RT8577 High Voltage 4-CH LED Driver General Description Features The RT8577 is an 4-CH LED driver capable of delivering 200mA for each channel. The RT8577 is a current mode boost converter with an adjustable switching frequency via the RT pin from 100kHz to 1MHz and a wide VIN range from 9V to 28V. z Wide Input Supply Voltage Range : 9V to 28V z Adjustable Boost Controller Switching Frequency from 100kHz to 1MHz Programmable Channel Current Channel Current Matching : ±1.5% External Dimming Control Boost MOSFET Over Current Protection Automatic LED Open/Short Protection to Avoid Output Over Voltage VCC Under Voltage Lockout Adjustable Over Voltage Protection Under Voltage Protection Thermal Shutdown Protection Abnormal Status Indicator for Open/Short/Thermal Condition RoHS Compliant and Halogen Free The PWM output voltage loop selects and regulates the LED pin with the highest voltage string to 0.6V, hence allowing voltage mismatches between LED strings. The RT8577 automatically detects and disconnects any unconnected and/or broken strings during operation from PWM loop to prevent VOUT from over voltage. The 1.5% matched LED currents on all channels are simply programmed with a resistor. A very high contrast ratio true digital PWM dimming can be achieved by driving the PWM pin with a PWM signal. z z z z z z z z z z z When an abnormal situation (open/short/thermal) occurs, a status signal will be sent to the system to shut down the IC. Applications z Ordering Information z RT8577 z Package Type QW : WQFN-20L 5x5 (W-Type) Pin Configurations Note : LED4 GND GND VCC (TOP VIEW) LED3 Lead Plating System Z : ECO (Ecological Element with Halogen Free and Pb free) LCD TV, Monitor Display Backlight LED Driver Application General Purpose Constant Current Source Richtek products are : 20 19 18 17 16 ` 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 LED2 1 15 CREG LED1 2 14 DRV 13 PGND 12 SEN 11 EN GND 3 OVP/UVP 4 RISET 5 GND 21 6 7 8 9 10 RT VC SS YMDNN : Date Code STATUS RT8577 ZQW YMDNN PWM RT8577ZQW : Product Number WQFN-20L 5x5 DS8577-00 November 2011 www.richtek.com 1 RT8577 Typical Application Circuit L1 10µH VIN 9V to 28V D1 CIN 47µF CVCC 1µF MSW RT8577 12 2 LED1 1 LED2 SEN CF 1nF RDRV 5 External/Internal PWM Chip Enable LED4 RC 560 9 CC 0.22µF RISET 6 PWM 1k 11 19 RSTATUS 100k STATUS 7 14 DRV 1k : : : …… : : : : : : : : : COUT 4.7µF x 6 …… RSENSE 100m RF2 0 ROVP1 82k …… RF1 100 OVP/UVP 4 16 VCC VOUT 45V ROVP2 3.3M RT EN 5 8 RRT 56k CREG 15 VC PGND 13 FLT RISET 9.1k SS 10 VIN CSS 0.1µF CREG 1µF GND 3, 17, 18, 21 (Exposed Pad) Figure 1. General Application FLT VIN 9V to 28V L1 10µH D1 CIN 47µF CVCC 1µF MSW External/Internal PWM Chip Enable RC 560 CC 0.22µF RT8577 12 SEN LED4 14 DRV 1k 6 PWM 1k 11 EN : : : …… : : : : : 19 RSTATUS 100k STATUS 7 RISET RT VC PGND 13 VIN FLT 5 RISET 9.1k 8 RRT 56k SS 10 9 : : : : 2 LED1 1 LED2 CF 1nF RDRV 5 ROVP1 82k COUT 4.7µF x 6 …… RSENSE 100m RF2 0 16 VCC …… RF1 100 OVP/UVP 1 VOUT 45V ROVP2 3.3M CSS 0.1µF CREG 1µF 15 CREG GND 3, 17, 18, 21 (Exposed Pad) Figure 2. External P-MOSFET Isolation Application www.richtek.com 2 DS8577-00 November 2011 RT8577 Functional Pin Description Pin No. Pin Name 1 LED2 2 LED1 3, 17, 18, GND 21 (Exposed pad) Pin Function Channel 2 LED Current Sink, Leave this pin unconnected if it is not used. 4 OVP/UVP 5 RISET 6 PWM Channel 1 LED Current Sink, Leave this pin unconnected if it is not used. Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. Over Voltage and Under Voltage Protection. PWM boost converter turns off when VOVP or VUVP goes higher than 1.2V or lower than 0.6V, respectively. LED Current Set Pin. A resistor or a current from DAC on this pin programs the full LED current. Dimming Control Input. 7 STATUS Boost Converter Operation Status Output. 8 RT Switching Frequency Set. Connect a resistor between RT and GND to set the boost converter switching frequency. 