RT8572 High Voltage 8-CH LED Driver General Description Features The RT8572 is an 8-CH LED driver capable of delivering 60mA for each channel. The RT8572 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 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 RT8572 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 z When an abnormal situation (open/short/thermal) occurs, a status signal will be sent to the system to shut down the IC. The RT8572 is available in an SOP-24 package. Ordering Information RT8572 Package Type S : SOP-24 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 RT8572GS : Product Number RT8572 GSYMDNN Wide Input Supply Voltage Range : 9V to 28V 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 Applications z z z LCD TV, Monitor Display Backlight LED Driver Application General Purpose Constant Current Source Pin Configurations (TOP VIEW) LED1 OVP/UVP RISET NC PWM NC STATUS RT VC SS EN SEN 24 2 23 3 22 4 21 5 20 6 19 7 18 8 17 9 16 10 15 11 14 12 13 LED2 LED3 LED4 LED5 LED6 LED7 LED8 GND VCC CREG DRV PGND SOP-24 YMDNN : Date Code DS8572-01 March 2011 www.richtek.com 1 RT8572 Typical Application Circuit VIN 9 to 28V L1 10µH D1 VOUT CIN 47µF OVP/UVP 2 16 VCC CVCC 1µF RF1 100 LED1 12 SEN CF 1nF RDRV 1k PWM Signal 5 1k 11 Chip Enable RC 560 CC 0.22µF LED8 14 DRV 5 PWM EN 18 RSTATUS 100k STATUS 7 RISET RT VC CREG PGND 13 : : : …… : : : : : 1 RISET 9.09k 8 RRT 56k 15 VIN FLT 3 SS 10 9 COUT 4.7µF x 6 LED2 24 …… RSENSE 100m RF2 0 : : : : ROVP1 82k RT8572 MSW ROVP2 3.6M CSS 0.1µF CREG 1µF GND 17 Figure 1. General Application www.richtek.com 2 DS8572-01 March 2011 RT8572 FLT VIN 9 to 28V L1 10µH D1 CIN 47µF OVP/UVP 2 16 VCC CVCC 1µF RF1 100 SEN CF 1nF RSENSE 100m RDRV 1k PWM Signal PWM 1k 11 EN Chip Enable RC 560 CC 0.22µF 5 9 : : : …… : : : : : LED2 24 LED8 14 DRV 5 : : : : COUT 4.7µF x 6 LED1 1 …… RF2 0 12 ROVP2 3.6M ROVP1 82k RT8572 MSW VOUT 45V VC PGND 13 18 RSTATUS 100k STATUS 7 RISET RT CREG FLT 3 RISET 9.09k 8 RRT 56k SS 10 15 VIN CSS 0.1µF CREG 1µF GND 17 Figure 2. External P-MOSFET Isolation Application DS8572-01 March 2011 www.richtek.com 3 RT8572 Functional Pin Description Pin No. Pin Name 1 LED1 2 OVP/UVP 3 RISET Pin Function Channel 1 LED Current Sink. Leave this pin unconnected if it is not used. NC 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. No Internal Connection. 5 PWM 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 Chip Enable. When EN is pulled low, chip will be in shutdown mode. 12 SEN 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. 13 PGND 14 DRV 15 CREG 16 VCC Power Supply of the Chip. For good bypass, a low ESR capacitor is required. 17 GND LED8 to LED2 Ground Pin. Channel 8 to Channel 2 LED Current Sink. Leave the pins unconnected if not in use. 4, 6 18 to 24 www.richtek.com 4 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. Boost Converter Power Ground. Boost Converter Power Switch Gate Output. This pin drives the external power N-MOSFET device. 1μF capacitor should be placed on this pin to stabilize the 5V output of the internal regulator. This regulator is for chip internal use only. DS8572-01 March 2011 RT8572 Function Block Diagram DRV SEN OSC RT VCC STATUS OVP - 6V UVP OTP S + R OVP/UVP + - 0.6V + R LED Short LED1 UVP + - EN - 5V LDO VC Shutdown + CREG + - 1.2V ………………… 1.2V LED8 VOUT Regulation Unit + 5V - 6µA SS PWM GND PGND + - RISET DS8572-01 March 2011 www.richtek.com 5 RT8572 Absolute Maximum Ratings z z z z z z z z z (Note 1) Supply Voltage, VCC , STATUS ----------------------------------------------------------------------------------------LED1 to LED8 --------------------------------------------------------------------------------------------------------------PWM, EN, DRV, SEN, SS, VC, RT, CREG, OVP/UVP, RISET ------------------------------------------------Power Dissipation, PD @ TA = 25°C SOP-24 ----------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) SOP-24, θJA -----------------------------------------------------------------------------------------------------------------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 33V 50V 5.5V 1.111W 90°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Voltage, VCC ------------------------------------------------------------------------------------------------------LED1 to LED8 --------------------------------------------------------------------------------------------------------------Junction Temperature Range --------------------------------------------------------------------------------------------Ambient Temperature Range --------------------------------------------------------------------------------------------- 9V to 28V 45V −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 -- -- 8 Hysteresis -- 1.4 -- 1.5 -- -- -- -- 0.8 57 60 63 mA -- ±1.5 ±3 % EN Input Threshold Voltage Logic-High VENH Logic-Low VENL V V LED Current Programming LED Current RISET = 9.09kΩ, V PWM > 1.2V LED Current Matching ILED = 60mA LED1 to LED8 Regulation Voltage ILED = 60mA -- 0.6 -- V VLED Threshold No Connection -- 0.1 -- V -- 1.2 -- V RISET Pin Voltage I(MAX ) − I(MIN ) 2 ×I( Avg) ×100% To be continued www.richtek.com 6 DS8572-01 March 2011 RT8572 Parameter Symbol Test Conditions Min Typ Max Unit Dimming PWM Input Threshold Voltage Logic-High VPWMH 1.2 -- -- Logic-Low VPWML -- -- 0.4 R RT = 24kΩ -- 1 -- MHz R RT = 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, OTP and Soft-Start OVP Threshold VOVP 1.1 1.2 1.3 V UVP Threshold VUVP 0.57 0.6 0.63 V SCP Threshold VSCP LED1 to LED8 -- 4.3 -- V Soft-Start Current ISS V SS < 2.5V -- 6 -- μA Thermal Shutdown Temperature TSD Lockout Temperature Point -- 150 -- °C Thermal Shutdown Hysteresis ΔT SD 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 low-effective thermal conductivity single-layer test board of JEDEC 51-3 thermal measurement standard. