® RT8464 Isolated Secondary Side 6-CH LED Driver with Optocoupler Driver General Description The RT8464 is an isolated secondary side 6-CH LED driver that delivers well matched LED current to each channel of LED strings. A built-in optocoupler driver supports the transformer primary side control circuit to regulate the LED current. The LED current is linearly controlled by a high gain amplifier that drives the optocoupler to control the primary side, thus making the conventional secondary DC/DC PWM switching stage unnecessary. The RT8464 selects and regulates the LED strings with the highest voltage, hence it allows voltage mismatches between LED strings. The RT8464 automatically detects and disconnects any unconnected and/or broken strings during operation from the PWM loop to prevent VOUT from over voltage. The LED currents on all channels can be simply programmed with a resistor on each channel. Three convenient dimming methods are provided : 1. Analog dimming is linearly controlled by an external voltage; 2. True digitally controlled PWM dimming controls duty cycle of LED current; 3. PWM dimming signal can also be easily filtered into analog dimming signal for noise-free PWM dimming by the low pass filter with an external capacitor at ACTL pin. Other protecting features include output over voltage protection and thermal shutdown. The RT8464 is available in the SOP-28 package. Features z z z z z z z z z z High Voltage : VCC up to 32V Channel Current Programmable 3% Current Sense Threshold Voltage Easy Analog and Digital Dimming Control Adjustable Soft-Start to Avoid Inrush Current Automatic Detecting Unconnected Channel Adjustable Over Voltage Protection to Limit Output Voltage Thermal Shutdown Under Voltage Lockout SOP-28 Package RoHS Compliant and Halogen Free Applications z z z z Building and Street Lighting LED TV Backlight LED Monitor Backlight Industrial Display Backlight Simplified Application Circuit Isolation Barrier VIN EN RT8464 Primary Side Controller VCC LEDx x6 GATEx x6 SENSEx x6 VC SS Dimming Signal ACTL RSENSE OPTO x6 GND Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8464-00 November 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8464 Ordering Information Pin Configurations RT8464 (TOP VIEW) Package Type S : SOP-28 Lead Plating System G : Green (Halogen Free and Pb Free) 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. Marking Information VCC OPTO CL EN OVP SS VC ACTL DCTL SENSE6 GATE6 LED6 SENSE5 GATE5 RT8464GS : Product Number RichTek RT8464 GSYMDNN 28 2 27 3 26 4 25 5 24 6 23 7 22 8 21 9 20 10 19 11 18 12 17 13 16 14 15 GND LED1 GATE1 SENSE1 LED2 GATE2 SENSE2 LED3 GATE3 SENSE3 LED4 GATE4 SENSE4 LED5 SOP-28 YMDNN : Date Code Functional Pin Description Pin No. Pin Name Pin Function 1 VCC Power Supply Input of the Chip. For good bypass, a low ESR capacitor is needed between this pin and GND. 2 OPTO Optocoupler Driver. It is an open collector output of the internal NPN transistor. 3 CL Current Limit. Current limit threshold is 100mV. 4 EN Chip Enable. The chip is active when VEN > 1.4V. 5 OVP Over Voltage Detection Input. OVP pin threshold is 1.2V (typ.). 6 SS Soft-Start. There is an internal constant current (6μA) to the SS pin during startup. Connect a capacitor to set soft-start time. 7 VC PWM Loop Compensation. 8 ACTL Analog/PWM Dimming Control Input. When using in analog dimming, ACTL control range is from 0.4V to 1.2V. 9 DCTL Digital Dimming Control Input. By adding a 0.1μF filter capacitor at ACTL, the digital dimming signal at DCTL pin will be averaged out and converted into analog dimming signal at ACTL pin. 10, 13, 16, 19, 22, 25 11, 14, 17, 20, 23, 26 12, 15, 18, 21, 24, 27 28 SENSE6 to SENSE1 GATE6 to GATE1 LED6 to LED1 Current Sense Input for LED Current. The threshold is 215mV and ILED = 0.215V / RSENSE GND Ground. Gate Drive Output for External Current Source MOSFETs. LED String Voltage Sensing Input. This pin is connected to the Drain of external current source MOSFET. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8464-00 November 2012 RT8464 Function Block Diagram OVP + 1.2V - VCC OPTO 1.2V + + 0.7V Min Select + VC -0.1V CL 6 LEDx 6 GATEx - + 6 - SENSEx 2.5V/6µA SS 1.4V + EN - Shutdown 5V 1.2V DCTL + - + - GND ACTL Operation The RT8464 regulates the lowest cathode voltage of LED strings and generates a feedback control signal to a primary controller to regulate LED current. Each LED channel current is accurately matched and controlled by sensing an external resistor in series with the MOSEFT. LED current dimming in all six channels can be precisely controlled by either a PWM signal via the DCTL input pin or by an analog dimming voltage applied at the ACTL pin. Internal protection is provided for over current and over voltage at the secondary output. The device can be operated in low current shutdown mode by pulling the EN pin low. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8464-00 November 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8464 Absolute Maximum Ratings (Note 1) Supply Voltage, VCC ----------------------------------------------------------------------------------------------------SENSEx --------------------------------------------------------------------------------------------------------------------(1) z LEDx ---------------------------------------------------------------------------------------------------------------------(2) z DCTL, ACTL, EN, OVP ------------------------------------------------------------------------------------------------z CL ----------------------------------------------------------------------------------------------------------------------------z Power Dissipation, PD @ TA = 25°C SOP-28 ---------------------------------------------------------------------------------------------------------------------z Package Thermal Resistance (Note 2) SOP-28, θJA ----------------------------------------------------------------------------------------------------------------z Junction Temperature ----------------------------------------------------------------------------------------------------z Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------z Storage Temperature Range -------------------------------------------------------------------------------------------z ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------Notes : (1) Add a series resistor of at least 30kΩ for higher pin voltage. (2) Add a series resistor of at least 20kΩ for higher pin voltage. z z Recommended Operating Conditions z z z −0.3V to 34V −0.3V to 12V −0.3V to 20V −0.3V to 10V −1V to 0.3V 1.31W 76.5°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Voltage, VCC ----------------------------------------------------------------------------------------------------- 10V to 32V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VCC = 24V, No load on any output, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit OVERALL VCC Supply Current IVCC VVC ≤ 0.4V (Not Switching) -- 5 8 mA Shutdown Current ISHDN VEN ≤ 0.7V -- 15 -- μA Shutdown Threshold at EN VEN -- 1.4 1.6 V Input Current at EN IEN -- -- 0.1 μA 204 215 226 mV -- -- 0.5 μA -- 0.4 0.5 V -- -- 0.5 μA -- 0.1 -- V Highest Voltage LED String -- -- 0.1 V 2.4V > VVC > 0.2V -- ±30 -- μA VEN ≤ 5V LED Current Programming SENSEx Threshold Analog Dimming Input Current at ACTL LED Current Off Threshold at ACTL 6V > VGATEx > 2V VACTL Input Current at DCTL IDCTL 0.3V ≤ VACTL ≤ 1.3V 0.3V ≤ VDCTL ≤ 6V VSENSEx Threshold for No Connection Regulated VLEDx Amplifier gm Output Current IVC Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8464-00 November 2012 RT8464 Parameter Symbol Test Conditions Min Typ Max Unit -- 0.15 -- V IGATEx = −1mA 4.2 4.8 -- IGATEx = −0.1mA 4.5 5 -- IGATEx = 1mA -- 0.8 1.1 IGATEx = 0.1mA -- 0.7 1 1.17 1.2 1.25 V Optocoupler Driver OPTO Low Voltage IOPTO = 2mA, VVC = 1.6V LED Current Sources Gate Driver GATEx High Voltage GATEx Low Voltage V V OVP and Soft-Start OVP Threshold VOVP OVP Input Current VOVP ≤ 1.2V -- -- −100 nA Soft-Start Current at SS VSS ≤ 2.5V -- 6 -- μA -- 150 -- °C 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. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8464-00 November 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8464 Typical Application Circuit 10V to 32V AC Power Input Primary Side Control Circuit R1 5 OVP RT8464 6 x N LEDs R2 1 VCC LED1 27 GATE1 26 25 SENSE1 2 Optocoupler 3 CVC RS1 OPTO CL LED2 24 GATE2 RVC CSS 7 VC M1 23 M2 SENSE2 22 RS2 6 SS LED3 21 GATE3 20 Chip Enable 4 EN 9 DCTL Dimming Signal 8 ACTL 28 GND M3 SENSE3 19 RS3 LED4 18 GATE4 17 SENSE4 M4 16 RS4 LED5 15 GATE5 14 M5 SENSE5 13 RS5 LED6 12 GATE6 11 10 SENSE6 M6 RS6 Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8464-00 November 2012 RT8464 Typical Operating Characteristics OVP vs. Input Voltage 1.240 9 1.235 8 1.230 7 OVP (V) Supply Current (mA) Supply Current vs. Input Voltage 10 6 5 4 3 1.225 1.220 OVP_H 1.215 1.210 OVP_L 2 1.205 1 VIN = 10V to 32V VIN = 10V to 32V 1.200 0 10 12 14 16 18 20 22 24 26 28 30 10 32 12.75 15.5 18.25 Input Voltage (V) LED Current vs. ACTL PWM Duty 225 225 125 LED Current (mA) LED Current (mA) LED1 (mA) LED2 (mA) LED3 (mA) LED4 (mA) LED5 (mA) LED6 (mA) 150 100 75 50 LED1 (mA) LED2 (mA) LED3 (mA) LED4 (mA) LED5 (mA) LED6 (mA) 200 175 150 125 100 75 50 25 25 VIN = 15V, PWM = 100Hz, RSENSE = 0.95Ω 0 VIN = 15V, RSENSE = 0.95Ω 0 0 10 20 30 40 50 60 70 80 90 100 0.4 0.6 0.7 0.8 0.9 100 7.2 95 6.4 Gate Voltage (V) 90 85 80 75 70 VIN = 10V to 32V, IOPTO = 1mA 60 14 16 18 20 22 24 26 28 Input Voltage (V) Copyright © 2012 Richtek Technology Corporation. All rights reserved. November 2012 1.2 1.3 1.4 4.8 4.0 3.2 2.4 Low 0.8 VIN = 10V to 32V 0.0 12 1.1 High 5.6 1.6 65 10 1 Gate Voltage vs. Input Voltage OPTO Voltage (Low) vs. Input Voltage OPTO Voltage Low (mV) 0.5 ACTL Voltage (V) ACTL PWM Duty (%) DS8464-00 32 LED Current vs. ACTL Voltage 250 175 23.75 26.5 29.25 Input Voltage (V) 250 200 21 30 32 10 12.75 15.5 18.25 21 23.75 26.5 29.25 32 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8464 Application Information The RT8464 regulates the lowest cathode voltage of LED strings and generates a feedback control signal to a primary controller to regulate LED current. Each LED channel current is accurately matched and controlled by sensing an external resistor in series with the MOSEFT. LED current dimming in all six channels can be precisely controlled by either a PWM signal via the DCTL input pin or by an analog dimming voltage applied at the ACTL pin. Internal protection is provided for over current and over voltage at the secondary output. The device can be operated in low current shutdown mode by pulling the EN pin low. Under Voltage Lockout The input operating voltage range of the RT8464 is from 10V to 32V. Placing an input capacitor at the VCC pin can reduce ripple voltage. It is recommended to use a 10μF ceramic capacitor or larger capacitance as the input capacitor. This IC provides an Under Voltage Lockout (UVLO) function to enhance the stability when start up. The UVLO rising input voltage threshold is set at 7V typically with a 0.7V hysteresis. Soft-Start Soft-start of the RT8464 can be achieved by connecting a capacitor from the SS pin to GND. The built-in soft-start circuit reduces the start-up current spike and output voltage overshoot. The soft-start time is determined by the external capacitor charged by an internal 6μA constant charging current. The SS pin directly limits the rate of voltage rise at the VC pin, which in turn limits the peak switch current. The soft-start interval is set by the soft-start capacitor selection according to the following equation : tSS = CSS x 2.5V 6μ A A typical value for the soft-start capacitor is 0.1μF. The soft-start pin reduces the oscillator frequency and the maximum current in the switch. The soft-start capacitor is discharged when EN/UVLO falls below its threshold or during an over temperature event. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 Optocouple Driver The OPTO output directly drives the diode side of an optocoupler to give isolated feedback control of a primary side PWM controller. The guaranteed sink current of the opto pin is 1mA. When the LEDx (x = 1 to 6) voltage decreases to below 0.7V, the error amplifier reacts by lowering the VC pin voltage, thereby decreasing the current from optocoupler. The decreased optocoupler bias signals the primary side controller to increase the amount of power and then raise the output voltage back to its regulated value. The output voltage of OPTO also responds to different protection conditions. When OVP or current limit occurs, the OPTO pin will sink more current immediately from the optocoupler. The increased optocoupler bias signal on the primary side controller will decrease secondary side output power. Compensation The RT8464 uses an internal error amplifier, in which through its compensation pin (VC) allows the loop response to be optimized for specific application. The error amplifier is a true voltage mode error amplifier and frequency compensation is performed around the amplifier. VC also ties to the overshoot control amplifier logic that detects if the VC pin is at its high clamp level. 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. LED Current Setting The maximum current of channel 1 to 6 is programmed by placing an appropriate sense resistor for each LED string. When the voltage of ACTL is higher than 1.4V, the LED current can be calculated by the following equation : 215mV ILED, MAX = (mA) RSENSE where R SENSE is the resistor between the external regulating N-MOSFET and GND. The ACTL pin should be tied to a voltage higher than 1.4V to get the full-scale 215mV (typical) threshold across the sense resistor. The ACTL pin can also be used to dim the is a registered trademark of Richtek Technology Corporation. DS8464-00 November 2012 RT8464 LED current to zero, although relative accuracy decreases with decreasing voltage sense threshold. When the ACTL pin