9 VC 10 SS 11 EN 12 SEN 13 PGND 14 15 16 19, 20 DS8577-00 PWM Boost Converter Loop Compensation Node. Soft-Start Pin. Place a capacitor of at least 10nF from this pin to GND to set the soft-start time period. Chip Enable. When EN is pulled low, the chip will be shut down. Current Sense Input. During normal operation, this pin senses the voltage across the external inductor current sensing resistor for peak current mode control and also to limit the inductor current during every switching cycle. Boost Converter Power Ground. Boost Converter Power Switch Gate Output. This pin drives the external power DRV N-MOSFET device. 1μF capacitor should be placed on this pin to stabilize the 5V output of the CREG internal regulator. This regulator is for chip internal use only. Power Supply of the Chip. For good bypass, a low ESR capacitor close to the VCC pin is required. LED4, LED3 Channel 4 and 3 LED Current Sinks. Leave the pins unconnected if not used. November 2011 www.richtek.com 3 RT8577 Function Block Diagram DRV SEN OSC RT VCC STATUS OVP - UVLO UVP OTP S + R OVP/UVP + - 0.6V + R LED Short LED1 UVP + - EN - 5V LDO VC Shutdown + CREG + - 1.2V ………………… 1.2V LED4 VOUT Regulation Unit + 5V - 6µA SS PWM GND PGND + - RISET www.richtek.com 4 DS8577-00 November 2011 RT8577 Absolute Maximum Ratings z z z z z z z z z (Note 1) Supply Voltage, VCC, STATUS -----------------------------------------------------------------------------------------LED1 to LED4 --------------------------------------------------------------------------------------------------------------PWM, EN, DRV, SEN, SS, VC, RT, CREG, OVP/UVP, RISET ------------------------------------------------Power Dissipation, PD @ TA = 25°C WQFN-20L 5x5 ------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) WQFN-20L 5x5, θJA -------------------------------------------------------------------------------------------------------WQFN-20L 5x5, θJC ------------------------------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Mode) ----------------------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------------------- Recommended Operating Conditions z z z z z 33V 50V 5.5V 2.778W 36°C/W 6°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Voltage, VCC ------------------------------------------------------------------------------------------------------LED1 to LED4 --------------------------------------------------------------------------------------------------------------ILED1 to ILED4 ----------------------------------------------------------------------------------------------------------------Junction Temperature Range --------------------------------------------------------------------------------------------Ambient Temperature Range --------------------------------------------------------------------------------------------- 9V to 28V 45V 10mA to 200mA −40°C to 125°C −40°C to 85°C Electrical Characteristics (VCC = 12V, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Supply Voltage Supply Current IVCC Switching Off -- 5 -- mA Shutdown Current ISHDN VEN < 0.7V -- -- 10 μA VDD LDO Output VCREG -- 5 -- V VDD LDO Capability ICREG 30 -- -- mA VCC UVLO Threshold VUVLO VCC Rising -- 6.7 8 Hysteresis -- 1.4 -- EN Threshold Voltage V Logic-High VENH 1.5 -- -- Logic-Low VENL -- -- 0.8 114 120 126 mA -- ±1.5 ±3 % V LED Current Programming LED Current Accuracy RISET = 9.1kΩ, VPWM > 1.2V RISET = 9.1kΩ, VPWM > 1.2V LED Current Matching ILEDx − ILED_AVE ILED_AVE ×100% LED1 to LED4 Regulation Voltage ILED = 200mA -- 0.6 -- V VLED Threshold No Connection -- 0.1 -- V -- 1.2 -- V RISET Pin Voltage To be continued DS8577-00 November 2011 www.richtek.com 5 RT8577 Parameter Symbol Test Conditions Min Typ Max Unit Dimming PWM Threshold Voltage Logic-High VPWMH 1.2 -- -- Logic-Low VPWML -- -- 0.35 RRT = 24kΩ -- 1 -- MHz RRT = Open -- 100 -- kHz V PWM Boost Controller Switching Frequency fSW Minimum On Time tON -- 100 -- ns Maximum Duty Cycle Dmax 80 -- -- % -- 0.5 -- V Gate Driver Source -- 2.5 -- A Gate Driver Sink -- 3 -- A SEN Current Sense Limit Input Current Limit OVP, UVP, SCP, OTP and Soft-Start OVP Threshold VOVP -- 1.2 -- V UVP Threshold VUVP -- 0.6 -- V SCP Threshold VSCP LED1 to LED4 -- 4.3 -- V Soft-Start Current ISS VSS < 2.5V -- 6 -- μA Thermal Shutdown Temperature TSD Lockout Temperature Point -- 150 -- °C Thermal Shutdown Hysteresis ΔTSD Resume Temperature Point -- 20 -- °C STATUS Low Voltage VSTATUS Open Drain at 10mA -- -- 0.5 V Note 1. Stresses listed as the above “Absolute Maximum Ratings” may cause permanent damage to the device. These are for stress ratings. 