® RT8480 High Voltage Boost/SEPIC Controller General Description Features The RT8480 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications. With a low side current sense amplifier threshold of 190mV, the LED current is programmable with one external current sense resistor. High Voltage Capability : VIN Up to 36V, VOUT is limited by External MOSFET Switch Boost Operation Current Mode PWM with Programmable Switching Frequency Easy Dimming Control : Analog or Digital Converting to Analog with One External Capacitor True PWM Dimming : External FET Driver is BuildIn Programmable Soft-Start to Avoid Inrush Current Programmable Over Voltage Protection VIN Under Voltage Lockout and Thermal Shutdown 16-Lead SOP Package RoHS Compliant and Halogen Free With programmable operating frequency up to 800kHz, the external inductor and capacitors can be small while maintaining high frequency. Dimming can be done by either analog or digital. A built-in clamping comparator and filter allow easy low noise analog dimming conversion from digital signal with only one external capacitor. An unique True PWM dimming control is made easy with MOSFET under LED string. A very high dimming ratio can be achieved by adopting both analog/ digital dimming and True PWM dimming together. The RT8480 is available in a SOP-16 package. Applications Ordering Information RT8480 Package Type S : SOP-16 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 General Industrial High Power LED Lighting Desk Lights and Room Lighting Building and Street Lighting Industrial Display Backlight Pin Configurations (TOP VIEW) GBIAS GATE PWMOUT ISW PWMDIM ISP ISN VC 2 16 15 3 14 4 5 13 12 6 7 11 10 8 9 GND VCC RSET OVP EN SS DCTL ACTL SOP-16 RT8480GS : Product Number RT8480 GSYMDNN YMDNN : Date Code Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8480 Typical Application Circuit L1 47µH VIN 4.5V to 36V VOUT CIN 10µF RT8480 15 5V RRSET 30k Analog Dimming GATE 2 VCC 12 EN ISW 4 14 RSET ISP 9 ACTL 10 DCTL CSS 0.1µF ISN 8 VC 11 SS 1 GBIAS RVC 1.8k CVC 10nF D1 CB 1µF OVP PWMOUT COUT 1µF M1 14 LEDs RSW 0.05 6 RSENSE 7 R1 13 3 VOUT R2 PWMDIM 5 GND 16 Figure 1. Analog Dimming in Boost Configuration L1 47µH VIN 4.5V to 36V VOUT CIN 10µF RT8480 15 GATE 2 VCC 12 EN 14 RSET 5V RRSET 30k D1 PWM Dimming control CSS 0.1µF RSENSE ISN SS 1 GBIAS 9 ACTL CB 1µF 14 LEDs RSW 0.05 10 DCTL 11 CVC 10nF M1 ISW 4 6 ISP 8 VC RVC 1.8k COUT 1µF OVP PWMOUT 7 R1 13 VOUT 3 R2 PWMDIM 5 GND 16 CA 0.47µF Figure 2. PWM to Analog Dimming in Boost Configuration L1 47µH VIN 4.5V to 36V VOUT CIN 10µF RT8480 15 14 RRSET 30k RSET 9 ACTL 10 DCTL RVC 1.8k CVC 10nF VCC 12 EN 5V CSS 0.1µF D1 GATE 2 M1 ISP 14 LEDs RSW 0.05 ISW 4 PWMOUT COUT 1µF 3 M2 6 7 8 VC ISN 5 11 PWMDIM SS 13 1 GBIAS OVP GND CB 16 1µF RSENSE R1 VOUT R2 Figure 3. True PWM Dimming in Boost Configuration Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 L1 47µH VIN 10V to 36V CIN 10µF RT8480 15 5V RRSET 30k Analog GATE 2 VCC 12 EN ISW 4 14 RSET ISP CSS 0.1µF ISN 8 VC 11 SS 1 GBIAS RVC 1.8k VOUT 100V M1 RSW 0.05 100k 6 190 9 ACTL 10 DCTL Dimming CVC 10nF D1 CB 1µF OVP PWMOUT 7 R1 13 3 VOUT R2 PWMDIM 5 GND 16 Figure 4. Constant Voltage Output of Boost Converter L1 47µH VIN 10V to 36V CIN 10µF RT8480 15 5V RRSET 30k Analog Dimming GATE 2 VCC 12 EN ISW 4 14 RSET ISP CSS 0.1µF ISN 8 VC 11 SS 1 GBIAS CB 1µF M1 RSW 0.05 D1 L2 47µH VOUT 24V 24k 6 190 9 ACTL 10 DCTL RVC 1.8k CVC 10nF 1µF / 100V OVP PWMOUT 7 R1 13 3 VOUT R2 PWMDIM 5 GND 16 Figure 5. Constant Voltage Output of SEPIC Converter L1 47µH VIN 10V to 36V CIN 10µF RT8480 GATE 2 15 VCC 12 EN 5V ISW 4 14 RSET RRSET 30k Analog Dimming RVC 1.8k CVC 10nF PWMOUT ISP 9 ACTL 10 DCTL 8 VC 11 CSS 0.1µF 1µF / 100V RSW 0.05 L2 47µH 3 VOUT 24V COUT 1µF M2 6 190 ISN SS 1 GBIAS CB 1µF M1 D1 PWMDIM GND 16 OVP 7 5 R1 13 VOUT R2 Figure 6. True PWM Dimming in SEPIC Application Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8480 Functional Pin Description Pin No. Pin Name Pin Function 1 GBIAS Internal Gate Driver Bias Pin. A good bypass capacitor is required. 