® RT8477 High Voltage High Current LED Driver General Description Features The RT8477 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications with wide input voltage (4.5V to 50V) and output voltage (up to 50V) ranges. With internal 380kHz 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 by analog. Buck, Boost Constant Current Converter High Voltage : VIN up to 50V, VOUT up to 50V 380kHz Fixed Switching Frequency Analog or PWM Control Signal for LED Dimming Internal Soft-Start to Avoid Inrush Current Under-Voltage Lockout Thermal Shutdown RoHS Compliant and Halogen Free The RT8477 is now available in the SOP-8 package. Applications Ordering Information Desk Lights and Room Lighting Industrial Display Backlight RT8477 Package Type S : SOP-8 Pin Configurations (TOP VIEW) Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : 8 VCC 2 7 DRV ISN 3 6 GND CTL 4 5 SENS RoHS compliant and compatible with the current requireSOP-8 ments of IPC/JEDEC J-STD-020. CREG ISP Suitable for use in SnPb or Pb-free soldering processes. Marking Information RT8477GS : Product Number RT8477 GSYMDNN YMDNN : Date Code Simplified Application Circuit D1 R1 VIN C1 C4 RT8477 C5 1µF VCC ... R4 10 ISP LEDs L1 ISN Analog Dimming CTL CREG GND Copyright © 2014 Richtek Technology Corporation. All rights reserved. December 2014 R3 SENS C2 DS8477-02 M1 DRV C3 R2 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8477 Functional Pin Description Pin No. Pin Name Pin Function 1 VCC Supply Voltage Input. For good bypass, a low ESR capacitor is required. 2 ISP Positive Input Current Sense. 3 ISN Negative Input Current Sense. Voltage threshold between ISP and ISN is 100mV. 4 CTL Analog Dimming Control Input. Effective programming range is 0.33V to 2V. 5 SENS Current Sense Input for LED Current. Connect the current sense resistor between external N-MOSFET switch and the ground. 6 GND Ground. 7 DRV External MOSFET Switch Gate Driver Output. 8 CREG Regulator Output. Placed 1F capacitor to stabilize the 5V regulator output. Function Block Diagram S OSC - VCC 4.5V CREG R + DRV R 5V LDO + - SENS Soft-Start GM + CTL ISN ISP + - GND Opertation The RT8477 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications. This device uses a fixed frequency, currentmode control scheme to provide excellent line and load regulation. by the control loop. The control loop has a current sense amplifier which senses the voltage between the ISP and ISN pins. linearity. The max sense threshold of 100mV can be obtained with CTL pin voltage greater than 2V (max dimming point). The sense threshold is intentionally forced to zero by an internal comparator when the CTL pin voltage is less than around 0.33V (min dimming point). Because of that, the actual sense threshold right before cut off may vary from part to part over process variation. A PWM comparator then turns off the external power switch when the sensed power switch current exceeds the internal compensated 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 Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 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 with good dimming The RT8477 provides protection functions which include over-temperature, and switch current limit to prevent abnormal situations. is a registered trademark of Richtek Technology Corporation. DS8477-02 December 2014 RT8477 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VCC ------------------------------------------------------------------------------------ −0.3V to 60V ISP, ISN ------------------------------------------------------------------------------------------------------------ −0.3V to 60V SENS, DRV, CREG Pin Voltage ----------------------------------------------------------------------------- −0.3V to 5.5V CTL Pin Voltage ------------------------------------------------------------------------------------------------- −0.3V to 20V (Note 2) Power Dissipation, PD @ TA = 25°C SOP-8 -------------------------------------------------------------------------------------------------------------- 0.53W Package Thermal Resistance (Note 3) SOP-8, θJA -------------------------------------------------------------------------------------------------------- 188°C/W Junction Temperature ------------------------------------------------------------------------------------------- 150°C Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------- 260°C Storage Temperature Range ---------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 4) HBM (Human Body Model) ------------------------------------------------------------------------------------ 2kV Recommended