® RT8452 High Voltage High Current LED Driver Controller for Buck Boost or Buck-Boost Topology General Description Features The RT8452 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications. With a current sense amplifier threshold of 190mV, the LED current is programmable with one external current sense resistor. With the maximum operating input voltage of 36V and output voltage up to 48V, the RT8452 is ideal for Buck, Boost or Buck-Boost operation. z With 350kHz operating frequency, the external inductor and capacitors can be small while maintaining high efficiency. z z z z z z z 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 RT8452 is available in WQFN-16L 3x3 and SOP-16 packages. Ordering Information RT8452 Package Type S : SOP-16 QW : WQFN-16L 3x3 (W-Type) Lead Plating System G : Green (Halogen Free and Pb Free) z z High Voltage Capability : VIN Up to 36V, VOUT Up to 48V Buck, Boost or Buck-Boost Operation Current Mode PWM with 350kHz Switching Frenquency 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 Undervoltage Lockout and Thermal Shutdown 16-Lead WQFN and SOP Packages. RoHS Compliant and Halogen Free Applications z z z z 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 Note : SOP-16 RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. ` GND VCC OVP EN Richtek products are : ` Suitable for use in SnPb or Pb-free soldering processes. GND VCC OVP EN NC SS DCTL ACTL 16 15 14 13 GBIAS GATE PWMOUT ISW 1 12 2 11 GND 3 10 17 4 6 7 9 8 PWMDIM ISP ISN VC 5 NC SS DCTL ACTL WQFN-16L 3x3 Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8452 Marking Information RT8452GS RT8452GQW RT8452GS : Product Number RT8452 GSYMDNN F4= : Product Code YMDNN : Date Code YMDNN : Date Code F4=YM DNN Typical Application Circuit L1 47µH VIN 4.5V to 36V CIN 10µF RT8452 15 13 EN 5V Analog Dimming 9 ACTL 10 DCTL 8 VC 11 SS 1 GBIAS RVC 10k CVC 3.3nF GATE 2 VCC CSS 0.1µF CB 1µF RSENSE 0.09 D1 COUT 1µF M1 14 LEDs RSW 0.05 ISW 4 6 ISP 7 ISN 14 OVP 3 PWMOUT PWMDIM 5 VOUT 48V (Max.) R1 VOUT R2 GND 16, 17 (Exposed Pad) Figure 1. Analog Dimming in Boost Configuration VIN 4.5V to 36V L1 47µH CIN 10µF RT8452 15 13 EN 5V 10 DCTL PWM Dimming control 8 VC 11 SS 1 GBIAS RVC 10k CVC 3.3nF CSS 0.1µF VCC CB 1µF 9 ACTL CA 0.47µF RSENSE 0.09 D1 GATE 2 ISW 4 6 ISP 7 ISN 14 OVP 3 PWMOUT PWMDIM 5 COUT 1µF M1 VOUT 48V (Max.) 14 LEDs RSW 0.05 R1 VOUT R2 GND 16, 17 (Exposed Pad) Figure 2. PWM to Analog Dimming in Boost Configuration Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 VIN 4.5V to 36V L1 47µH CIN 10µF RT8452 15 GATE 2 VCC 13 EN 5V 9 ACTL 10 DCTL 8 VC 11 SS 1 GBIAS RVC 10k CVC 3.3nF CSS 0.1µF CB 1µF RSENSE 0.09 D1 ISW 4 6 ISP 7 ISN 3 PWMOUT PWMDIM 5 OVP VOUT 48V (Max.) COUT 1µF M1 14 LEDs RSW 0.05 M2 R1 14 GND 16, 17 (Exposed Pad) VOUT R2 Figure 3. True PWM Dimming in Boost Configuration L1 47µH VIN 4.5V to 36V VOUT 48V (Max.) CIN 10µF RT8452 15 GATE VCC 13 EN 5V Analog Dimming CSS 0.1µF 2 8 VC 11 SS 1 GBIAS CB 1µF ISN COUT 1µF M1 RSW 0.05 ISW 4 9 ACTL 10 DCTL RVC 10k CVC 3.3nF D1 10 LEDs RSENS 0.09 7 ISP 6 PWMOUT 3 PWMDIM 5 OVP 14 R1 GND 16, 17 (Exposed Pad) VOUT R2 Figure 4. Analog Dimming in Buck-Boost Configuration D1 COUT 1µF RSENSE VIN CIN 10µF 0.09 8 LEDs L1 47µH RT8452 15 13 EN 5V Analog Dimming 9 ACTL 10 DCTL RVC 10k CVC 3.3nF ISP 6 VCC CSS 0.1µF 8 VC 11 SS 1 GBIAS CB 1µF ISN 7 GATE M1 2 RSW 0.05 ISW 4 PWMOUT 3 PWMDIM 5 OVP 14 GND 16, 17 (Exposed Pad) Figure 5. Analog Dimming in Buck Configuration Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8452 Functional Pin Description Pin No. Pin Name Pin Function SOP-16 WQFN-16L 3x3 1 1 GBIAS Internal Gate Driver Bias. A good bypass capacitor is required. 2 2 GATE External MOSFET Switch Gate Driver Output. 3 3 PWMOUT Output Pin for the PWM Dimming FET Driver. 