® RT8280 3A, 24V, 3MHz Step-Down Converter General Description The RT8280 is a high voltage buck converter that can support an input voltage range from 4.5V to 24V with output current up to 3A. Current mode operation provides fast transient response and eases loop stabilization. The chip provides protection functions such as cycle-bycycle current limiting and thermal shutdown protection. In shutdown mode, the regulator only draws 25μA of supply current. The RT8280 is available in a SOP-8 (Exposed Pad) package. Features Wide Operating Input Range : 4.5V to 24V Adjustable Output Voltage Range : 0.8V to 15V Output Current up to 3A 25μ μA Low Shutdown Current High Efficiency up to 90% at 2.2MHz Programmable Frequency : 220kHz to 3MHz Internal Soft-Start Stable with Low ESR Output Ceramic Capacitors Thermal Shutdown Protection Cycle-By-Cycle Over Current Protection RoHS Compliant and Halogen Free Ordering Information RT8280 Applications Package Type SP : SOP-8 (Exposed Pad-Option 1) Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : ` DSL Modem for ADSL2+ Standard Distributed Power Systems Pre-Regulator for Linear Regulators Pin Configurations (TOP VIEW) 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 RT8280GSP : Product Number RT8280 GSPYMDNN YMDNN : Date Code Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 BOOT VIN 2 SW GND 3 4 GND 9 8 RT 7 EN 6 COMP 5 FB SOP-8 (Exposed Pad) is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8280 Typical Application Circuit 2 VIN 4.5V to 24V CIN 10µF Chip Enable VIN BOOT RT8280 7 EN 1 SW 3 CBOOT L 10nF 2.2µH B330A R1 31.6k 8 RT RRT 24k 4, 9 (Exposed Pad) VOUT 3.3V/3A FB 5 GND COMP 6 CC 1.8nF RC 24k COUT 22µF R2 10k NC CP VOUT (V) 10 8 5 3.3 2.5 1.8 1.5 1.2 Table 1. Recommended Component Selection for fSW = 2.2MHz R1 (kΩ) R2 (kΩ) RC (kΩ) C C (nF) L (μH) 115 10 68 0.82 8.2 91 10 51 1 6.8 52.3 10 36 1.2 4.7 31.6 10 24 1.8 2.2 21.5 10 18 2.2 2 12.4 10 13 2.2 1.5 8.87 10 12 2.2 1.5 4.99 10 9 1.8 1 COUT (μF) 22 22 22 22 22 22 22 22 Functional Pin Description Pin No. Pin Name Pin Function 1 BOOT Bootstrap Power. BOOT supplies the drive for the high side N-MOSFET switch. Connect a 10nF or greater capacitor from SW to BOOT to power the high side switch. 2 VIN Supply Input. VIN supplies the power to the IC, as well as the step-down converter switches. Drive VIN with a 4.5V to 24V power source. Bypass VIN to GND with a suitably large capacitor to eliminate noise on the input to the IC. 3 SW Switch Node. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BOOT to power the high side switch. 4, 9 (Exposed Pad) GND 5 FB 6 COMP 7 EN 8 RT Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation. Feedback Input. FB senses the output voltage to regulate. Drive FB with a resistive voltage divider from the output voltage. Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND to compensate the regulation control loop. In some cases, an additional capacitor from COMP to GND is required. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN higher than 1.4V to turn on the regulator, lower than 0.4V to turn off. For automatic startup, leave EN unconnected. Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the switching frequency. Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Function Block Diagram VIN Internal Regulator 1µA Oscillator - VA VCC EN 10k Foldback Control 0.4V - 0.8V + - EA VA BOOT + UV Comparator + Shutdown Comparator 1V 3V Current Sense Slope Comp Amplifier + S Q R Q SW + Current Comparator + GND 0.8V - FB COMP RT Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8280 Absolute Maximum Ratings (Note 1) VIN ---------------------------------------------------------------------------------------------------------------- −0.3V to 26V SW --------------------------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) BOOT ----------------------------------------------------------------------------------------------------------- (SW − 0.3V) to (SW + 6V) All Other Pins ------------------------------------------------------------------------------------------------- −0.3V to 6V Power Dissipation, PD @ TA = 25°C SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------Package Thermal Resistance (Note 2) SOP-8 (Exposed Pad) , θJA -------------------------------------------------------------------------------SOP-8 (Exposed Pad) , θJC ------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------Storage Temperature Range ------------------------------------------------------------------------------ESD Susceptibility (Note 3) HBM (Human Body Mode) --------------------------------------------------------------------------------MM (Machine Mode) ----------------------------------------------------------------------------------------- Recommended Operating Conditions 1.333W 75°C/W 15°C/W 260°C 150°C −65°C to 150°C 2kV 200V (Note 4) Supply Input Voltage, VIN ----------------------------------------------------------------------------------- 4.5V to 24V Junction Temperature Range ------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range ------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VIN = 12V, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions 4.5V ≤ VIN ≤ 24V Min Typ Max Unit 0.784 0.8 0.816 V Feedback Reference Voltage VFB Upper Switch On Resistance RDS(ON)1 -- 0.11 -- Ω Lower Switch On Resistance RDS(ON)2 -- 10 -- Ω Upper Switch Leakage ILEAK -- 0 10 μA Current Limit ILIM -- 5 -- A -- 3.8 -- A/V Current Sense Transconductance Output Current to Comp Pin Voltage Error Amplifier Transconductance Oscillator Frequency VEN = 0V, VSW = 0V Duty = 90%, VBOOT − VSW = 4.8V gCS gEA ΔIC = ±10μA -- 920 -- μA/V fSW RRT = 24kΩ -- 2.2 -- MHz VFB = 0V, RRT = 24kΩ -- 230 -- kHz VUVLO -- 4.2 -- V ΔVUVLO -- 430 -- mV -- 65 -- % -- 70 -- ns Short Circuit Frequency Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis Maximum Duty Cycle DMAX Minimum On Time tON VFB = 0.7V, RRT = 24kΩ Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Parameter EN Threshold Voltage Symbol Test Conditions Min Typ Max Unit Logic-High VIH 1.4 -- 5.5 Logic-Low VIL -- -- 0.4 -- 1 -- μA Enable Pull Up Current V Quiescent Current IQ VEN = 2V, VFB = 1V -- 0.8 1 mA Shutdown Current ISHDN VEN = 0V -- 25 -- μA -- 150 -- °C Thermal Shutdown 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. Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8280 Typical Operating Characteristics Efficiency vs. Output Current 100 90 90 80 80 70 70 Efficiency (%) Efficiency (%) Efficiency vs. Output Current 100 60 50 40 30 20 VIN = 12V VIN = 19V 60 50 40 30 20 10 VIN = 12V, VOUT = 3.3V, fSW = 2.2MHz 10 0 0.0 0.5 1.0 1.5 2.0 2.5 VOUT = 5V, fSW = 2.2MHz 0 3.0 0.0 0.5 1.0 Output Current (A) 1.5 2.0 2.5 3.0 Output Current (A) Output Voltage vs. Output Current Output Voltage vs. Input Voltage 5.30 3.35 5.25 3.34 5.15 Output Voltage (V) Output Voltage (V) 5.20 5.10 VIN = 19V 5.05 5.00 VIN = 12V 4.95 4.90 4.85 3.33 3.32 3.31 3.30 4.80 IOUT = 0A 4.75 3.29 4.70 0.0 0.5 1.0 1.5 2.0 2.5 3 3.0 6 9 15 18 21 24 Input Voltage (V) Output Current (A) Reference Voltage vs. Temperature Quiescent Current vs. Temperature 0.810 0.90 0.805 0.85 Quiescent Current (mA) Reference Voltage (V) 12 0.800 0.795 0.790 0.785 0.80 0.75 0.70 0.65 VIN = 12V, IOUT = 0A VIN = 12V, RRT = Open 0.60 0.780 -50 -25 0 25 50 75 100 Temperature (°C) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 125 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Current Limit vs. Temperature Current Limit vs. Duty Cycle 6.0 6.50 6.25 5.5 5.75 Peak Current (A) Peak Current (A) 6.00 fSW = 2.2MHz 5.50 5.25 fSW = 400kHz 5.00 4.75 4.50 4.25 5.0 4.5 4.0 3.5 4.00 3.75 VIN = 12V, VOUT = 3.3V, fSW = 2.2MHz 3.0 3.50 0 20 40 60 80 -50 100 -25 0 25 50 75 Duty Cycle (%) Temperature (°C) Switching Frequency vs. Temperature Output Ripple Voltage 100 125 Switching Freq. (MHz)1 2.4 V OUT (10mV/Div) 2.3 2.2 VSW (5V/Div) 2.1 2.0 1.9 VIN = 12V, VOUT = 3.3V, RRT = 24kΩ IL (2A/Div) 1.8 -50 -25 0 25 50 75 100 125 VIN = 12V, VOUT = 3.3V lOUT = 3A, fSW = 2.2MHz Time (200ns/Div) Temperature (°C) Load Transient Response VIN = 12V, VOUT = 3.3V lOUT = 0A to 3A, fSW = 2.2MHz Load Transient Response VOUT (200mV/Div) VOUT (200mV/Div) IOUT (1A/Div) IOUT (1A/Div) Time (100μs/Div) Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 VIN = 12V, VOUT = 3.3V lOUT = 1.5A to 3A, fSW = 2.2MHz Time (100μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8280 Power On from EN Power off from EN VIN = 12V, VOUT = 3.3V lOUT = 3A, fS = 2.2MHz VEN (2V/Div) VEN (2V/Div) VOUT (1V/Div) VOUT (1V/Div) IL (2V/Div) VIN = 12V, VOUT = 3.3V lOUT = 3A, fSW = 2.2MHz Time (400μs/Div) Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 IL (2V/Div) Time (40μs/Div) is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Application Information The RT8280 is an asynchronous high voltage buck converter that supports an input voltage range from 4.5V to 24V with output current up to 3A. Output Voltage Setting The resistive voltage divider allows the FB pin to sense the output voltage as shown in Figure 1. VOUT R1 FB RT8280 R2 GND Figure 1. Output Voltage Setting The output voltage is set by an external resistive voltage divider according to the following equation : VOUT = VFB ⎛⎜ 1+ R1 ⎞⎟ ⎝ R2 ⎠ where VFB is the feedback reference voltage (0.8V typ.). External Bootstrap Diode Connect a 10nF low ESR ceramic capacitor between the BOOT pin and SW pin. This capacitor provides the gate driver voltage for the high side MOSFET. It is recommended to add an external bootstrap diode between an external 5V voltage source and the BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty cycle is higher than 65% .The bootstrap diode can be a low cost one such as IN4148 or BAT54. The external voltage source must be fixed at 5V and can be provided from the system or the output of the RT8280. Note that the external boot voltage must be lower than 5.5V. 5V BOOT RT8280 10nF Operating Frequency Selection of the operating frequency is a trade off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequency improves efficiency by reducing internal gate charge and switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The operating frequency of the RT8280 is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator. Selection of the RT resistor value can be determined by examining the curve below in Figure3. Although frequencies as high as 3MHz are available, the minimum on-time of the RT8280 imposes a limit on the operating duty cycle. Figure 4 shows the examples of minimum on-time constraint for output voltages 3.3V and 1.8V. It is recommended to operate the RT8280 in the region under the corresponding Vout curve. Except the minimum on-time constraint, the limit of maximum duty also needs to be considered. In ideal case, the duty cycle of the RT8280 can be calculated by below equation, But in practical case it will be higher than the calculation result since all the components in a converter circuit are not ideal. Figure 5 shows an example for the limit of maximum duty. With 5V input voltage, the 3.3V output voltage of the RT8280 becomes out of regulation when the output current is increased. However, when the input voltage is changed to 12V, the 3.3V output voltage of the RT8280 remains in regulation even with 3A output current. According to equation below, the duty cycle is 0.67 for the RT8280 operated with 5V input voltage and 3.3V output voltage in 2.2MHz switching frequency. The ideal case duty cycle calculation is already over the limit of maximum duty (65%). Thus, it is obvious that the RT8280 can't support 3A output current in such conditions : Duty Cycle = 1 − 0.15 x fSW (MHz) SW Figure 2. External Bootstrap Diode Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8280 Switching Frequency vs. RRT Switching Frequency (kHz)1 3000 Chip Enable Operation 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0 0 200 400 600 800 1000 RRRT (kΩ) RT (kΩ) Figure 3. Switching Frequency vs. RRT Minimum On-Time Constraint 30.0 27.5 Input Voltage (V) 25.0 22.5 Inductor Selection 20.0 The inductor value and operating frequency determine the ripple current according to a specific input and output voltage. The ripple current ΔIL increases with higher VIN and decreases with higher inductance : VOUT = 3.3V 17.5 15.0 12.5 VOUT = 1.8V 10.0 7.5 V V ΔIL = ⎡⎢ OUT ⎤⎥ x ⎡⎢1− OUT ⎤⎥ f x L VIN ⎦ ⎣ SW ⎦ ⎣ 5.0 2.5 0.0 1000 1400 1800 2200 2600 3000 Switching Frequency (kHz) Figure 4. Minimum On-Time Constraint to Input Voltage Output Voltage vs. Load Current 3.5 3.4 3.3 Output Voltage (V) The EN pin is the enable input. Pull the EN pin low (<0.4V) to shutdown the device. During shutdown mode, the RT8280 quiescent current drops to lower than 25μA. Drive the EN pin high (>1.4V, < 5.5V), to turn on the device. If the EN pin is open, it will be pulled high by the internal circuit. For external timing control (e.g.RC), the EN pin can also be externally pulled high by adding a 100kΩ or greater resistor from the VIN pin (see Figure 6). In some cases, the output voltage of the RT8280 may still be under UVP threshold when soft-start finishes. Then the RT8280 will restart again and the output voltage of the RT8280 will rise to the regulation voltage. This phenomenon often happens in high frequency operation and with slow rising input voltage. It can easily be solved by adding a voltage divider on the EN pin. The RT8280 will be enabled when the input voltage rises close to the nominal input voltage. VIN = 12V 3.2 3.1 3.0 ⎡ ⎤ ⎡ VOUT VOUT ⎤ L =⎢ x ⎢1 − ⎥ ⎥ ⎣ fSW x ΔIL(MAX) ⎦ ⎣ VIN(MAX) ⎦ 2.9 VIN = 5V 2.8 2.7 2.6 2.5 0.0 0.5 1.0 1.5 2.0 2.5 Load Current (A) Figure 5. Limit of Maximum Duty Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. Higher frequency combined with smaller ripple current is necessary to achieve high efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, setting the maximum value of the ripple current ΔIL = 0.24(IMAX) is a reasonable starting point. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : 3.0 The inductor's current rating (defined by that which causes a temperature rise from 25°C ambient to 40°C) should be greater than the maximum load current and its saturation current should be greater than the short circuit peak current limit. Refer to Table 2 for the suggested inductor selection. is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Table2. Suggested Inductors for Typical Application Circuit Component Supplier TDK TDK TAIYO YUDEN Series Dimensions (mm) VLC6045 SLF12565 6 x 6 x 4.5 12.5 x 12.5 x 6.5 NR8040 8x8x4 When the power switch turns off, the path for the current is through the diode connected between the switch output and ground. This forward biased diode must have a minimal voltage drop and recovery time. Schottky diodes are recommended and should be able to handle those current. The reverse voltage rating of the diode should be greater than the maximum input voltage, and the current rating should be greater than the maximum load current. For details, please refer to Table 3. Table 3. Suggested Diode Component Series VRRM (V) IOUT (A) Supplier Package DIODES B330A 30 3 SMA DIODES B340A 40 3 SMA PANJIT SK33 30 3 DO-214AB PANJIT SK34 40 3 DO-214AB CIN and COUT Selection The input capacitance, C IN, is needed to filter the trapezoidal current at the source of the high side MOSFET. To prevent large ripple current, a low ESR input capacitor sized for the maximum RMS current should be used. The RMS current is given by : VIN −1 VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = I OUT / 2. This simple worst-case condition is commonly used for design. Choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. For the input capacitor, one 10μF low ESR ceramic capacitors is recommended. For the recommended Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 The selection of COUT is determined by the required ESR to minimize voltage ripple. Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT , is determined by : Diode Selection V IRMS = IOUT(MAX) OUT VIN capacitor, please refer to Table 4 below for more details. 