RT8472 1A, Hysteretic, High Brightness LED Driver with Internal Switch General Description Features The RT8472 is a high efficiency, continuous mode inductive step-down converter, designed for driving single or multiple series connected LEDs from a voltage source higher than the LED voltage. It operates from an input voltage of 7V to 30V and employs hysteretic control with a high side current sense resistor to set the constant output current. z The RT8472 includes an output switch and a high side output current sensing circuit, which uses an external resistor to set the nominal average output current. LED brightness control is achieved with PWM dimming from an analog or PWM input signal. z z z z z z z z z z 7V to 30V Input Voltage Range Hysteretic Control with High Side Current Sensing Internal N-MOSFET with 350m Low RDS(ON) 1A Output Current Up to 97% Efficiency Typical 5% LED Current Accuracy Analog or PWM Control Signal for LED Dimming 300Hz On-Board Ramp Generator Input Under Voltage Lockout Thermal Shutdown Protection RoHS Compliant and Halogen Free The RT8472 is available in a small TSOT-23-5 package. Applications Ordering Information z RT8472 z Package Type J5 : TSOT-23-5 z Lead Plating System G : Green (Halogen Free and Pb Free) z z z Automotive LED Lighting High Power LED Lighting Indicator and Emergency Lighting Architectural Lighting Low Voltage Industrial Lighting Signage and Decorative LED Lighting Note : Pin Configurations Richtek products are : RoHS compliant and compatible with the current require- (TOP VIEW) ments of IPC/JEDEC J-STD-020. VIN SENSE 5 4 Suitable for use in SnPb or Pb-free soldering processes. Marking Information 07= : Product Code 07=DNN DS8472-00 YMDNN : Date Code December 2011 2 3 LX GND ADJ TSOT-23-5 www.richtek.com 1 RT8472 Typical Application Circuit VIN 7V to 30V CIN 10F/50V RS RT8472 5 VIN SENSE 4 optional 3 ADJ 2 D GND L LX 1 Functional Pin Description Pin No. Pin Name Pin Function 1 LX Switching Node. Open drain output of internal N-MOSFET. 2 GND Ground. 3 ADJ Dimming Control Input : --- Analog signal input for analog PWM dimming. --- PWM signal input for digital PWM dimming. 4 SENSE Output Current Sense Terminal. Sense LED string current. 5 VIN Supply Input Voltage. Function Block Diagram VIN Regulator Bandgap SENSE VCC 1.25V Ramp Gen. UVLO Dimming LX GND + www.richtek.com 2 MOSFET + - ADJ + UVLO - UVLO Dimming - DS8472-00 December 2011 RT8472 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN ------------------------------------------------------------------------------------Switch Voltage, LX --------------------------------------------------------------------------------------------z Sense Voltage, SENSE -------------------------------------------------------------------------------------z All Other Pins ----------------------------------------------------------------------------------------------------z Power Dissipation, PD @ TA = 25C TSOT-23-5 (Single-layer PCB) ------------------------------------------------------------------------------TSOT-23-5 (Four-layer PCB) --------------------------------------------------------------------------------z Package Thermal Resistance (Note 2) TSOT-23-5, JA (Single-layer PCB) ------------------------------------------------------------------------TSOT-23-5, JC (Single-layer PCB) ------------------------------------------------------------------------TSOT-23-5, JA (Four-layer PCB) --------------------------------------------------------------------------z Junction Temperature -----------------------------------------------------------------------------------------z Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------z Storage Temperature Range --------------------------------------------------------------------------------z ESD Susceptibility (Note 3) HBM (Human Body Mode) ----------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------z z Recommended Operating Conditions z z -0.3V to 33V -0.3V to (VIN + 0.7V) (VIN - 5V) to (VIN + 0.3V) -0.3V to 6V 0.400W 0.625W 250C/W 130C/W 160C/W 150C 260C -65C to 150C 2kV 200V (Note 4) Supply Input Voltage, VIN ------------------------------------------------------------------------------------- 7V to 30V