voltage is less than 1.4V, the LED current is : ILED = (VACTL − 0.4) × 215mV RSENSE (mA) The ACTL pin can also be used in conjunction with a thermistor to provide over temperature protection for the LED load, or with a resistive voltage divider to VCC to reduce output power and switching current when VCC is low. Brightness Control For LED applications where a wide dimming range is required, two methods are available: analog dimming and PWM dimming. The easier method is to simply vary the DC current through the LED by analog dimming. The other dimming method is PWM dimming, which turns the LED on and off by different duty cycle to control the average LED current. The PWM dimming offers several advantages over analog dimming and is more preferred by LED manufacturers. One advantage is the chromaticity of the LEDs which remains unchanged in this scheme since the LED current is either zero or at a programmed current. Another advantage of PWM dimming is that a wider dimming range is available. The RT8464 features both analog and digital dimming control. Analog dimming is linearly controlled by an external voltage (0.4V to 1.2V) at the ACTL pin. A very high contrast ratio can be obtained by true digital PWM dimming which is achieved by driving the ACTL pin with a PWM signal. The recommended PWM frequency is from 100Hz to 10kHz. The PWM dimming frequency can be sufficiently adjusted from 100Hz to 30kHz. However, the LED current cannot be 100% proportional to duty cycle, especially for high frequency and low duty ratio because of physical limitation caused by the internal switching frequency. Output Over Current Protection The current limit amplifier senses the voltage drop across an external sense resistor via the CL pin to implement over current protection. A voltage drop of 100 mV, gives a maximum current limit of 2A with a 0.5Ω sense resistor. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8464-00 November 2012 Output Over Voltage Protection The RT8464 is equipped with Over Voltage Protection (OVP) function. When the voltage at the OVP pin exceeds threshold value typically 1.2V, the power switch is turned off. The power switch can be turned on again once the voltage at the OVP pin drops below 1.2V. The output voltage can be clamped at a certain voltage level set by the following equation : VOUT, OVP = 1.2 × (1+ R1 ) R2 where R1 and R2 are the resistors in the resistive voltage divider from VOUT to GND with the divider center node connected to the OVP pin. If at least one string is in normal operation, the controller will automatically ignore the open strings and continue to regulate the current for the string(s) in normal operation. Over Temperature Protection The RT8464 has Over Temperature Protection (OTP) function to prevent excessive power dissipation from overheating the device. 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 down by approximately 20°C. 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-28 package, the thermal resistance, θ JA , is is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8464 76.5°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 : Layout Considerations PCB layout is very important when designing power switching converter circuits. Some recommended layout guidelines are as follows : PD(MAX) = (125°C − 25°C) / (76.5°C/W) = 1.31W for SOP-28 package Maximum Power Dissipation (W)1 The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. ` The input capacitor CVCC must be placed as close to the VCC pin as possible. ` Place the compensation components as close to the VC pin as possible to avoid noise pick up. 1.4 Four-Layer PCB 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 1. Derating Curve of Maximum Power Dissipation Power Path should be Wide as possible Isolation Barrier VIN EN Primary Side Controller RT8464 GND VCC Place the compensation components as close to the VC pin as possible GND Dimming Signal VC LEDx x6 GATEx x6 SENSEx x6 SS ACTL OPTO GND RSENSE x6 GND Figure 2. PCB Layout Guide Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 is a registered trademark of Richtek Technology Corporation. DS8464-00 November 2012 RT8464 Outline Dimension H A M J B F C I D Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 17.704 18.110 0.697 0.713 B 7.391 7.595 0.291 0.299 C 2.362 2.642 0.093 0.104 D 0.330 0.508 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.229 0.330 0.009 0.013 I 0.102 0.305 0.004 0.012 J 10.008 10.643 0.394 0.419 M 0.381 1.270 0.015 0.050 28–Lead SOP Plastic Package Richtek Technology Corporation 5F, No. 20, Taiyuen 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. DS8464-00 November 2012 www.richtek.com 11