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 for extended periods may remain possibility to affect device reliability. Note 2. θJA is measured in natural convection at TA = 25°C on a high-effective thermal conductivity four-layer test board of JEDEC 51-7 thermal measurement standard. The measurement case position of θJC is on 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. www.richtek.com 6 DS8577-00 November 2011 RT8577 Typical Operating Characteristics LED Current vs. Input Voltage LED Current vs. PWM Duty Cycle 160 120 100 LED Current (mA) LED Current (mA) 140 120 LED1 LED2 LED3 LED4 100 80 80 PWM = 200Hz PWM = 1kHz PWM = 10kHz 60 40 20 96LEDs, RISET = 9.1kΩ 60 VIN = 12V, 96LEDs, RISET = 9.1kΩ 0 8 10 12 14 16 18 20 22 24 26 28 0 10 20 30 40 50 60 70 Input Voltage (V) PWM Duty Cycle (%) Efficiency vs. Input Voltage Power On from VIN 80 90 100 100 Efficiency (%) 95 90 VIN (5V/Div) 85 DRV (5V/Div) 80 75 96LEDs, RISET = 9.1kΩ 70 8 10 12 14 16 18 20 22 24 26 I IN (1A/Div) VIN = 12V, CSS = 0.1μF, 96LEDs, RISET = 18.2kΩ Time (5ms/Div) 28 Input Voltage (V) Power On from EN Power On from PWM VEN (2V/Div) PWM (2V/Div) DRV (5V/Div) DRV (5V/Div) I IN (1A/Div) VIN = 12V, CSS = 0.1μF, 96LEDs, RISET = 18.2kΩ Time (5ms/Div) DS8577-00 November 2011 I IN (1A/Div) VIN = 12V, CSS = 0.1μF, 96LEDs, RISET = 18.2kΩ Time (5ms/Div) www.richtek.com 7 RT8577 Application information The RT8577 is an 4-CH driver controller that delivers well matched LED current to each channel of LED strings. The external N-MOSFET current source will accommodate the power dissipation difference among channels resulting from the forward voltage difference between the LED strings. With high speed current source N-MOSFET drivers, the RT8577 features highly accurate current matching, while also providing very fast turn-on and turn-off times. This allows a very narrow minimum on or off pulse. The RT8577 integrates adjustable switching frequency and soft-start and provides circuitry for over temperature, over voltage, under voltage and current limit protection. Soft-Start The RT8577 employs a soft-start feature to limit the inrush current. The soft-start circuit prevents excessive inrush current and input voltage droop. The soft-start time is determined by a capacitor, CSS, connected between SS and GND and charged with a 6μA constant current as shown in the following equation. tSS (max) = CSS x 4.8 x 105 (s) The value of capacitor CSS is user-defined to satisfy the designer' requirement. Compensation Setting and Regulation of LED current The LED current can be calculated by the following equation : 1092 ILED ≅ RISET where RISET is the resistor between the RISET pin and GND. This setting is the reference for the LED current at pin LEDx and represents the sensed LED current for each string. The DC/DC converter regulates the LED current according to the setting. Over Voltage and Under Voltage Protection The RT8577 integrates Over Voltage Protection (OVP) and Under Voltage Protection (UVP). When the voltage at the OVP/UVP pin rises above the threshold voltage of approximately 1.2V or falls below the threshold voltage of approximately 0.6V, the internal switch will be turned off and STATUS pin will be pulled high. The internal switch will be turned on again once the voltage at the OVP/UVP pin returns to normal range. The output voltage can be clamped at a certain voltage level and can be calculated by the following equations : ⎛ ⎞ R VOUT(OVP) = VOVP × ⎜ 1 + OVP2 ⎟ R OVP1 ⎠ ⎝ ⎛ ⎞ R VOUT(UVP) = VUVP × ⎜ 1 + OVP2 ⎟ R OVP1 ⎠ ⎝ The regulator loop can be