Junction Temperature Range --------------------------------------------------------------------------------- -40C to 125C Electrical Characteristics (VIN = 12V, TA = 25C, unless otherwise specified) Parameter Symbol Test Conditions Measure on SENSE Pin with Respect to VIN. ADJ pin is Floating. Min Typ Max Unit 95 100 105 mV Mean Current Sense Threshold Voltage VSENSE Sense Threshold Hysteresis VSENSEHYS -- 15 -- % MOSFET On-Resistance MOSFET Leakage Current RDS(ON) VLX = 5V --- 350 0.01 -10 m A Under Voltage Lockout Threshold VUVLO VIN Rising -- 4 -- V Under Voltage Lockout Threshold Hysteresis Ramp Frequency VUVLO -- 200 -- mV f RAMP -- 300 -- Hz ADJ Input Threshold Voltage Logic-High VADJ_H 1.4 -- 5.5 Logic-Low VADJ_L -- -- 0.2 0.4 -- 1.2 V Analog Dimming Range V Minimum Switch On Time tON(MIN) LX Switch On -- 210 -- ns Minimum Switch Off Time tOFF(MIN) LX Switch Off -- 170 -- ns To be continued DS8472-00 December 2011 www.richtek.com 3 RT8472 Parameter Quiescent Supply Current with Output Off Quiescent Supply Current with Output Switching Internal Propagation Delay Symbol tPD Sense Pin Input Current ISENSE Thermal Shutdown Thermal Shutdown Hysteresis TSD TSD Test Conditions Min Typ Max Unit IVIN_Off VADJ = 0V -- 450 -- A IVIN_On ADJ Pin is Floating, f SW = 250kHz, VIN = 8V -- 1000 -- A -- 25 -- ns -- 300 -- nA --- 150 30 --- C C VSENSE = VIN - 0.1V 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. JA is measured at TA = 25C on a single-layer and four-layer test board of JEDEC 51. The measurement case position of JC is on the lead 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 4 DS8472-00 December 2011 RT8472 Typical Operating Characteristics Output Current Deviation vs. Input Voltage Efficiency vs. Input Voltage 100 100% 6% 6 Output Current Deviation (%)1 7 LED 95 95% Efficiency (%) 3 LED 90 90% 85% 85 1 LED 80% 80 75% 75 VIN = 7V to 30V, ILED = 1A, L = 33H 4 4% 7 LED 2 2% 3 LED 0% 0 1 LED -2% -2 -4% -4 VIN = 7V to 30V, ILED = 1A, L = 33H -6 -6% 70% 70 0 7 14 21 28 0 35 7 Output Current vs. Input Voltage 1.01 1.00 Output Current Deviation (%)1 Output Current (A) 1.02 0.99 0.98 0.97 VIN = 7V to 30V, ILED = 1A, L = 33H 0.96 35 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 3 3% 2 2% 1 1% 0% 0 -1% -1 -2% -2 -3% -3 VIN = 7V to 30V, ILED = 1A, L = 33H -4% -4 0 7 14 21 28 35 0 7 Input Voltage (V) 14 21 28 35 Input Voltage (V) Switching Frequency vs. Input Voltage Duty Cycle vs. Input Voltage 850 100% 100 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 750 650 550 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 90% 90 80 80% Duty cycle (%) Switching Frequency (kHz)1 28 Output Current Deviation vs. Input Voltage 4 4% 1 LED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 1.03 21 Input Voltage (V) Input Voltage (V) 1.04 14 450 350 70 70% 60 60% 50 50% 40 40% 30 30% 20 20% 250 150 0 7 14 21 Input Voltage (V) DS8472-00 10 10% VIN = 7V to 30V, ILED = 1A, L = 33H December 2011 28 VIN = 7V to 30V, ILED = 1A, L = 33H 0%0 35 0 7 14 21 28 35 Input Voltage (V) www.richtek.com 5 RT8472 Quiescent Input Current vs. Input Voltage Quiescent Input Current vs. Input Voltage 480 Quiescent Input Current (A)1 Quiescent Input Current (A) 1200 1000 800 600 400 200 475 470 465 Output Switching Output Off 0 460 0 7 14 21 28 35 0 7 14 Input Voltage (V) 35 Output Current vs. PWM Duty Cycle 1200 1200 1000 1000 Output Current (mA) Output Current (mA) 28 Input Voltage (V) Output Current vs. PWM Duty Cycle 800 600 400 200 800 600 400 200 RS = 0.1, fDIMMING = 500Hz RS = 0.1, fDIMMING = 10kHz 0 0 20 40 60 80 0 100 0 20 40 PWM Duty Cycle (%) 60 80 100 PWM Duty Cycle (%) MOSFET On-Resistance vs. Temperature Output Current vs. ADJ Voltage 600 1200 RS = 100m 550 800 On-Resistance (m) (m ) 1000 Output Current (mA) 21 RS = 150m 600 400 RS = 350m 200 500 450 400 350 300 VIN = 12V, 1LED 0 0.2 0.5 0.8 1.1 ADJ Voltage (V) www.richtek.com 6 1.4 1.7 250 -50 -25 0 25 50 75 100 125 150 Temperature (C) DS8472-00 December 2011 RT8472 Ramp Frequency vs. Temperature 330 325 325 Ramp Frequency (Hz)1 Ramp Frequency (Hz) Ramp Frequency vs. Input Voltage 330 320 315 310 305 300 320 315 310 305 300 295 295 290 290 0 7 14 21 28 -50 35 0 25 50 75 100 125 Input Voltage (V) Temperature (C) Digital Dimming from ADJ On Digital Dimming from ADJ Off VADJ (2V/Div) VADJ (2V/Div) IOUT (500mA/Div) IOUT (500mA/Div) Time (5s/Div) Time (5s/Div) Power On from VIN Power Off from VIN VIN (5V/Div) VIN (5V/Div) IOUT (500mA/Div) IOUT (500mA/Div) RS = 0.1, 1 LED Time (500s/Div) December 2011 150 VIN = 12V, RS = 0.1, fDIMMING = 500Hz, 1 LED VIN = 12V, RS = 0.1, fDIMMING = 500Hz, 1 LED DS8472-00 -25 RS = 0.1, 1 LED Time (500s/Div) www.richtek.com 7 RT8472 Application Information The RT8472 is a simple high efficiency, continuous mode inductive step-down converter. The device operates with an input voltage range from 7V to 30V and delivers up to 1A of output current. A high side current sense resistor sets the output current and a dedicated PWM dimming input enables pulsed LED dimming over a wide range of brightness levels. A high side current sensing scheme and an onboard current setting circuitry minimize the number of external components. A 1% sense resistor performs a 3% LED current accuracy for the best performance. Under Voltage Lockout (UVLO) The RT8472 includes a UVLO feature with 200mV hysteresis. The internal MOSFET turns off when VIN falls below 3.8V (typ.). Setting Average Output Current The RT8472 output current which flows through the LEDs is set by an external resistor (RS) connected between the VIN and SENSE terminal. The relationship between output current (IOUT) and RS is shown as below : IOUTavg = 0.1V ( A ) RS Analog Dimming Control The ADJ terminal can be driven by an external voltage (VADJ) to adjust the output current to an average value set by RS. The average output current is given by : V - 0.4 IOUTavg = 0.1V x ADJ 0.8 RS where VADJ is ranged from 0.4V to 1.2V. When VADJ is larger than 1.2V, the output current value will just be set by the external resistor (RS). Digital Dimming Control A Pulse Width Modulated (PWM) signal can drive the ADJ terminal directly. Notice that the PWM signal logic high level must be above 1.4V and the logic low level must be below 0.2V at the ADJ terminal. It's recommended to maintain the PWM dimming at low frequency (ex. 500Hz ) in order to obtain a linear dimming curve. PWM Soft-Start Behavior The RT8472 features an optional PWM soft-start behavior that allows for gradual brightness transition. This is achieved by simply connecting an external capacitor between the ADJ pin and GND. An internal current source will then charge this capacitor for soft-start behavior, resulting in steady LED current increase and decrease during power on and power off, as shown in Figure 1. 1.2V Internal VRAMP 0.4V VADJ 0V 1A ILED 0A Figure 1. PWM Soft-Start Behavior Mechanism www.richtek.com 8 DS8472-00 December 2011 RT8472 The capacitor can be selected according to below equation : -6 where VD is the rectifier diode forward voltage (V) C = 1.5 x 10 x tSS VSEN is the voltage cross current sense resistor (V) where tSS is the soft-start period. RL is the inductor DC resistance () LED Current Ripple Reduction L is the inductance (H) Higher LED current ripple will shorten the LED life time and increase heat accumulation of LED. By adding an output capacitor in parallel with the LED. This will then allow the use of a smaller inductor. The saturation current of the selected inductor must be higher than the peak output LED current, and the continuous current rating must be above the average output LED current. In general, the inductor saturation current should be 1.5 times the LED current. In order to reduce the output current ripple, a higher inductance is recommended at higher supply voltages. However, it could also cause a higher line resistance and result in a lower efficiency. Inductor Selection The inductance is determined by inductor current ripple, switching frequency, duty ratio, circuit specifications and component parameters, as expressed in the following equation : D L > VIN - VOUT - VSEN - RDS(ON) x IOUT x f SW x IL ( ) where fSW is the switching frequency (Hz) RDS(ON) is the on-resistance of internal MOSFET ( = 0.35 typical) Diode Selection To obtain better efficiency, the Schottky diode is recommended for its low reverse leakage current, low recovery time and low forward voltage. With