RT A, Hysteretic, High Brightness LED Driver with Internal Switch. Features. General Description. Applications. Ordering Information

Transcription

1 RT A, Hysteretic, High Brightness LED Driver with Internal Switch General Description The RT8420 is a high-efficiency, continuous mode, inductive step-down converter, designed for driving single or multiple series-connected LED strings from a voltage source higher than the LED string voltage. It operates from an input voltage range from 7V to 50V and employs a hysteretic control loop with a high side current sense resistor to set the constant LED output current. The RT8420 includes a low side power switch and a high side output current sensing circuit, which uses an external resistor to set the nominal average output current. The LED brightness control is achieved by analog dimming method and the maximum switching frequency is clamped when the dimming percentage is low. The RT8420 is available in MSOP-8 (Exposed Pad) and SOP-8 (Exposed Pad) package that makes the chip more thermal efficient. Ordering Information RT8420 Package Type FP : MSOP-8 (Exposed Pad) SP : SOP-8 (Exposed Pad-Option 1) 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. Features 7V to 50V Input Voltage Range Hysteretic Control with High Side Current Sensing Internal N-MOSFET with 350mΩ Low R DS(ON) 1.2A Output Current Up to 97% Efficiency Typical ±5% LED Current Accuracy ADJ Pin for Analog Dimming and PWM Dimming Limited Max Switching Frequency at Low Percentage Dimming Input Under Voltage Lockout Thermal Shutdown Protection RoHS Compliant and Halogen Free Applications Automotive LED Lighting High Power LED Lighting Indicator and Emergency Lighting Architectural Lighting Low Voltage Industrial Lighting Signage and Decorative LED Lighting Marking Information RT8420GFP 11=YM DNN RT8420GSP RT8420 GSPYMDNN 11= : Product Code YMDNN : Date Code RT8420GSP : Product Number YMDNN : Date Code 1

2 Pin Configuration (TOP VIEW) SENSE ADJ VIN PVCC SENSE ADJ VIN PVCC MSOP-8 (Exposed Pad) SOP-8 (Exposed Pad) Typical Application Circuit V IN 7V to 50V C IN R S C VIN 8 RT8420 VIN SENSE 1 optional optional 7 PVCC 4 ADJ 2, 3, 9 (Exposed Pad) C PVCC 5, 6 L D Functional Pin Description Pin No. Pin Name Pin Function 1 SENSE 2, 3, 9 (Exposed Pad) 4 ADJ Output current sense. Sense LED string current with an external resistor connected between VIN and SENSE. For accurate LED current sensing, a separate VCC PCB trace used for Kelvin sensing is recommended. Ground. The exposed pad must be soldered to a large PCB and connected to for maximum power dissipation. Analog or PWM dimming input. Analog dimming range is 0.44V to 2.5V. Apply 3.3V square wave for PWM dimming 5, 6 Switch output terminal. Drain of internal N-MOSFET. 7 PVCC 8 VIN Regulator output for internal circuit. At least 1F bypass ceramic capacitor should be placed closely between PVCC and pins. Supply input voltage. At least 1F bypass ceramic capacitor should be placed closely between VIN and pins. 2

3 Functional Block Diagram VIN Regulator Bandgap SENSE PVCC UVLO UVLO V CC UVLO Dimming ADJ Analog and PWM Dimming Operation The RT8420 is a simple high efficiency, continuous mode inductive step-down converter. The device operates with an input voltage range from 7V to 50V and delivers up to 1.2A of output current. A high side current sense resistor sets the output current. A high side current sensing scheme and an onboard current setting circuitry minimize the number of external components. A 1% sense resistor performs a ±5% LED current accuracy for the best performance. Under Voltage Lockout (UVLO) The RT8420 includes a UVLO feature with 90mV (typ.) hysteresis. The internal MOSFET turns off when VIN falls below 4.91V (typ.). Thermal Protection A thermal protection feature is included to protect the RT8420 from excessive heat damage. When the junction temperature exceeds a threshold of 150 C (typ.), the thermal protection will turn off the terminal. When the junction temperature drops below 120 C (typ.), the RT8420 will turn back on the terminal and return to normal operations. 3