2 GATE External MOSFET Switch Gate Driver Output Pin. 3 PWMOUT 4 ISW 5 PWMDIM Output Pin for the PWM Dimming MOSFET Driver. External MOSFET Switch Current Sense Pin. Connect the current sense resistor between external N-MOSFET switch and the ground. Control Input Pin for the PWM Dimming MOSRET Driver. 6 ISP 7 ISN 8 VC 9 ACTL Analog Dimming Control Pin. The effective programming voltage range of the pin is between 0.3V and 1.2V. 10 DCTL PWM Dimming Control Pin, By adding a 0.47F filtering capacitor on the ACTL pin, the PWM dimming signal on the DCTL pin can be averaged and converted into analog dimming signal on the ACTL pin following the formula below. V ACTL = 1.2V x PWM Dimming Duty Cycle. 11 SS Soft-Start Pin. A capacitor of at least 100nF is required for proper soft-start. 12 EN Chip Enable (Active High). When this pin voltage is low, the chip is in shutdown mode. 13 OVP Over Voltage Protection Pin. The PWM converter turns off when the voltage of the pin goes to higher than 1.2V. 14 RSET Switching Frequency Set Pin connect a Resistor from RSET to GND. fRSET = 30k will set f SW = 380kHz. 15 VCC The Power Supply Pin of the Chip. For good bypass, a low ESR capacitor is required. 16 GND Ground. LED Current Sense Amplifier Positive Input. LED Current Sense Amplifier Negative Input. Voltage threshold between ISP and ISN is 190mV. PWM Control Loop Compensation Pin. Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Function Block Diagram VOC 8.5V EN + 1.4V + - Shutdown - RSET VCC GBIAS S OSC GATE - 4.5V 5V + R OVP 1.2V + R - R 100k GBIAS R PWMOUT PWMDIM + - - 110mV + VC ISW ISP ISN GM + 6µA SS DCTL 1.2V + + - - GND ACTL VISP – VISN (mV) 190 0 0.25 Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 1.25 VACTL (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8480 Absolute Maximum Ratings (Note 1) Supply Input Voltage, VCC ------------------------------------------------------------------------------------------------- 38V GBIAS, GATE, PWMDIM, PWMOUT ---------------------------------------------------------------------------------- 10V ISW DC ------------------------------------------------------------------------------------------------------------------------------- 1V < 200ns ------------------------------------------------------------------------------------------------------------------------ 6V ISP, ISN DC ------------------------------------------------------------------------------------------------------------------------------- 2V < 200ns ------------------------------------------------------------------------------------------------------------------------ 6V DCTL, ACTL, OVP Pin Voltage ------------------------------------------------------------------------------------------ 8V (Note 2) EN Pin Voltage --------------------------------------------------------------------------------------------------------------- 20V Power Dissipation, PD @ TA = 25°C SOP-16 ------------------------------------------------------------------------------------------------------------------------ 1.176W Package Thermal Resistance (Note 3) SOP-16, θJA ------------------------------------------------------------------------------------------------------------------ 85°C/W Junction Temperature ------------------------------------------------------------------------------------------------------- 150°C Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------------------- 260°C Storage Temperature Range ---------------------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 4) HBM (Human Body Mode) ------------------------------------------------------------------------------------------------ 2kV MM (Machine Mode) -------------------------------------------------------------------------------------------------------- 200V Recommended Operating Conditions (Note 5) Supply Input Voltage Range, VCC --------------------------------------------------------------------------------------- 4.5V to 36V Junction Temperature Range ---------------------------------------------------------------------------------------------- −40°C to 125°C Electrical Characteristics (VCC = 24V, No Load, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Overall Supply Current ICC VC 0.4V (Switching Off) -- 6 7.2 mA Shutdown Current ISHDN VEN 0.7V -- 12 -- A EN Threshold Voltage Logic-High VIH 2 -- -- Logic-Low VIL -- -- 0.5 -- -- 3 A 170 190 210 mV -- 20 -- A -- 20 -- A -- 0.7 -- V VEN 3V EN Input Current V Current Sense Amplifier Input Threshold (VISP VISN) ISP / ISN Input Current IISP / IISN VC Output Current IVC VISP VISN = 0V VISP VISN = 190mV, 0.5V VC 2.4V VC Threshold for PWM Switch Off Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Parameter Symbol Test Conditions Min Typ Max Unit VACTL = 1.2V -- 1 -- VACTL = 0.3V -- 10 -- VACTL_ON -- 1.25 -- V VACTL_OFF -- 0.25 -- V -- -- 0.5 A LED Dimming ACTL Input Current I ACTL LED Current On Threshold at ACTL LED Current Off Threshold at ACTL DCTL Input Current A I DCTL 0.3V VDCTL 6V f SW RRSET = 30k 300 -- 380 280 460 -- kHz ns GBIAS Voltage VGBIAS I GBIAS = 20mA -- 8.2 -- V Gate Voltage High VGate_H I Gate = 20mA -- 7.2 -- I Gate = 100A -- 7.5 -- Gate Voltage Low VGate_L I Gate = 100A -- 0.5 -- V 1nF Load at GATE -- 15 -- ns I LIM_SW -- 90 -- mV VPWMDIMH 2 -- -- VPWMDIML -- -- 0.5 PWM Control Switching Frequency Minimum OFF Time (Note 6) Switch Gate Driver GATE Drive Rise and Fall Time PWM Switch Current Limit Threshold PWM Dimming Gate Driver PWMDIM Logic-High Threshold Logic-Low Voltage PWMOUT Output Voltage V V VPWMOUTH I PWMOUT = 1mA -- 7.5 -- VPWMOUTL I PWMOUT = 100A -- 0.45 -- 1nF Load at PWMOUT -- 40 -- ns 1.12 1.18 1.24 V PWMOUT Drive Rise and Fall Time OVP and Soft-Start V OVP Threshold VOVP_th OVP Input Current I OVP 0.7V VOVP 1.5V -- -- 0.1 A Soft-Start Current I SS VSS 2V -- 6 -- A Thermal Protection Thermal Shutdown TSD -- 145 -- C Thermal Shutdown Hysteresis TSD -- 10 -- C Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8480 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. If connected with a 20kΩ serial resistor, ACTL and DCTL can go up to 36V. Note 3. θJA is measured at TA = 25°C on a low effective thermal conductivity single-layer test board per JEDEC 51-3. Note 4. Devices are ESD sensitive. Handling precaution is recommended. Note 5. The device is not guaranteed to function outside its operating conditions. Note 6. When the natural maximum duty cycle of the switching frequency is reached, the switching cycle will be skipped (not reset) as the operating condition requires to effectively stretch and achieve higher on cycle than the natural maximum duty cycle set by the switching frequency. Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Typical Operating Characteristics Efficiency vs. Input Voltage LED Current vs. ACTL PWM Duty 100.0 350 97.5 300 250 LED Current (mA) Efficiency (%) 95.0 92.5 90.0 87.5 85.0 200 150 100 50 82.5 VIN = 24V 15 LEDs, IOUT = 300mA 80.0 12 15 18 21 24 27 30 33 0 0 36 20 Input Voltage (V) LED Current vs. Input Voltage 100 RSET vs. Switching Frequency Switching Frequency (kHz)1 LED Current (mA) 80 1200 350 300 250 200 150 100 50 1000 800 600 400 200 VIN = 24V VIN = 24V to 36V 0 0 12 16 20 24 28 32 36 10 20 30 40 50 60 70 80 90 100 (kΩ) RSET (K) Input Voltage (V) Switching Frequency vs. Input Voltage VISP – VISN Threshold vs. Input Voltage 220 840 205 760 Switching Frequency (kHz)1 VISP – VISN Threshold (mV) 1 60 PWM Duty (%) 400 190 175 160 145 130 115 RRSET = 20kΩ 680 600 RRSET = 36kΩ 520 440 360 RRSET = 10kΩ 280 100 4 11 18 25 32 Input Voltage(V) Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 40 November 2013 39 4 11 18 25 32 39 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8480 Supply Current vs. Input Voltage 10 390 9 380 8 Supply Current (mA) Switching Frequency (kHz)1 Switching Frequency vs. Temperature 400 370 360 350 340 330 7 6 5 4 3 2 320 310 1 VIN = 24V 300 -50 -25 0 25 50 75 100 0 4 125 11 9 9 8 7 6 5 4 3 2 -25 0 25 50 39 75 100 8 7 6 5 4 3 2 1 VIN = 24V -50 32 Shutdown Current