4 4 ISW External MOSFET Switch Current Sense. Connect the current sense resistor between external N-MOSFET switch and the ground. 5 5 PWMDIM Control Input Pin for the PWM Dimming FET Driver. 6 6 ISP LED Current Sense Amplifier Positive Input. 7 7 ISN LED Current Sense Amplifier Negative Input. Voltage threshold between ISP and ISN is 190mV. 8 8 VC PWM Control Loop Compensation. 9 9 ACTL 10 10 DCTL 11 11 SS Soft-Start. A capacitor of at least 10nF is required for proper soft start. 12 12 NC No Internal Connection. 13 13 EN 14 14 OVP 15 15 VCC 16 16 GND 17 (Exposed Pad) Analog Dimming Control. The effective programming voltage range of the pin is between 0.2V and 1.2V. By adding a 0.47μF filtering capacitor on ACTL pin, the PWM dimming signal on DCTL pin can be averaged and converted into analog dimming signal on the ACTL pin. Chip Enable (Active High). When this pin voltage is low, the chip is in shutdown mode. Over Voltage Protection. The PWM converter turns off when the voltage of the pin goes to higher than 1.2V. Power Supply Pin of the Chip. For good bypass, a low ESR capacitor is required. Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 Function Block Diagram VOC 5.7V EN + 1.4V VCC - Shutdown GBIAS - GATE S OSC - 4.5V + 5V + R OVP 1.2V + R - R 100k GBIAS PWMOUT R PWMDIM + - - 100mV + VC ISW ISN GM + 6µA ISP SS DCTL 1.2V + 1.2V + - - GND ACTL VISP – VISN (mV) 190 0 0.2 1.2 VACTL (V) Figure 6 Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8452 Absolute Maximum Ratings z z z z z z z z z z z z (Note 1) Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------GBIAS, GATE, PWMDIM, PWMOUT -------------------------------------------------------------------------------ISW --------------------------------------------------------------------------------------------------------------------------ISP, ISN ---------------------------------------------------------------------------------------------------------------------DCTL, ACTL, OVP Pin Voltage ---------------------------------------------------------------------------------------EN Pin Voltage ------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C SOP-16 ---------------------------------------------------------------------------------------------------------------------WQFN-16L 3x3 -----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) SOP-16, θJA ----------------------------------------------------------------------------------------------------------------WQFN-16L 3x3, θJA ------------------------------------------------------------------------------------------------------WQFN-16L 3x3, θJC -----------------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------- Recommended Operating Conditions z z 38V 10V 1V 54V 8V (Note 6) 20V 1.0W 1.4W 95°C/W 68°C/W 7.5°C/W 150°C 260°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage Range, VCC ------------------------------------------------------------------------------------- 4.5V to 36V Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C Electrical Characteristics (VCC = 24V, No Load on any Output, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit Overall Supply Current IVCC VC ≤ 0.4V (Switching off) -- 6 7.2 mA Shutdown Current ISHDN VEN ≤ 0.7V -- 12 -- μA EN Threshold Logic-High VIH 2 -- -- Voltage Logic-Low VIL -- -- 0.5 -- -- 1.2 μA 180 190 200 mV VEN ≤ 3V EN Input Current V Current Sense Amplifier Input Threshold (VISP − VISN) 12V ≤ common mode ≤ 36V ISP / ISN Input Current IISP / IISN 4.5V ≤ VISP = VISN ≤ 48V -- 140 -- μA VC Output Current IVC VISP − VISN = 190mV, 0.5V ≤ VC ≤ 2.4V -- ±20 -- μA -- 0.7 -- V VC Threshold for PWM Switch Off Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 Parameter Symbol Test Conditions Min Typ Max Unit VACTL = 1.2V -- 1 -- VACTL = 0.2V -- 10 -- -- 0.3 -- V -- -- 0.5 μA 280 350 420 kHz -- 250 -- ns IGBIAS = 20mA -- 8.5 -- V IGate = −50mA -- 7.2 -- IGate = −100μA -- 7.8 -- IGate = 50mA -- 0.25 -- IGate = 100μA -- 0.1 -- 1nF Load at GATE -- 15 -- ns -- 110 -- mV 2 -- -- -- -- 0.5 VPWMOUT_H IPWMOUT = 1mA -- 7.5 -- VPWMOUT_L IPWMOUT = −100μA -- 0.45 -- -- 40 -- ns -- 1.18 -- V -- 0.1 μA μA LED Dimming Analog Dimming ACTL Pin Input Current IACTL LED Current Off Threshold at ACTL VACTL_Off DCTL Input Current IDCTL 0.3V ≤ VDCTL ≤ 6V μA PWM Control Switching Frequency fSW Minimum Off Time (Note 5) Switch Gate Driver GBIAS Voltage VGBIAS Gate Voltage High VGate_H Gate Voltage Low VGate_L GATE Drive Rise and Fall Time PWM Switch Current Limit Threshold ISW_LIM V V PWM Dimming Gate Driver Logic-High VPWMDIM_H PWMDIM Threshold Voltage Logic-Low VPWMDIM_L PWMOUT Output Voltage PWMOUT Drive Rise and Fall Time 1nF Load at PWMOUT V V OVP and Soft Start OVP Threshold VOVP_th OVP Input Current IOVP 0.7V ≤ VOVP ≤ 1.5V -- Soft-Start Pin Current ISS VSS ≤ 2V -- 6 -- Thermal Shutdown Protection T SD -- 145 -- Thermal Shutdown Hysteresis ΔTSD -- 10 -- Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 °C is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8452 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. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC 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. Note 5. When the natural maximum duty cycle of 350kHz 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 350kHz switching frequency. Note 6. If connected with a 20kΩ serial resistor, ACTL and DCTL can go up to 36V. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 Typical Operating Characteristics Efficiency vs. Output Current 90 90 Efficiency (%) 100 Efficiency (%) 100 80 70 80 70 Boost Application, VIN = 24V, VOUT = 48V 60 Boost Application, VOUT = 48V, IOUT = 1A 60 0 200 400 600 800 1000 1200 9 Output Current(mA) 12 15 18 21 24 400 Switching Frequency (kHz) 380 360 340 320 380 360 340 320 VIN = 24V 300 300 8 12 16 20 24 30 Switching Frequency vs. Temperature Switching Frequency vs. Input Voltage 4 27 Input Voltage (V) 400 Switching Frequency (kHz) Efficiency vs. Input Voltage 28 32 -50 36 -25 0 25 50 75 100 125 Temperature (°C) Input Voltage (V) Supply Current vs. Input Voltage Shutdown Current vs. Input Voltage 10 30 Supply Current (mA) Shutdown Current (μA) 9 25 20 15 10 5 8 7 6 5 4 3 2 1 0 0 4 7 10 13 16 19 22 25 28 31 34 37 40 Input Voltage (V) Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 4 8 12 16 20 24 28 32 36 Input Voltage (V) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8452 LED Current vs. DCTL Duty LED Current vs. ACTL 2.5 2.5 2.0 LED Current (A) 2.0 LED Current (A) DCTL high level is 3V and low level is 0V, DCTL = 10kHz, VIN = 24V, RSENSE = 90mΩ 1.5 1.0 1.5 1.0 0.5 0.5 VIN = 24V, RSENSE = 90mΩ 0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.0 0 1.1 1.2 1.3 1.4 10 20 30 1.20 1.20 1.19 OVP_H 1.18 OVP_L 1.16 1.16 1.15 1.15 20 90 100 24 28 32 OVP_H OVP_L 36 VIN = 24V -40 -25 -10 5 20 35 50 65 80 Input Voltage (V) Temperature (°C) Power Off from EN Power On from EN 95 110 125 Boost Application, VIN = 24V, VOUT = 48V Boost Application, VIN = 24V, VOUT = 48V IOUT (200mA/Div) IOUT (500mA/Div) VOUT (50V/Div) EN (2V/Div) VGate (10V/Div) VIN = 24V Time (100ms/Div) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 80 1.18 1.17 16 70 1.19 1.17 12 60 OVP vs. Temperature 1.21 OVP (V) OVP (V) OVP vs. Input Voltage 1.21 8 50 DCTL Duty (%) ACTL (V) 4 40 VOUT (20V/Div) EN (2V/Div) VGate (10V/Div) Time (250μs/Div) is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 Applications Information The RT8452 is a current mode PWM controller designed to drive an external MOSFET for high current LED applications. The LED current can be programmed by an external resistor. The input voltage range of the RT8452 can be up to 36V and the output voltage can be up to 