compensated by adjusting the external components connected to the VC pin. The VC pin is the output of the internal error amplifier. The compensation capacitor will adjust the integrator zero to maintain stability and the resistor value will adjust the frequency integrator gain for fast transient response. Typical values of the compensation components are RC = 560Ω, CC = 0.22μF. where ROVP1 and ROVP2 are the resistors in the resistive voltage divider connected to the OVP/UVP 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 strings in normal operation. Suggested value for ROVP2 is up to 3MΩ to prevent loading effect. LED Connection LED Short Circuit Protection The RT8577 equips 4-CH LED drivers and each channel supports up to 15 LEDs. The LED strings are connected from the output of the boost converter to pin LEDx (x = 1 to 4) respectively. If one of the LED channel is not in use, the LED pin should be opened directly. The RT8577 integrates LED Short Circuit Protection (SCP). If one of the LED1 to LED4 pin voltages exceeds a threshold of approximately 4.3V during normal operation, the STATUS pin will be pulled high for a fault signal. STATUS After the IC is enable. STATUS will output logic high if LED Short/OVP/UVP/OTP conditions exist. STATUS will be reset after VIN or EN is re-applied. www.richtek.com 8 DS8577-00 November 2011 RT8577 Setting the Switching Frequency The RT8577 switching frequency is programmable from 100kHz to 1MHz by adjusting the oscillator resistor, RRT. The switching frequency can be calculated by the following equation : fSW ≅ 100k + 9 21.6 × 10 RRT Current Limit Protection The RT8577 can sense the RSENSE voltage between the SEN pin and GND to achieve over current protection. The boost converter senses the inductor current during the on period. The duty cycle depends on the current signal and internal slope compensation compared with the error signal. The external switch will be turned off when the current signal is larger than the internal slope compensation. In the off period, the inductor current will decrease until the internal switch is turned on by the oscillator. The current limit value can be calculated by the following equation : 0.5V Current Limit (A) ≅ RSENSE Brightness Control The RT8577 features a digital dimming control scheme. A very high contrast ratio true digital PWM dimming is achieved by driving the PWM pin with a PWM signal. The recommended PWM frequency is 200Hz to 10kHz, but the LED current cannot be 100% proportional to duty cycle, especially for high frequency and low duty ratio. Over Temperature Protection The RT8577 has over temperature protection function to prevent the IC from overheating due to excessive power dissipation. The IC will shut down and the STATUS pin will be pulled high when junction temperature exceeds 150°C. Main converter starts switching after junction temperature cools down by approximately 20°C. Inductor Selection The value of the inductance, L, can be approximated by the following equation, where the transition is from Discontinuous Conduction Mode (DCM) to Continuous Conduction Mode (CCM) : 2 L= D × (1 − D ) × VOUT 2 × f × IOUT DS8577-00 November 2011 The duty cycle can be calculated as the following equation : D= VOUT − VIN VOUT where VOUT is the maximum output voltage, VIN is the minimum input voltage, f is the operating frequency, and IOUT is the sum of current from all LED strings. The boost converter operates in DCM over the entire input voltage range when the inductor value is less than this value, L. With an inductance greater than L, the converter operates in CCM at the minimum input voltage and may be discontinuous at higher voltages. The inductor must be selected with a saturated current rating that is greater than the peak current as provided by the following equation : IPEAK = VOUT × IOUT VIN × D × T + η × VIN 2×L where η is the efficiency of the power converter. Diode Selection Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. Power dissipation, reverse voltage