vs. Input Voltage 10 Shutdown Current (μA)1 Supply Current (mA) 1 Supply Current vs. Temperature 10 0 25 Input Voltage (V) Temperature (°C) 1 18 VEN = 0V 0 125 4 11 Temperature (°C) 18 25 32 39 32 39 Input Voltage(V) Soft-Start Current vs. Input Voltage OVP vs. Input Voltage 1.20 8.0 1.19 7.0 6.5 OVP (V) Soft-Start Current (μA) 7.5 6.0 5.5 1.18 OVP_H 1.17 OVP_L 5.0 1.16 4.5 4.0 1.15 3.5 1.14 3.0 4 11 18 25 32 Input Voltage(V) Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 39 4 11 18 25 Input Voltage(V) is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 GBIAS Voltage vs. Input Voltage 10 0.235 9 0.230 8 GBIAS Voltage (V) ACTLThreshold (V) 1 ACTLThreshold vs. Input Voltage 0.240 0.225 0.220 0.215 0.210 7 6 5 4 0.205 3 0.200 2 1nF Load 4 11 18 25 32 39 4 11 Input Voltage(V) ISW Threshold (mV) Gate Output Voltage (V) 39 140 8 Gate_High 7 6 5 4 3 2 Gate_Low 1 130 120 110 100 90 80 70 60 50 0 4 11 18 25 32 4 39 ACTL (5V/Div) EN (5V/Div) GATE (5V/Div) GATE (5V/Div) VOUT (50V/Div) I LED (500mA/Div) VOUT (50V/Div) I LED (500mA/Div) Time (500μs/Div) Copyright © 2013 Richtek Technology Corporation. All rights reserved. November 2013 18 25 32 39 Power On from EN PWM Dimming Response VIN = 24V, fPWM = 1kHz, Duty = 50% 11 Input Voltage (V) Input Voltage(V) DS8480-02 32 150 1nF Load 9 25 ISW Threshold vs. Input Voltage Gate Output Voltage vs. Input Voltage 10 18 Input Voltage (V) VIN = 24V, IOUT = 360mA, CSS = 0.1μF Time (10ms/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8480 Application Information The RT8480 is a constant frequency, current mode controller which drives an external MOSFET for PWM LED applications, DC/DC Boost, SEPIC and Flyback converter. When using an external load switch, the PWMDIM input not only drives PWMOUT, but also enables controller GATE switching and error amplifier operation. This feature provides extremely fast, true PWM load switching with no transient overvoltage. In normal operation with PWMDIM high, GATE goes high and the power MOSFET is turned on. When the oscillator sets the PWM latch, the power MOSFET is turned off when the VC current comparator resets the latch. When the load current increases, a fall in the ISN voltage relative to the reference voltage at ISP causes the VC pin to rise and the average inductor current will therefore rise until it equals the load current. When PWMDIM goes low, PWMOUT goes low, VC opens and GATE switching is disabled. Lowering PWMOUT and disabling GATE causes the output capacitor, COUT, to hold the output voltage constant in the absence of load current. Power on sequence VIN UVLO PWM EN must be turned on later than VIN and PWM signal EN must be turned off early than VIN and PWM signal No Soft-Start Soft-Start if PWM turns on later EN VOUT Figure 7. Power On Sequence Control by EN Power on sequence Power off sequence UVLO PWM EN VIN must be turned off parlier than EN and PWM signal VIN must be turned on later than EN and PWM signal Soft-Start VOUT Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 No Soft-Start if PWM turns on late Figure 8. Power On Sequence Control by VIN EN and/or VIN should be pulled low once PWM pulls low for over 10ms EN/VIN Power Sequence Please refer to below Figure 7 and 7. The recommended power-on sequence suggests the PWM to be ready before EN and/or VIN is ready. If not, the soft-start function will be disabled. As for power-off sequence, EN/VIN must be pulled low within 10ms to prevent “Hard-Start” as shown Figure 9. Abnormal Power on sequence VIN Input UVLO The input operating voltage range of the RT8480 is 4.5V to 36V. An input capacitor at the VCC pin can reduce ripple voltage. It is recommended to use a ceramic 10μF or larger capacitance as the input capacitor. This IC provides an Under Voltage Lockout (UVLO) function to enhance the stability when startup. The UVLO threshold of input rising voltage is set at 4.5V typically with a 0.7V hysteresis. Abnormal Power on sequence Power off sequence PWM 10ms Figure 9. To Prevent “Hard-Start” Sequence