48V. The RT8452 provides analog and PWM dimming to achieve LED current control. GBIAS Regulator and Bypass Capacitor 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 a 1μF capacitor will be adequate for many 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 output protects the RT8452 from excessive on-chip power dissipation. The GBIAS pin has its own under-voltage disable (UVLO) set to 4.3V(typical) to protect the external FETs from excessive power dissipation caused by not being fully enhanced. If the input voltage, VIN, will not exceed 8V, then the GBIAS pin should be connected to the input supply. Be aware if GBIAS supply is used to drive extra circuits besides RT8452, typically the extra GBIAS load should be limited to less than 10mA. Loop Compensation The RT8452 uses an internal error amplifier whose compensation pin (VC) allowing the loop response optimized for specific application. The external inductor, output capacitor and the 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. For typical LED applications, a 3.3nF compensation capacitor at VC is adequate, and a series resistor should always be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on the input supply to the converter an external resistor in series with a capacitor is connected Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 from the VC pin to GND to provide a pole and a zero for proper loop compensation. The typical compensation forthe RT8452 is 10k and 3.3nF. Soft-Start The soft-start of the RT8452 can be achieved by connecting a capacitor from 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 : 2.4V TSS = CSS × 6μA A typical value for the soft-start capacitor is 0.1μF. The soft-start capacitor is discharged when EN/UVLO falls below its threshold, during an over-temperature event or during an GBIAS under-voltage event. LED current Setting The LED current is programmed by placing an appropriate value current sense resistor between the ISP and ISN pins. Typically, sensing of the current should be done at the top of the LED string. The ACTL pin should be tied to a voltage higher than 1.2V 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 1.2V, the LED current is : (VACTL - 0.2) × 0.19 ILED = RSENSE Where, RSENSE is the resister between ISP and ISN. When the voltage of ACTL is higher than 1.2V, the LED current is regulated to : 190mV ILED(MAX) = RSENSE is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8452 The ACTL pin can also be used in conjunction with a thermistor to provide over temperature protection for the LED load, or with a resistor divider to VIN to reduce output power and switching current when VIN is low. The presence of a time varying differential voltage signal (ripple) across ISP and ISN at the switching frequency is expected. The amplitude of this signal is increased by high LED load current, low switching frequency and/or a smaller value output filter capacitor. The compensation capacitor on the VC pin filters the signal so the average difference between ISP and ISN is regulated on the user-programmed value. Dimming Control For LED applications where a wide dimming range is required, two competing methods are available : analog/ digital dimming and True PWM dimming. The easiest method is to simply vary the DC current through the LED by analog dimming. The other better dimming method is the True PWM dimming, which switches the LED on and off by different duty cycle to control the average LED current. The True PWM dimming offers several advantages over analog dimming and is much preferred by LED manufacturers. One advantage is the chromaticity of the LEDs remains unchanged in this scheme since the LED current is either zero or at the programmed current. Another profit of True PWM dimming over analog/digital dimming is that a wider dimming