Operating Conditions (Note 5) Supply Input Voltage, VCC ------------------------------------------------------------------------------------ 4.5V to 50V Junction Temperature Range ---------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range ---------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VCC = 12V, 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 I VCC VCTL = 3V -- -- 3 VIN Under Voltage Lockout Threshold VUVLO VIN Rising -- 4.25 4.5 VIN Falling -- 4.2 -- 4.5 Common Mode 20V 95 100 105 I ISP VISP = 24V -- 150 -- I ISN VISN = 24V -- 50 -- Input Current of CTL Pin ICTL 0.2V VCTL 1.2V -- 1 2 A LED Current Off Threshold at CTL VCTL_OFF -- 0.33 0.5 V LED Current On Threshold at CTL VCTL_ON -- 2 2.5 V V Current Sense Amplifier Input Threshold (VISP VISN) Input Current mV A LED Dimming Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8477-02 December 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8477 Parameter Symbol Test Conditions Min Typ Max Unit 330 380 430 kHz -- -- 100 % -- 200 -- ns 4.5 5 5.5 V Gate Driver Source 1 2.5 -- A Gate Driver Sink 1 3.5 -- A -- 2 -- ms 100 150 -- mV PWM Converter Switch Frequency f SW Maximum Duty Cycle DMAX (Note 6) Minimum On-Time Gate High Voltage VGATE_H Soft-Start Time Sense Current Limit Threshold IGATE = 20mA (Note 7) ISENS_LIM Over-Temperature Protection Thermal Shutdown Temperature TSD -- 150 -- C Thermal Shutdown Hysteresis T SD -- 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, CTL 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. 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. When the natural maximum duty cycle of the switching frequency is reached, the switching cycle will be skipped (not reset) as the operating condition requires to effectively stretch and achieve higher on cycle than the natural maximum duty cycle set by the switching frequency. Note 7. Guaranteed by design, not subjected to production test. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8477-02 December 2014 RT8477 Typical Application Circuit D1 R1 VIN 4.5V to 50V C1 10µF 0.1 RT8477 1 C5 1µF ISP VCC ISN 4 CTL 8 CREG Analog Dimming DRV C2 1µF 6 GND SENS C4 1µF LEDs ... R4 10 2 L1 22µH 3 7 M1 R3 51 5 C3 1nF R2 0.03 Figure 1. Buck Configuration L1 22µH VIN VOUT 50V(Max) C1 10µF C5 1µF 1 VCC RT8477 7 DRV SENS 4 CTL 8 CREG C2 1µF 6 GND 5 M1 ISN LEDs R3 51 C3 1nF ISP C4 1µF ... R4 10 Analog Dimming R1 0.1 D1 R2 0.03 51 2 3 VF (VF > VLEDs) Figure 2. Boost Configuration Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8477-02 December 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8477 Typical Operating Characteristics Efficiency vs. Input Voltage Efficiency vs. Input Voltage 100 100 Buck, LED Current = 2A, L = 22μH VOUT VOUT VOUT VOUT 98 94 21V 18V 15V 12V 96 92 VOUT = 9V 90 88 86 Boost, LED Current = 1A, L = 22μH 98 94 Efficiency (%) Efficiency (%) 96 = = = = VOUT = 6V 92 90 VOUT VOUT VOUT VOUT 88 86 84 84 82 82 80 22V 28V 35V 40V 80 5 15 25 35 45 55 5 10 15 Input Voltage (V) 20 25 30 Input Voltage (V) LED Current vs. VCTL Supply Current vs. VCC 450 1.5 LED Current = 300mA, LED = 6pcs 400 1.4 350 LED Current (mA) Supply Current (mA) = = = = 1.3 1.2 300 250 200 150 100 1.1 50 0 1.0 0 5 10 15 20 25 30 35 40 45 0.3 50 0.6 0.9 1.2 VCC (V) ISP - ISN Threshold vs. Temperature 1.8 2.1 2.4 2.7 3 45 50 Frequency vs. VCC 120 410 110 400 Frequency (kHz)1 ISP - ISN Threshold (mV) 1.5 VCTL (V) 100 90 80 390 380 370 VCC = 24V 70 360 -40 -10 20 50 80 110 Temperature (°C) Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 140 0 5 10 15 20 25 30 35 40 VCC (V) is a registered trademark of Richtek Technology Corporation. DS8477-02 December 2014 RT8477 Power On from VIN Power Off from VIN VIN (20V/Div) VIN (20V/Div) VOUT (10V/Div) VOUT (10V/Div) IOUT (2A/Div) VIN = 30V, IOUT = 2A, LED = 42pcs, L = 47μH Time (2.5ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8477-02 December 2014 IOUT (2A/Div) VIN = 30V, IOUT = 2A, LED = 42pcs, L = 47μH Time (25ms/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8477 Application Information The RT8477 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications. This device 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. 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. 