1 ⎤ ΔVOUT ≤ ΔIL ⎡⎢ESR + 8fCOUT ⎥⎦ ⎣ The output ripple will be highest at the maximum input voltage since ΔIL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirement. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR value. However, it provides lower capacitance density than other types. Although Tantalum capacitors have the highest capacitance density, it is important to only use types that pass the surge test for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR. However, it can be used in cost sensitive applications for ripple current rating and long term reliability considerations. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. Nevertheless, high value low cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8280 Checking Transient Response Choose a capacitor that is greater than the above calculation result. The frequency of the zero, which consists of RC and CC, should be lower than one fourth of fC to get a sufficient phase margin. If the zero moves close to fC, the phase margin decreases. The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD (ESR) and COUT also begins to be charged or discharged generating a feedback error signal for the regulator to return VOUT to its steady state value. During this recovery time, VOUT can be monitored for overshoot or ringing to indicate any stability problem. In some applications, the output capacitor will be an electrolytic capacitor, not a ceramic capacitor. A zero will be produced by the electrolytic capacitor and its ESR. CP can be used to produce a pole with RC to cancel the zero. To calculate CP, follow the equation below : Compensation Parameters CP = The switching frequency of the RT8280 can be programmed from free running frequency to 3MHz. Table 1 only lists the recommended compensation parameters for 2.2MHz switching frequency. Optimized compensation parameters for other switching frequency can also be determined through below procedures. The first step is to decide the crossover frequency, fc. In general, the crossover frequency is one tenth of the switching frequency. Then, Rc can be obtained through the following equation : 2π × COUT × fC × VOUT RC = gCS × gEA × VFB EMI Consideration Since parasitic inductance and capacitance effects in PCB circuitry would cause a spike voltage on the SW pin when the high side MOSFET is turned-on/off, this spike voltage on SW may impact EMI performance in the system. In order to enhance EMI performance, there are two methods to suppress the spike voltage. One is to place an R-C snubber between SW and GND and place them as close as possible to the SW pin (see Figure 6). Another method is to add a resistor in series with the bootstrap capacitor, CBOOT. But this method will decrease the driving capability to the high side MOSFET. It is strongly recommended to reserve the R-C snubber during PCB layout for EMI improvement. Moreover, reducing the SW trace area and keeping the main power in a small loop will be helpful for EMI performance. For detailed PCB layout guide, please refer to the section on Layout Consideration. where gCS is Current SenseTransconductance = 1.8 (A/V) gEA is Error Amplifier Tansconductance = 920 (μA/V) Once the value of Rc has been determined, the value of Cc can be obtained by the following equation : 1 CC = f 2π × RC × C 4 2 VIN 4.5V to 24V CIN 10µF REN* Chip Enable VIN BOOT RT8280 7 EN 1 8 RT 4, 9 (Exposed Pad) GND * : Optional RBOOT* CBOOT L 10nF 2.2µH SW 3 RS* CEN* RRT 24k COUT × ESR RC B330A R1 31.6k CS* FB 5 COMP 6 VOUT 3.3V/3A CC 1.8nF RC 24k COUT 22µF R2 10k NC CP Figure 6. Reference Circuit with Snubber and Enable Timing Control Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient designer to see the effect of rising ambient temperature on the maximum power dissipation. (a) Copper Area = (2.3 x 2.3) mm2, θJA = 75°C/W thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP-8 (Exposed Pad) packages, the thermal resistance, θJA, is 