range is possible. True PWM method modulate the current source between zero and full current to achieve a precisely programmed average current. For best current accuracy, the minimum True PWM low or high time should be at least 18μS. The maximum True PWM period is determined by the system and suggested shorter than 9ms. The maximum True PWM dimming ratio (PWMRATIO) can be calculated from the maximum True PWM period (tMAX) and the minimum True PWM pulse width (tMIN) as follows : PW MRATIO = tMAX tMIN The RT8452 features both analog and digital dimming control. Analog dimming is linearly controlled by an external voltage (0.2V < VACTL < 1.2V). With an on chip output clamping amplifier and a resistor, digital dimming signal fed at DCTL pin can be easily low-pass filtered to an analog dimming signal with one external capacitor from ACTL pin to GND for noise-free digital dimming. A high contrast ratio digital dimming can also be achieved by driving ACTL pin with a digital signal from 100Hz to10kHz. Output Over Voltage Setting The RT8452 is equipped with over voltage protection (OVP) function. When the voltage at OVP pin exceeds a threshold of approximately1.18V, the power switch is turned off. The power switch can be turned on again once the voltage at OVP pin drops below 1.18V. For the Boost and Buck-Boost application, the output voltage could be clamped at a certain voltage level. The OVP voltage can be set by the following equation : ⎛ R1 ⎞ VOUT, OVP = 1.18 × ⎜1 + ⎟ ⎝ R2 ⎠ Where, R1 and R2 are the voltage divider from Vout to GND with the divider center node connected to OVP pin. ISW Sense Resistor Selection The resistor, RSW, between the source of the external N-MOSFET and GND should be selected to provide adequate switch current to drive the application without exceeding the 110mV (typical) current limit threshold on the ISW pin of RT8452. For real applications, select a resistor that gives a switch current at least 30% greater than the required LED current. For Buck application, select a resistor according to : ⎛ 0.08V ⎞ RSW, Buck = ⎜ ⎟ ⎝ IOUT ⎠ For Buck-Boost application, select a resistor according to : ⎛ ⎞ VIN × 0.08V RSW, Buck-Boost = ⎜ ⎟ ⎝ (VIN + VOUT ) × IOUT ⎠ tMAX = 9ms, tMIN = 18μs (fSW = 350kHz) For Boost application, select a resistor according to : PWMRATIO = 9ms / 12μs = 500 : 1 ⎛ VIN × 0.08V ⎞ RSW, Boost = ⎜ ⎟ ⎝ VOUT × IOUT ⎠ Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 The placement of RSW should be close to the source of the N-MOSFET and GND of the RT8452. The ISW pin input to RT8452 should be a Kelvin connection to the positive terminal of RSW. Over Temperature Protection The RT8452 has over temperature protection (OTP) function to prevent the excessive power dissipation from overheating. The OTP function will shut down switching operation when the die junction temperature exceeds 150°C. The chip will automatically start to switch again when the die junction temperature cools off. Inductor Selection The inductor used with the RT8452 should have a saturation current rating appropriate to the maximum switch current selected with the RSW resistor. Choose an inductor value based on operating frequency, input and output voltage to provide a current mode ramp on ISW pin during the switch on-time of approximately 20mV magnitude. The following equations are useful to estimate the inductor value. For Buck application : RSW × VOUT × ( VIN − VOUT ) LBuck = 0.02 × VIN × fSW For Boost application LBoost = RSW × VIN × ( VOUT − VIN ) 0.02 × VOUT × fSW For Buck-Boost application LBuck −Boost = RSW × VIN × VOUT 0.02 × ( VIN + VOUT ) × fSW Power MOSFET Selection For applications operating at high input or output voltages, the power N-MOS FET 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 RT8452 has a fixed current limit to protect the IC from excessive power dissipation at high VIN, so the N-MOSFET should be chosen so that the product of Qg at 5V and switching frequency does not exceed the GBIAS current limit. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 Schottky Diode Selection The Schottky diode, with their low forward voltage drop and fast switching speed, is necessary for the RT8452 applications. In addition, power dissipation, reverse voltage rating and pulsating peak current are the important parameters for the Schottky diode selection. Choose a suitable Schottky diode whose reverse voltage rating is greater than 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 the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. Capacitor Selection The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For most the RT8452 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 : IOUT × VOUT COUT = VIN × VRIPPLE × fSW 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 operation junction temperature. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8452 The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) − TA ) / θJA Where T J(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating conditions specification, the maximum junction temperature is 125°C. The junction to ambient thermal resistance θJA is layout dependent. For WQFN-16L 3x3 packages, the thermal resistance θJA is 68°C/W on the standard JEDEC 51-7 four layers thermal test board. For SOP-16 packages, the thermal resistance θJA is 95°C/W on the standard JEDEC 51-7 four layers thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : PD(MAX) = (125°C − 25°C) / (68°C/W) =1.471W for WQFN-16L 3x3 packages PD(MAX) = (125°C − 25°C) / (95°C/W) =1.053W for SOP-16 packages Maximum Power Dissipation (W) The maximum power dissipation depends on operating ambient temperature for fixed T J(MAX) and thermal resistance θJA. The Figure 6 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Layout Consideration PCB layout is very important to design power switching converter circuits. Some recommended layout guide lines are suggested as follows : ` 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. The PCB trace between power components must be as short and wide as possible due to large current flow through these traces during operation. ` The input capacitors CVCC must be placed as close to VCC pin as possible. ` Place the compensation components to VC pin as close as possible to avoid noise pick up. ` Connect GND pin ane Exposed Pad to a large ground plane for maximum power dissipation and noise reduction. Place these components as close as possible D1 VIN CIN M1 COUT RSW GND RSENS Four-Layer PCB L1 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 OVP EN NC SS DCTL ACTL Locate input capacitor as close VCC as possible CVCC CSS RVC CVC WQFN-16L 3x3 GND Locate the compensation components to VC pin as close as possible SOP-16 0 25 50 Figure 8. PCB Layout Guide 75 100 125 Ambient Temperature (°C) Figure 7. Derating Curve of Maximum Power Dissipation Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8452-03 May 2012 RT8452 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 Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8452-03 May 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8452 D SEE DETAIL A D2 L 1 E E2 e 1 2 DETAIL A Pin #1 ID and Tie Bar Mark Options b A A1 1 2 Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. A3 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 0.180 0.300 0.007 0.012 D 2.950 3.050 0.116 0.120 D2 1.300 1.750 0.051 0.069 E 2.950 3.050 0.116 0.120 E2 1.300 1.750 0.051 0.069 e L 0.500 0.350 0.020 0.450 0.014 0.018 W-Type 16L QFN 3x3 Package Richtek Technology Corporation 5F, No. 20, Taiyuen 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. www.richtek.com 16 DS8452-03 May 2012