its low power dissipation, the Schottky diode outperforms other silicon diodes and increase overall efficiency. D is the duty cycle determined by VOUT/VIN Input Capacitor selection IOUT is the required LED current (A) Input capacitor has to supply peak current to the inductor and flatten the current ripple on the input. The low ESR condition is required to avoid increasing power loss. The ceramic capacitor is recommended due to its excellent high frequency characteristic and low ESR, which are suitable for the RT8472. For maximum stability over the entire operating temperature range, capacitors with better dielectric are suggested. IL is the inductor peak-peak ripple current (internally set to 0.3 x IOUT) VIN is the input supply voltage (V) VOUT is the total LED forward voltage (V) Besides, the selected inductance has also to satisfy the limit of the minimum switch on/off time. The calculated on time must be greater than 210ns of the minimum on time, and the off time must be greater than 170ns of the minimum off time. The following equation can be used to verify the suitability of the inductor value. tON = L x IL VIN - VOUT - IOUT (RSEN + RL + RDS(ON) ) > tON(MIN) (210ns typ.) Thermal Protection A thermal protection feature is included to protect the RT8472 from excessive heat damage. When the junction temperature exceeds a threshold of 150C, the thermal protection will turn off the LX terminal. When the junction temperature drops below 125C, the RT8472 will turn back on the LX terminal and return to normal operations. tOFF = L x IL > tOFF(MIN) (170ns typ.) VOUT + VD + VSEN + (IOUT x RL ) DS8472-00 December 2011 www.richtek.com 9 RT8472 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) / JA 0.8 Maximum Power Dissipation (W)1 Thermal Considerations TSOT-23-5 package, the thermal resistance, JA, is 250C/ W on a standard JEDEC 51-3 single-layer thermal test board and 160C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25C can be calculated by the following formulas : PD(MAX) = (125C - 25C) / (250C/W) = 0.4W for 0.4 0.3 0.2 TSOT23-5 (Single-Layer PCB) 0.1 25 50 75 100 125 Ambient Temperature (C) Figure 2. Derating Curves for the RT8472 Packages Layout Considerations For best performance of the RT8472, please abide the following layout guide. The capacitor CIN, CADJ and external resistor, RS, must be placed as close as possible to the VIN and SENSE pins of the device respectively. The GND should be connected to a strong ground plane. Keep the main current traces as short and wide as possible. The inductor (L) should be mounted as close to the device with low resistance connections. The ADJ pin trace need to be kept far away from LX terminal. TSOT-23-5 package (four-layer PCB) The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, JA. For the RT8472 package, the derating curves in Figure 2 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation. TSOT23-5 (Four-Layer PCB) 0.5 0 TSOT-23-5 package (single-layer PCB) PD(MAX) = (125C - 25C) / (160C/W) = 0.625W for 0.6 0.0 where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and JA is the junction to ambient thermal resistance. For recommended operating condition specifications of the RT8472, the maximum junction temperature is 125C and TA is the ambient temperature. The junction to ambient thermal resistance, JA , is layout dependent. For 0.7 Place the resistor RS as close as possible to VIN and SENSE pins. RS VIN CIN GND D Place the capacitor CIN as close as possible to VIN pin. LED+ VIN SENSE 5 4 1 2 Place the capacitor CADJ as close as possible to the ADJ pin. 3 LX GND ADJ L CADJ LED- Figure 3. PCB Layout Guide www.richtek.com 10 DS8472-00 December 2011 RT8472 Outline Dimension H D L B C b A A1 e Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A 0.700 1.000 0.028 0.039 A1 0.000 0.100 0.000 0.004 B 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 C 2.591 3.000 0.102 0.118 D 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 H 0.080 0.254 0.003 0.010 L 0.300 0.610 0.012 0.024 TSOT-23-5 Surface Mount 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: marketing@richtek.com 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. DS8472-00 December 2011 www.richtek.com 11