rating, and pulsating peak current are important parameters for consideration when making a Schottky diode selection. Make sure that the diode's peak current rating exceeds IPEAK and reverse voltage rating exceeds the maximum output voltage. Capacitor Selection The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For general applications, six 4.7μF ceramic capacitors are 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. It is recommended to choose a ceramic capacitor based on the output voltage ripple requirements. The minimum value of the output capacitor, COUT, can be calculated by the following equation : COUT = IOUT × D ΔVOUT × f where ΔVOUT is the peak-to-peak ripple voltage at the output. www.richtek.com 9 RT8577 Thermal Considerations Layout 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 : Careful PCB layout is very important for designing switching power converter circuits. The following layout guidelines should be strictly followed for best performance of the RT8577. ` The power components L1, D1, CIN, COUT must be placed as close as possible to the IC to reduce current loop. The PCB trace between power components must be as short and wide as possible. ` The compensation circuit should be kept away from the power loops and shielded with a ground trace to prevent any noise coupling. Place the compensation components, RC and CC, as close as possible to pin 9. ` The exposed pad of the chip should be connected to ground plane for thermal consideration. 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 of RT8577, the maximum junction temperature is 125°C and TA is the ambient temperature. The junction to ambient thermal resistance, θJA, is layout dependent. For WQFN20L 5x5 packages, the thermal resistance, θJA, is 36°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 / (36°C/W) = 2.778W for WQFN-20L 5x5 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. For RT8577 package, the derating curve in Figure 3 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. Maximum Power Dissipation (W)1 3.20 Four-Layer PCB 2.80 2.40 2.00 1.60 1.20 0.80 0.40 0.00 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curve for RT8577 Packages www.richtek.com 10 DS8577-00 November 2011 RT8577 Locate the CVCC as close to VCC as possible. AGND LED3 LED4 GND GND VCC CVCC R1 20 19 18 17 16 VIN 1 15 CREG LED1 2 14 DRV GND 3 13 PGND OVP/UVP RISET RISET 4 12 SEN 11 EN GND 8 9 STATUS RT VC MSW RF2 CF RSENSE PGND 10 RC RRT AGND RF1 D1 VOUT RDRV SS 7 PWM 21 6 COUT L1 LED2 5 Place the power components as Close as possible. The traces should be wide and short especially for the high current loop. PGND VIN CIN + Separate power ground (PGND) and analog ground (AGND). Connect AGND and PGND islands at a single end. Make sure there are no other connections between these separate ground planes. The PGND should be wide and short enough to connect ground plane. CSS The exposed pad of the chip should be connected to ground plane for thermal consideration. CC The compensation circuit and RISET resistor should be kept away from the power loops and should be shielded with a ground trace to prevent any noise coupling. Figure 4. PCB Layout Guide DS8577-00 November 2011 www.richtek.com 11 RT8577 Outline Dimension Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 0.700 0.800 0.028 0.031 A1 0.000 0.050 0.000 0.002 A3 0.175 0.250 0.007 0.010 b 0.250 0.350 0.010 0.014 D 4.900 5.100 0.193 0.201 D2 3.100 3.200 0.122 0.126 E 4.900 5.100 0.193 0.201 E2 3.100 3.200 0.122 0.126 e L 0.650 0.550 0.026 0.650 0.022 0.026 W-Type 20L QFN 5x5 Package Richtek Technology Corporation Richtek Technology Corporation Headquarter Taipei Office (Marketing) 5F, No. 20, Taiyuen Street, Chupei City 5F, No. 95, Minchiuan Road, Hsintien City Hsinchu, Taiwan, R.O.C. Taipei County, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611 Tel: (8862)86672399 Fax: (8862)86672377 Email: [email protected] Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek. www.richtek.com 12 DS8577-00 November 2011