75°C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA=25°C can be calculated by the following formulas : (b) Copper Area = 10mm2, θJA = 64°C/W P D(MAX) = (125°C − 25°C) / (75°C/W) = 1.333W (min. copper area PCB layout) P D(MAX) = (125°C − 25°C) / (49°C/W) = 2.04W (70mm2 copper area PCB layout) The thermal resistance, θJA, of SOP-8 (Exposed Pad) is determined by the package architectural design and the PCB layout design. The package architectural design is fixed. However, it's possible to increase thermal performance via better PCB layout copper design. The thermal resistance, θJA, can be decreased by adding copper area under the exposed pad of the SOP-8 (Exposed Pad) package. As shown in Figure 7, the amount of copper area to which the SOP-8 (Exposed Pad) is mounted on affects thermal performance. When mounted to the standard SOP-8 (Exposed Pad) (Figure 7a), θJA is 75°C/W. Adding copper area under the SOP-8 (Exposed Pad) (Figure 7b) reduces θJA to 64°C/W. Further increasing the copper area to 70mm2 (Figure 7e) will reduce θJA to 49°C/W. The maximum power dissipation depends on operating ambient temperature for fixed T J (MAX) and thermal resistance, θJA. The derating curves in Figure 8 allow the Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 (c) Copper Area = 30mm2 , θJA = 54°C/W (d) Copper Area = 50mm2 , θJA = 51°C/W (e) Copper Area = 70mm2 , θJA = 49°C/W Figure 7. Themal Resistance vs. Copper Area Layout Design is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8280 2.2 Four Layer PCB Power Dissipation (W) 2.0 1.8 Copper Area 70mm2 50mm2 30mm2 10mm2 Min.Layout 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 8. Derating Curves for RT8280 Package Layout Consideration ` Follow the PCB layout guidelines for optimum performance of the RT8280. ` ` Keep the traces of the main current paths as short and wide as possible. Place the input capacitor as close as possible to the device pins (VIN and GND). ` SW node experiences high frequency voltage swing and Connect the feedback network behind the output capacitors. Keep the loop area small. Place the feedback components near the RT8280. ` Connect all analog grounds to a common node and then connect the common node to ground behind the output capacitors. ` An example of the PCB layout guide is shown in Figure 9 for reference. should be kept in a small area. Keep analog components away from the SW node to prevent stray capacitive noise pick up. VIN GND Input capacitor must be placed as close to the IC as possible. RRT CIN BOOT D CS COUT The feedback components must be connected as close to the device as possible. SW GND RS 8 VIN 2 SW GND 3 4 GND 9 CC RT 7 EN 6 COMP 5 FB CP R1 R2 L VOUT RC VOUT GND SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. Figure 9. PCB Layout Guide Copyright © 2012 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8280-02 March 2012 RT8280 Table 4. Suggested Capacitors for CIN and COUT Location Component Supplier Part No. Capacitance (μF) Case Size CIN MURATA GRM31CR61E106K 10 1206 CIN TDK C3225X5R1E106K 10 1206 CIN TAIYO YUDEN TMK316BJ106ML 10 1206 COUT MURATA GRM31CR60J476M 47 1206 COUT TDK C3225X5R0J476M 47 1210 COUT MURATA GRM32ER71C226M 22 1210 COUT TDK C3225X5R1C226M 22 1210 Copyright © 2012 Richtek Technology Corporation. All rights reserved. DS8280-02 March 2012 is a registered trademark of Richtek Technology Corporation. www.richtek.com 15 RT8280 Outline Dimension H A M EXPOSED THERMAL PAD (Bottom of Package) Y J X B F C I D Dimensions In Millimeters Symbol Dimensions In Inches Min Max Min Max A 4.801 5.004 0.189 0.197 B 3.810 4.000 0.150 0.157 C 1.346 1.753 0.053 0.069 D 0.330 0.510 0.013 0.020 F 1.194 1.346 0.047 0.053 H 0.170 0.254 0.007 0.010 I 0.000 0.152 0.000 0.006 J 5.791 6.200 0.228 0.244 M 0.406 1.270 0.016 0.050 X 2.000 2.300 0.079 0.091 Y 2.000 2.300 0.079 0.091 X 2.100 2.500 0.083 0.098 Y 3.000 3.500 0.118 0.138 Option 1 Option 2 8-Lead SOP (Exposed Pad) Plastic 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 DS8280-02 March 2012