is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Soft-Start The soft-start of the RT8480 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 on 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 equation : tSS = CSS 2.4V 6 A (s) 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 during an over-temperature event or during a GBIAS under voltage event. GBIAS Regulator Operation The GBIAS pin requires a capacitor for stable operation and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R or X5R ceramic capacitor for best performance. The value of the capacitor is determined primarily by the stability of the regulator rather than the gate charge of the switching N-MOSFET. A 1μF capacitor will be adequate for most applications. Place the capacitor close to the IC to minimize the trace length to the GBIAS pin and also to the IC ground. An internal current limit on the GBIAS protects the RT8480 from excessive on-chip power dissipation. If the input voltage, VIN, is less than 8V, then the GBIAS pin should be connected to the input supply. Be aware that if GBIAS supply is used to drive extra circuits besides RT8480, typically the extra GBIAS load should be limited to less than 10mA. Loop Compensation The RT8480 uses an internal error amplifier via the compensation pin (VC) to optimize the loop response for specific application. The external inductor, output capacitor, Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 and compensation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop response and stability. 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. The typical compensation for the RT8480 is 1.8kΩ and 10nF. LED Current Setting The maximum current is programmed by placing an appropriate valued sense resistor at the LED string. When the voltage of ACTL is higher than 1.25V, the LED current can be calculated by the following equation : 190mV 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 1.25V to get the full-scale 190mV (typical) threshold across the sense resistor. The ACTL pin can also be used to dim the LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. When the ACTL pin voltage is less than 1.25V, the LED current is : ILED = (VACTL 0.25) 190mV RSENSE (mA) The ACTL pin can also be connected with a thermistor to provide over-temperature protection for the LED load, or with a resistive voltage divider to VIN to reduce output power and switching current when VIN is low. Brightness Control For LED applications where a wide dimming range is required, two competing methods are available: analog dimming and PWM dimming. The easiest method is to simply vary the DC current through the LED by analog dimming. The RT8480 features both analog and digital dimming control. Analog dimming is linearly controlled by an external voltage (0.25V to 1.25V) at the ACTL pin. Digital dimming can be implemented by driving a PWM signal at the DCTL pin for linear current regulator. A very high is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8480 Dimming frequency can be sufficiently adjusted from 100Hz to 30kHz. However, LED current cannot be 100% proportional to duty cycle especially for high frequency and low duty ratio because of physical limitation caused by internal switching frequency. Typically, in order to avoid visible flicker, PWM dimming signal should be greater than 120Hz. Assuming inductor and capacitor sizing close to discontinuous operation, two fOSC cycles are sufficient for proper PWM operation. Thus, the minimum dimming duty can be as low as 1% for the frequency range from 100Hz to 300Hz. For the dimming frequency from 300Hz to 1kHz, the duty is about 5%. If the frequency is increased to 1kHz to 30kHz, the duty will be about 10%. 