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. DS8572-01 March 2011 www.richtek.com 7 RT8572 Typical Operating Characteristics LED Current vs. PWM Duty Cycle LED Current vs. Input Voltage 60 70 65 60 LED1 LED2 LED3 LED4 55 LED Current (mA) LED Current (mA) 50 LED5 LED6 LED7 LED8 40 PWM = 200Hz PWM = 1kHz PWM = 10kHz 30 20 10 96LEDs, RISET = 9.1kΩ VIN = 12V, 96LEDs, RISET = 9.1kΩ 0 50 8 10 12 14 16 18 20 22 24 26 0 28 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 = 9.1kΩ 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 = 9.1kΩ Time (5ms/Div) www.richtek.com 8 I IN (1A/Div) VIN = 12V, CSS = 0.1μF, 96LEDs, RISET = 9.1kΩ Time (5ms/Div) DS8572-01 March 2011 RT8572 Application Information The RT8572 is an 8-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 RT8572 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 RT8572 integrates adjustable switching frequency and soft-start and provides circuitry for over temperature, over voltage, under voltage and current limit protection. Soft-Start The RT8572 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 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 = 510Ω, CC = 0.22μF. LED Connection The RT8572 equips 8-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 8) respectively. If one of the LED channel is not in use, the LEDx pin should be opened directly. DS8572-01 March 2011 Setting and Regulation of LED current The LED current can be calculated by the following equation : 545.4 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 RT8572 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 ⎟ ROVP1 ⎠ ⎝ ⎛ ⎞ R VOUT(UVP) = VUVP × ⎜ 1 + OVP2 ⎟ R OVP1 ⎠ ⎝ 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 Short Circuit Protection The RT8572 integrates LED Short Circuit Protection (SCP). If one of the LED1 to LED8 pin voltages exceeds a threshold of approximately 4.3V during normal operation, the STATUS pin will be pulled high for a fault signal. www.richtek.com 9 RT8572 STATUS Inductor Selection After the IC is enabled, STATUS will output logic high if LED short/OVP/UVP/OTP conditions exist. STATUS will be reset after VIN or EN is re-applied. 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 Setting the Switching Frequency The RT8572 switching frequency is programmable from 100kHz to 1MHz by adjusting the oscillator resistor, RRT. The switching frequency can be calculated by the following equation : 21.6 × 109 fSW ≅ 100k + RRT Current Limit Protection The RT8572 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 : Current Limit (A) ≅ 0.5V RSENSE L= D × (1 − D ) × VOUT 2 × f × IOUT The duty cycle can be calculated as the following equation : V − VIN D = OUT 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 × IIOUT VIN × D × T + η × VIN 2×L where η is the efficiency of the power converter. Brightness Control Diode Selection The RT8572 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. 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. Over Temperature Protection The RT8572 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. www.richtek.com 10 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. DS8572-01 March 2011 RT8572 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 : I ×D COUT = OUT ΔVOUT × f Maximum Power Dissipation (W)1 1.2 where ΔVOUT is the peak-to-peak ripple voltage at the output. 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 of the RT8572, the maximum junction temperature is 125°C and TA is the ambient temperature. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP24 packages, the thermal resistance, θJA, is 90°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 : Single-Layer PCB 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curve for RT8572 Packages Layout Considerations PCB layout is very important for designing switching power converter circuits. The following layout guides should be strictly followed for best performance of the RT8572. ` The power components L1, D1, CIN, COUT must be placed as close to the chip as possible 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 as close to pin 9 as possible, regardless of whether it is RC or CC. PD(MAX) = (125°C − 25°C) / (90°C/W) = 1.111W for SOP-24 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. For the RT8572 package, the derating curve in Figure 3 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. DS8572-01 March 2011 www.richtek.com 11 RT8572 24 LED2 OVP/UVP 2 23 LED3 RISET 3 22 LED4 NC 4 21 LED5 PWM 5 20 LED6 NC 6 19 LED7 STATUS 7 18 LED8 RT 8 17 GND VC 9 16 VCC LED1 The compensation circuit should be kept away from the power loops and shielded with a ground trace to prevent any noise coupling. RC CC SS 10 15 CREG EN 11 14 DRV SEN 12 13 PGND Place the power components as close as possible to the IC. The traces should be wide and short especially for the high-current loop. Place the CVCC as close to VCC as possible. COUT VOUT D1 CVCC + CIN RDRV MSW RF2 VIN L1 GND RF1 CF RSENSE GND Figure 4. PCB Layout Guide www.richtek.com 12 DS8572-01 March 2011 RT8572 Outline Dimension H A M J B F C I D Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 15.189 15.596 0.598 0.614 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 24–Lead SOP Plastic 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. DS8572-01 March 2011 www.richtek.com 13