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 The converter operates in discontinuous conduction mode when the inductance value is less than the value LBCM. With an inductance greater than LBCM, the converter operates in Continuous Conduction Mode (CCM). The inductance LBCM is determined by the following equations. For Buck application : LBCM LED Current Setting The LED current can be calculated by the following equation : V(ISP ISN) ILED(MAX) = R1 where V(ISP − ISN) is the voltage between ISP and ISN (100mV typ. if CTL dimming is not applied) and the R1 is the resister between ISP and ISN. Sense Resistor Selection The resistor, R2, between the Source of the external NMOSFET and GND should be selected to provide adequate switch current to drive the application without exceeding the current limit threshold set by the SENSE pin sense threshold of RT8477. The Sense resistor value can be calculated according to the formula below : R2 Current Limlit Threshold Minimum Value IOCP where IOCP is about 1.33 to 1.5 times of inductor peak current IPEAK. The placement of R2 should be close to the source of the N-MOSFET and the IC GND of the RT8477. The SENSE pin input to RT8477 should be a Kelvin sense connection to the positive terminal of R2. Over-Temperature Protection The RT8477 has Over-Temperature Protection (OTP) function to prevent the excessive power dissipation from overheating. The OTP function will shut down switching Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 VOUT VIN VOUT 2 IOUT f VIN For Boost application : LBCM VIN VOUT VIN 2 IOUT f VOUT where VOUT = output voltage. VIN = input voltage. f = operating frequency. IOUT = LED current. Choose an inductance based on the operating frequency, input voltage and output voltage to provide a current mode ramp signal during the MOSFET on period for PWM control loop regulation. The inductance also determines the inductor ripple current. Operating the converter in CCM is recommended, which will have the smaller inductor ripple current and hence the less conduction losses from all converter components. As a design example, to design the peak to peak inductor ripple to be ±30% of the output current, the following equations can be used to estimate the size of the needed inductance : For Buck application : L= VOUT VIN VOUT 2 0.3 IOUT f VIN For Boost application : L= VIN VOUT VIN 2 0.3 IOUT f VOUT is a registered trademark of Richtek Technology Corporation. DS8477-02 December 2014 RT8477 For Buck application : VOUT VIN VOUT 2 L f VIN For Boost application : IPEAK = IOUT + IPEAK = VOUT IOUT VIN VOUT VIN + 2 L f VOUT VIN where η is the efficiency of the power converter. Schottky Diode Selection The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for RT8477 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). Capacitor Selection The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most RT8477 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 : 1 VOUT IL ESR + 8 f C OUT 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 SOP-8 package, the thermal resistance, θJA, is 188°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 : PD(MAX) = (125°C − 25°C) / (188°C/W) = 0.53W for SOP-8 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 4 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Maximum Power Dissipation (W)1 The inductor must also be selected with a saturation current rating greater than the maximum inductor current during normal operation. The maximum inductor current can be calculated by the following equations. 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 4. Derating Curve of Maximum Power Dissipation Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS8477-02 December 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8477 Layout Considerations PCB layout is very important when designing power switching converter circuits. Some recommended layout guide lines are as follows : The power components M1, 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 M1, 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 VCC pin as possible. Keep the ISP and ISN with the Kelvin sense connection VIN power trace to ISP must be wide and short. ISP VIN R1 C1 ISN D1 ... Locate input capacitor as close VCC as possible. GND L1 C2 8 CREG ISP 2 7 DRV ISN CTL 3 6 GND 4 5 SENS VCC C5 GND C4 R4 R3 C3 Power trace must be wide and short when compared to the normal trace. M1 Place these components as close as possible. R2 GND Normal trace. Figure 5. PCB Layout Guide Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8477-02 December 2014 RT8477 Outline Dimension H A M J 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 3.988 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.508 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.170 0.254 0.007 0.010 I 0.050 0.254 0.002 0.010 J 5.791 6.200 0.228 0.244 M 0.400 1.270 0.016 0.050 8-Lead SOP 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. DS8477-02 December 2014 www.richtek.com 11