4 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VIN V to 60V Switch Voltage, V to 60V Sense Voltage, SENSE (V IN 5V) to V IN All Other Pins V to 6V Power Dissipation, P T A = 25 C MSOP-8 (Exposed pad, Two-layer PCB) W MSOP-8 (Exposed pad, Four-layer PCB) W SOP-8 (Exposed pad, Two-layer PCB) W SOP-8 (Exposed pad, Four-layer PCB) W Package Thermal Resistance (Note 2) MSOP-8 (Exposed pad, Two-layer PCB), θ JA C/W MSOP-8 (Exposed pad, Two-layer PCB), θ JC C/W MSOP-8 (Exposed pad, Four-layer PCB), θ JA C/W MSOP-8 (Exposed pad, Four-layer PCB), θ JC C/W SOP-8 (Exposed pad, Two-layer PCB), θ JA C/W SOP-8 (Exposed pad, Two-layer PCB), θ JC C/W SOP-8 (Exposed pad, Four-layer PCB), θ JA C/W SOP-8 (Exposed pad, Four-layer PCB), θ JC C/W Junction Temperature C Lead Temperature (Soldering, 10 sec.) C Storage Temperature Range C to 150 C ESD Susceptibility (Note 3) HBM (Human Body Model) kV CDM (Charged Device Model) V Recommended Operating Conditions (Note 4) Supply Input Voltage, V IN V to 50V Junction Temperature Range C to 125 C Electrical Characteristics (V IN = 12V, T A = 25 C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Mean Current Sense Threshold Voltage VSENSE Measure on SENSE Pin with respecting to VIN. ADJ is floating mv Sense Threshold Hysteresis VSENSE -- ±15 -- % Low Side Switch On-Resistance RDS(ON) m Low Side Switch Leakage Current V = 12V, VADJ = 0V A Under Voltage Lockout Threshold VUVLO VIN rising V Under Voltage Lockout Threshold Hysteresis VUVLO mv Regulator Output Voltage VPVCC CPVCC = 1F V 4

5 ADJ Input Threshold Voltage Parameter Symbol Test Conditions Min Typ Max Unit Fully Turn On VADJ, H Turn Off VADJ, OFF Limited Max Switching Frequency fswl khz Quiescent Input Current with Output Off Quiescent Input Current with Output Switching IVIN, OFF VADJ = 0V A IVIN, ON ADJ is Floating, fsw = 250kHz, VIN = 8V Note 1. Stresses beyond those listed under 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. θ JA is measured under natural convection (still air) at T A = 25 C with the component mounted on a high effectivethermal-conductivity four-layer test board on a JEDEC 51-7 thermal measurement standard. θ JC is measured at the exposed pad of the package. θ JA is measured under natural convection (still air) at T A = 25 C with the component mounted on a low effectivethermal-conductivity two-layer test board on a JEDEC thermal measurement standard. θ JC is measured at 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. V A Sense Pin Input Current ISENSE VSENSE = VIN 0.1V na Thermal Shutdown TSD C Thermal Shutdown Hysteresis TSD C 5

6 Typical Operating Characteristics 98 Efficiency vs. Output Voltage 100 Efficiency vs. Input Voltage Efficiency (%) VIN = 34V, ILED = 480mA, L = 100μH Output Voltage (V) Efficiency (%) 98 6 LED 10 LED 96 3 LED ILED = 480mA, L = 100μH Input Voltage (V) 0.53 Output Current vs. Input Voltage Output Current Deviation vs. Input Voltage 3.0 ILED = 480mA, L = 100μH Output Current (A) LED 6 LED 10 LED ILED = 480mA, L = 100μH Input Voltage (V) Output Current Deviation (%) LED LED LED Input Voltage (V) Output Current (ma) Output Current vs. ADJ Voltage 30 VIN = 37V, 10 LED, ILED = 280mA, L = 100μH ADJ Voltage (V) Output Current (ma) Output Current vs. PWM Duty Cycle PWM Duty Cycle (%) PWM = 200Hz PWM = 1kHz PWM = 5kHz VIN = 37V, 10 LED, ILED = 280mA, L = 100μH 6

7 5.20 UVLO vs. Temperature 120 UVLO Hysteresis vs. Temperature UVLO (V) UVLO Hysteresis (mv) Temperature ( C) Temperature ( C) Current Sense Voltage (mv) Current Sense Voltage vs. Temperature Temperature ( C) ADJ Threshold Voltage (V) ADJ Threshold Voltage vs. Temperature Temperature ( C) 500 R DS(ON) vs. Temperature 460 RDS(ON) (mohm) (mω) Temperature ( C) 7