1/fPWM Duty/fPWM Table 1. Switching Frequency vs RT Value (1% Resistors) f OSC (kHZ) RRSET (k) 800 10.6 600 15.81 500 20.26 300 35.8 200 47.6 Frequency vs. RRSET 900 800 Frequency (kHz)1 contrast ratio can be obtained via true digital PWM dimming, which is achieved by driving ACTL pin with a PWM signal. The recommended PWM frequency rangle is 100Hz to 10kHz. 700 600 500 400 300 200 10 15 20 25 30 35 40 45 50 RRSET (Ω) (ٛ ) Figure 11. Switching Frequency vs RRSET N>2 1/fOSC N : the number of fOSC cycles per PWM cycle Figure 10. PWM Dimming Parameters Programmable Switching Frequency The RSET frequency adjust pin allows the user to program the switching frequency from 100kHz to 1MHz for optimized efficiency and performance or external component size. Higher frequency operation allows for smaller component size but increases switching losses and gate driving current, and may not allow sufficiently high or low duty cycle operation. Lower frequency operation gives better performance but with larger external component size. For an appropriate RRSET resistor value see Table 1 or Figure 11. An external resistor from the RSET pin to GND is required-do not leave this pin open. Input Over Current Protection The resistor, RSW, between the source of the external switching N-MOSFET and GND should be selected to provide adequate switch current. The RT8480 senses the inductor current through ISW pin in the switch on period. The duty cycle depends on the current sense signal summed with the internal slope compensation and compared to the VC pin signal. The external N-MOSFET will be turned off when the current signal is larger than the VC pin signal. In the off period, the inductor current will descend. The external N-MOSFET is turned on by the oscillator at the beginning of the next cycle. To drive the application without exceeding the 90mV (typical) current limit threshold on the ISW pin of the RT8480, it is recommended to select a resistor that gives a switch current of at least 20% greater than the required LED current according to : V 0.1V RSW =( IN ) VOUT IOUT The ISW pin input to the RT8480 should be a Kelvin connection to the positive terminal of RSW. Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Output Over Voltage Protection Setting Power MOSFET Selection The RT8480 is equipped with an Over Voltage Protection (OVP) function. When the voltage at the OVP pin exceeds a threshold of approximately1.18V, the power switch will be turned off. The power switch can be turned on again once the voltage at the OVP pin drops below 1.18V. The output voltage could be clamped at a certain voltage level set by the following equation : R1 VOUT, OVP = 1.18 (1 ) R2 For applications operating at high input or output voltages, the power N-MOSFET switch is typically chosen for drain voltage, VDS, rating and low gate charge. Consideration of switch on-resistance, R DS(ON), is usually secondary because switching losses dominate power loss. The GBIAS regulator on the RT8480 has a fixed current limit to protect the IC from excessive power dissipation at high VIN, so the N-MOSFET should be chosen such that the product of Qg at 5V and switching frequency does not exceed the GBIAS current limit. where R1 and R2 are the voltage divider resistors 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 RT8480 provides an over temperature protection (OTP) function to prevent the excessive power dissipation from overheating the device. The OTP function will shut down switching operation when the die junction temperature exceeds 145°C. The chip will automatically start to switch again when the die junction temperature is reduced by approximately 10°C. Inductor Selection The inductor used with the RT8480 should have a saturation current rating appropriate to the maximum switch current. Choose an inductor value based on operating frequency, input and output voltage to provide a current mode ramp of approximately 20mV magnitude on the ISW pin during the switch on-time. The following equations are useful to estimate the inductor value : L= (VOUT VIN ) (VIN )2 2 IOUT f (VOUT )2 where, VOUT = Maximum output voltage. VIN = Minimum input voltage. f = Operating frequency. IOUT = Sum of current from all LED strings. η is the efficiency of the power converter. Copyright © 2013 Richtek Technology