by the oscillator clock in each cycle. If the comparator does not turn off the switch in a cycle, the power switch will be on for more than a full switching period until the comparator is tripped. In this manner, the programmed voltage across the sense resistor is regulated by the control loop. Frequency Compensation The RT8474A has an external compensation pin, allowing the loop response to be optimized for specific applications. An external resistor in series with a capacitor is connected from the VC pin to GND to provide a pole and a zero for proper loop compensation. The typical value for the RT8474A is 10k and 3.3nF. LED Current Setting The LED current can be calculated by the following equation : VISP VISN ILED(MAX) = RSENSE where (VISP − VISN) is the voltage between the ISP and ISN pins (100mV typ. if CTL dimming is not applied) and the RSENSE is the resister between the ISP and ISN pins. Current Limit The RT8474A can limit the peak switch current with its internal over-current protection feature. In normal operation, the power switch is turned off when the switch current hits the loop-set value. The over-current protection function will turn off the power switch independent of the loop control when the peak switch current reaches around 2A. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 Output Over-Voltage Setting The RT8474A OVP pin provides the Over-Voltage Protection (OVP) function for buck topology only. The OVP sense threshold is referenced to the top side of the LED string with hysteresis. When the voltage difference between the ISP pin and the OVP pin exceeds a threshold of approximately 1.2V, the power switch will be turned off. The power switch can be turned on again once the voltage difference between the ISP pin and OVP pin drops below 1V. The OVP protection voltage level can be set by the resistor divider R3 and R4 across the output capacitor C4 between ISP pin and the bottom end of the LED string, with the center node of the resistor divider tied to the OVP pin. Typically, set R4 = 10kΩ˜is suggested. Over-Temperature Protection The RT8474A has Over-Temperature Protection (OTP) function to prevent the excessive power dissipation from overheating. The OTP function will shut down switching operation when the die junction temperature exceeds 150°C. The chip will automatically start to switch again when the die junction temperature cools off. Inductor Selection Choose an inductor that can handle the necessary peak current without saturating and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 4.7μH to 22μH inductor will meet the demand of most of the RT8474A applications. Inductor manufacturers specify the maximum current rating as the current where the inductance falls to certain percentage of its nominal value, typically 65%. In Multiple-Topology application where the transition between discontinuous and continuous modes occurs, the value of the required output inductor, L, can be approximated by the following equation : For Buck application : V VOUT L = OUT 1 f I V L IN(MAX) is a registered trademark of Richtek Technology Corporation. DS8474A-05 April 2015 RT8474A The ripple current ΔIL and peak current IPEAK can be calculated : Capacitor Selection V V L = IN 1 IN f I V L OUT The ripple current ΔIL and peak current IPEAK can be calculated : The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most RT8474A applications, a 4.7μF ceramic capacitor is sufficient. A value higher or lower may be used depending on the noise level from the input supply and the input current to the converter. In Buck application, the output capacitor is typically ceramic and selection is mainly based on the output voltage ripple requirements. The output ripple, ΔVOUT, is determined by the following equation : V V IL = IN 1 IN f L VOUT 1 VOUT IL ESR + 8 f COUT I VOUT IL IPEAK = OUT + 2 VIN Thermal Considerations V V IL = OUT 1 OUT VIN f L I IPEAK = IOUT + L 2 For Boost application : For Buck-Boost application : V VOUT L = OUT 1 I f V IN + VOUT L The ripple current ΔIL and peak current IPEAK can be calculated : V VOUT IL = OUT 1 L f VIN + VOUT VIN + VOUT IOUT IL IPEAK = + VIN 2 where, VOUT = output voltage. VIN = input voltage. IOUT = LED current. f = switching frequency. η = efficiency. Schottky Diode Selection The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for RT8474A applications. In addition, power dissipation, reverse voltage rating and pulsating peak current are important parameters of the Schottky diode that must be considered. The diode's average current rating must exceed the average output current. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle). Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8474A-05 April 2015 For continuous operation, do not exceed the maximum operation junction temperature 125°C. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) − TA ) / θJA where T J(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating conditions 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 the standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25°C can be calculated by 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 operating ambient temperature for fixed T J(MAX) and thermal resistance θJA. The deration curve in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power allowed. is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8474A Layout Considerations Maximum Power Dissipation (W)1 3.6 Four-Layer PCB PCB layout is very important when designing power switching converter circuits. Some recommended layout guide lines are as follows : 3.0 2.4 The power components L1, D1 and C4 must be placed as close to each other as possible to reduce the ac current loop area. The PCB trace between power components must be as short and wide as possible due to large current flow through these traces during operation. Place L1 and D1 as close to each other as possible. The trace should be as short and wide as possible. The input capacitor C5 must be placed as close to the VCC pin as possible. Place the compensation components to the VC pin as close as possible to avoid noise pickup. 1.8 1.2 0.6 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 1. Derating Curve of Maximum Power Dissipation Keep the ISP and ISN with the Kelvin sense connection. VIN power trace to ISP must be wide and short. ISP VIN RSENSE R4 C1 D1 R5 OVP C4 Locate input capacitor as close to VCC as possible. GND ISP 2 ISN OVP GND 3 GND 8 CREG 7 SW 6 CTL 9 4 5 L1 Place these components as close as possible. VC R1 Locate the compensation components to VC pin as close as possible. R3 ... C3 VCC C5 ISN R2 Power trace must be wide and short when compared to the normal trace. Normal trace. C2 GND Figure 2. PCB Layout Guide Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8474A-05 April 2015 RT8474A 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. DS8474A-05 April 2015 www.richtek.com 13