® RT4528 High Voltage Buck LED Driver with Dimming Control General Description Features The RT4528 is a current-mode LED driver supporting wide input voltage range from 4.5V to 50V and output voltage up to 50V. With 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 RT4528 provides an internal soft-start function to avoid inrush current and thermal shutdown to prevent the device from overheat. High Voltage : VIN Up to 50V, VOUT Up to 50V Built-In 3A 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 Under-Voltage Lockout Thermal Shutdown RoHS Compliant and Halogen Free Applications The RT4528 is available in the SOP-8 (Exposed pad) package. Ordering Information Desk Lights and Room Lighting Industrial Display Backlight Marking Information RT4528 RT4528GSP : Product Number Package Type SP : SOP-8 (Exposed-Option 2) RT4528 GSPYMDNN YMDNN : Date Code Lead Plating System G : Green (Halogen Free and Pb Free) Note : Pin Configurations Richtek products are : (TOP VIEW) RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. 8 VCC Suitable for use in SnPb or Pb-free soldering processes. ISP 2 ISN 3 NC 4 GND CREG 7 SW 6 CTL 5 VC 9 SOP-8 (Exposed Pad) Simplified Application Circuit D1 VIN C1 Analog Dimming or PWM Dimming RT4528 VCC ISP CTL ISN VC R2 C4 L1 SW R1 CREG C2 C3 Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS4528-00 April 2014 RSENSE GND is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT4528 Functional Pin Description Pin No. Pin Name Pin Function Supply Voltage Input. For good bypass, connect a low ESR capacitor between this pin and GND. Positive Current Sense Input. 1 VCC 2 ISP 3 ISN Negative Current Sense Input. Voltage threshold between ISP and ISN is 100mV. 4 NC No Internal Connection. 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) Function Block Diagram SW OSC - VCC 4.5V S R + R 5V LDO CREG + - VC Soft-Start GM + ISN ISP + - GND CTL Operation The RT4528 is specifically designed to be operated in Buck converter 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 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 Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 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. 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 RT4528 provides protection functions which include over- temperature, and switch current limit to prevent abnormal situations. is a registered trademark of Richtek Technology Corporation. DS4528-00 April 2014 RT4528 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------SW Pin Voltage at Switching Off, ISP, ISN -------------------------------------------------------------------------CREG Voltage -------------------------------------------------------------------------------------------------------------CTL Voltage (Note 2) ---------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C −0.3V to 60V −0.3V to 60V −0.3V to 5.5V −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 -- VIN Falling -- 3.8 -- 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 -- 0.2 0.25 V VC Threshold for CTL Switch Off LED Dimming Input Current of CTL Pin ICTL 0.2V VCTL 1.2V LED Current off Threshold at CTL VCTL_OFF Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS4528-00 April 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT4528 Parameter Symbol Test Conditions Min Typ Max Unit 440 490 540 kHz -- -- 100 % Minimum On-Time -- 100 200 ns SW RDS(ON) -- 0.15 -- 3.5 4.2 -- A -- 5.7 -- ms PWM Converter Switch Frequency f SW Maximum Duty Cycle DMAX SW Current Limit ILIM_SW Soft-Start Time Over-Temperature Protection Thermal Shutdown Threshold 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, 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. Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS4528-00 April 2014 RT4528 Typical Application Circuit D1 VIN 4.5V to 50V C1 RT4528 1 VCC 6 CTL Analog Dimming or PWM Dimming ISP ISN 2 RSENSE 100mV 3 C4 5 VC R2 5.1M R1 10k C2 3.3nF C3 1µF 8 CREG Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS4528-00 April 2014 SW 7 L1 GND 9 (Exposed Pad) is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT4528 Typical Operating Characteristics LED Current vs. VCTL 450 95 400 90 350 LED Current (mA) Efficiency (%) Efficiency vs. Input Voltage 100 85 80 75 LED = 6pcs LED = 5pcs LED = 4pcs LED = 3pcs LED = 2pcs LED = 1pcs 70 65 60 55 300 250 200 150 100 50 IOUT = 340mA, L = 47μH 50 RSENS = 300mΩ, LED = 6pcs 0 24 26 28 30 32 34 36 38 40 0 0.5 1 Input Voltage (V) Supply Current vs. VCC 2.5 3 ISP-ISN Threshold vs. Temperature 104 ISP-ISN Threshold (mV) 2.25 Supply Current (mA) 2 105 2.30 2.20 2.15 2.10 2.05 2.00 1.95 103 102 101 100 99 98 97 96 ICC VCC = 24V 95 1.90 0 5 10 15 20 25 30 35 40 45 -50 50 -25 0 25 50 75 100 VCC (V) Temperature (°C) SW RDS(ON) vs. VCC SW RDS(ON) vs. Temperature 125 0.30 0.180 0.178 0.26 RDS(ON) ( Ω) RDS(ON) (Ω) 1.5 VCTL (V) 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 © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 50 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS4528-00 April 2014 RT4528 Frequency vs. VCC Power On from VCC 535 534 VIN (20V/Div) Frequency (kHz)1 533 532 531 VOUT (20V/Div) 530 529 528 IOUT (200mA/Div) 527 526 VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs 525 0 5 10 15 20 25 30 35 40 45 50 Time (25ms/Div) VCC (V) Switching Power Off from VCC VIN (20V/Div) SW SW (20V/Div) V IN VOUT (20V/Div) IOUT (200mA/Div) V VIN = 24V, 24V, IIOUT = 340mA, 340mA, LL == 47μH, 47μH, LED LED == 6pcs 6pcs IN = OUT = Time (50ms/Div) Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS4528-00 April 2014 VIN (10V/Div) VOUT (10V/Div) IOUT (200mA/Div) V OUT IOUT VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs Time (1μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT4528 Application Information The RT4528 is specifically designed to be operated in Buck converter 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 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 RT4528 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 RT4528 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. Over-Temperature Protection The RT4528 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 RT4528 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 Buck 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 : VOUT VOUT L= 1 f I V L(MAX) IN(MAX) The ripple current ΔIL can be calculated : V V IL = OUT 1 OUT VIN f L where, VOUT = output voltage. VIN = input voltage. f = switching frequency. Schottky Diode Selection Current Limit The RT4528 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 © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for RT4528 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). is a registered trademark of Richtek Technology Corporation. DS4528-00 April 2014 RT4528 Capacitor Selection Maximum Power Dissipation (W)1 3.6 The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most RT4528 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 : Four-Layer PCB 3.0 2.4 1.8 1.2 0.6 0.0 0 1 VOUT IL ESR + 8 f COUT 25 50 75 100 125 Ambient Temperature (°C) Figure 1. Derating Curve of Maximum Power Dissipation Thermal Considerations 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 : Layout Considerations PCB layout is very important when designing power switching converter circuits. Some recommended layout guide lines are as follows : 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 C1 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. 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. Copyright © 2014 Richtek Technology Corporation. All rights reserved. DS4528-00 April 2014 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT4528 Keep the ISP and ISN with the Kelvin sense connection. ISP VIN C1 RSENSE GND D1 C3 2 ISN 3 NC 4 GND 8 CREG 7 SW 6 CTL 5 VC 9 Locate the compensation components to VC pin as close as possible. C4 ... GND Locate input capacitor as close to VCC as possible. VCC ISP ISN L1 Place these components as close as possible. R1 R2 C2 GND Figure 2. PCB Layout Guide Copyright © 2014 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS4528-00 April 2014 RT4528 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. DS4528-00 April 2014 www.richtek.com 11