8 Application Information Setting Average Output Current The RT8420 output current which flows through the LEDs is set by an external resistor (R S ) connected between the VIN and SENSE terminal. The relationship between output current (I OUT ) and R S is shown as below : I 0.1V OUTavg = (A) RS PWM 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 2.7V (Max.) and the logic low level must be below 0.39V (Min.) at the ADJ terminal. It's recommended to maintain the PWM dimming at low frequency in order to obtain a linear dimming curve. Soft-Start Behavior The RT8420 features an optional soft-start behavior that allows for gradual brightness transition. This is achieved by simply connecting an external capacitor between the ADJ pin and. An internal current source will then charge this capacitor for soft-start behavior. The capacitor can be selected according to below equation : C = 1.25μA (typ.) x t SS where t SS is the soft-start period. 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 > V IN VOUT VSEN RDS(ON) I OUT f I where f SW is the switching frequency SW R DS(ON) is the low side switch on-resistance of internal MOSFET ( = 0.35Ω typ.) D is the duty cycle determined by VOUT/VIN L I OUT is the required LED current ΔI L is the inductor peak-peak ripple current [internally set to 0.3 (typ.) x I OUT ] V IN is the input supply voltage V OUT is the total LED forward voltage 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 300ns (typ.) of the minimum on time, and the off time must be greater than 300ns (typ.) of the minimum off time. The following equation can be used to verify the suitability of the inductor value. t t ON OFF LIL V V I R R R IN OUT OUT SEN L DS(ON) t ON(MIN) (300ns typ.) LIL V V V I R OUT D SEN OUT L t OFF(MIN) (300ns typ.) where V D is the rectifier diode forward voltage V SEN is the voltage cross current sense resistor R L is the inductor DC resistance L is the inductance 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. 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 8

9 dissipation, the Schottky diode outperforms other silicon diodes and increase overall efficiency. Thermal Considerations The junction temperature should never exceed the absolute maximum junction temperature T J(MAX), listed under Absolute Maximum Ratings, to avoid permanent damage to the device. The maximum allowable power dissipation depends on the thermal resistance of the IC package, the PCB layout, the rate of surrounding airflow, and the difference between the junction and ambient temperatures. The maximum power dissipation can be calculated using the following formula : P D(MAX) = (T J(MAX) T A ) / θ JA where T J(MAX) is the maximum junction temperature, T A is the ambient temperature, and θ JA is the junction-to-ambient thermal resistance. For continuous operation, the maximum operating junction temperature indicated under Recommended Operating Conditions is 125 C. The junction-to-ambient thermal resistance, θ JA, is highly package dependent. For a MSOP-8 (Exposed Pad), the thermal resistance, θ JA, is 47.4 C/W on a standard JEDEC 51-7 high effective-thermalconductivity four-layer test board. For a SOP-8 (Exposed Pad), the thermal resistance, θ JA, is 30.6 C/W on a standard JEDEC 51-7 high effective-thermal-conductivity four-layer test board. The maximum power dissipation at T A = 25 C can be calculated as below : P D(MAX) = (125 C 25 C) / (47.4 C/W) = 2.1W for a MSOP-8 (Exposed Pad) package. P D(MAX) = (125 C 25 C) / (30.6 C/W) = 3.26W for a SOP-8 (Exposed Pad) package. Maximum Power Dissipation (W) Four-Layer PCB SOP-8 (Exposed Pad) MSOP-8 (Exposed Pad) Ambient Temperature ( C) Figure 1. Derating Curve of Maximum Power Dissipation Layout Considerations For best performance of the RT8420, please abide the following layout guide. The capacitor C VIN, C PVCC, C ADJ and external resistor, R S, must be placed as close as possible to the PVCC, VIN and SENSE pins of the device respectively. The should be connected to a strong ground plane. The IC thermal pad should be connected to a large ground copper area, preferably with vias underneath the IC connected to inner ground planes for optimal cooling. 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 terminal. The maximum power dissipation depends on the operating ambient temperature for the fixed T J(MAX) and the thermal resistance, θ JA. The derating curves in Figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 9

10 Place the resistor R S as close as possible to VIN and SENSE pins. R S SENSE RT VIN PVCC C PVCC C VIN C IN D ADJ 4 5 L C ADJ Place the capacitor C VIN and C PVCC as close as possible to VIN and PVCC pins Figure 2. PCB Layout Guide 10

11 Outline Dimension Symbol Dimensions In Millimeters Dimensions In Inches Min. Max. Min. Max. A A A b D e E E L U V Lead MSOP (Exposed Pad) Plastic Package 11

12 A H M EXPOSED THERMAL PAD (Bottom of Package) J Y X B F I D C Symbol Dimensions In Millimeters Dimensions In Inches Min Max Min Max A B C D F H I J M Option 1 X Y Option 2 X Y Lead SOP (Exposed Pad) Plastic Package 12

13 Footprint Information Package Number of Pin Footprint Dimension (mm) P A B C D Sx Sy M Tolerance MSOP-8(PP) ±0.10 Footprint Dimension (mm) Package Number of Pin P A B C D Sx Sy M Option PSOP Option Tolerance ±0.10 Richtek Technology Corporation 14F, No. 8, Tai Yuen 1 st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863) 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. 13