Corporation. All rights reserved. DS8480-02 November 2013 Schottky Diode Selection The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for RT8480 applications. In addition, power dissipation, reverse voltage rating and pulsating peak current are also important parameters for Schottky diode selection. Choose a suitable Schottky diode with reverse voltage rating greater than the maximum output voltage. The diode's average current rating must exceed the average output current. The diode conducts current only when the power switch is turned off (typically less than 50% duty cycle). If using the PWM feature for dimming, it is important to consider diode leakage, which increases with temperature, from the output during the PWM low interval. Therefore, a Schottky diode with sufficiently low leakage current is suggested. V VIN ID, PEAK = 1.2 IOUT ( OUT ) VOUT Capacitor Selection The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most of the RT8480 applications, a 10μF ceramic capacitor is 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. In Boost Application, the output capacitor is typically a ceramic capacitor and is selected based on the output voltage ripple requirements. The minimum value of the output capacitor, COUT, is approximately given by the following equation : COUT = IOUT (VOUT VIN ) VRIPPLE VOUT f is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8480 1.2 Maximum Power Dissipation (W)1 For LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the current ripple. Use of X7R type ceramic capacitors is recommended. Lower operating frequencies will require proportionately higher capacitor values. 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 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 12. Derating Curve for RT8480 Package Layout Consideration where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient PCB layout is very important when designing power switching converter circuits. Some recommended layout guidelines are suggested as follows : thermal resistance. The power components L1, D1, CIN, M1 and COUT must be placed as close to each other as possible to reduce the ac current loop area. At least one via to the ground plane immediately under the exposed pad. The ground trace on the top layer of the PC board should be as wide and short as possible to minimize series resistance and inductance. Place L1 and D1 connected to N-MOSFET as close to each other as possible. The trace should be as short and wide as possible. The input capacitor, CIN, 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. For recommended operating condition specifications of the RT8480, the maximum junction temperature is 125°C and TA is the ambient temperature. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP16 packages, the thermal resistance, θJA, is 85°C/W on a standard JEDEC 51-7 single-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) / (85°C/W) = 1.176W for SOP-16 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. For the RT8480 package, the derating curve in Figure 12 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. VOUT D1 L1 VIN M1 GND M2 GBIAS GATE PWMOUT ISW PWMDIM ISP ISN VC 16 2 15 3 14 4 13 5 12 6 11 7 10 8 9 VOUT GND VCC RSET OVP EN SS DCTL ACTL Analog Dimming Figure 13. PCB Layout Guide Copyright © 2013 Richtek Technology Corporation. All rights reserved. www.richtek.com 16 is a registered trademark of Richtek Technology Corporation. DS8480-02 November 2013 RT8480 Outline Dimension H A M B J F C I D Dimensions In Millimeters Dimensions In Inches Symbol Min Max Min Max A 9.804 10.008 0.386 0.394 B 3.810 3.988 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.508 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.178 0.254 0.007 0.010 I 0.102 0.254 0.004 0.010 J 5.791 6.198 0.228 0.244 M 0.406 1.270 0.016 0.050 16–Lead SOP Plastic Package Richtek Technology Corporation 14F, No. 8, Tai Yuen 1st 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. DS8480-02 November 2013 www.richtek.com 17