RT8265 3A, 24V, 1MHz Asynchronous Step-Down Converter General Description Features The RT8265 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75V to 24V and the output current can be up to 3A. The IC provides a Feed-forward Voltage Mode operation to achieve the goal of fast line transient response. The RT8265 also provides adjustable soft-start to be a flexible solution for customers. z Wide Operating Input Range : 4.75V to 24V z Adjustable Output Voltage Range : 0.8V to 15V Output Current up to 3A 25μ μA Low Shutdown Current Power MOSFET : 110mΩ Ω z z z z z The chip provides protection functions such as cycle-bycycle current limiting and thermal shutdown protection. In shutdown mode, the regulator draws 25μA of supply current. The RT8265 is available in a SOP-8 (Exposed Pad) surface mount package. z z z z z Ordering Information High Efficiency up to 93% 1MHz Fixed Switching Frequency Stable with Low ESR Output Ceramic Capacitors Programmable Soft-Start Thermal Shutdown Cycle-By-Cycle Over Current Protection RoHS Compliant and 100% Lead (Pb)-Free Applications RT8265 z Package Type SP : SOP-8 (Exposed Pad-Option 1) z z Lead Plating System P : Pb Free G : Green (Halogen Free and Pb Free) Distributed Power Systems Battery Charger Pre-Regulator for Linear Regulators Pin Configurations (TOP VIEW) Note : Richtek products are : ` BOOT RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. ` Suitable for use in SnPb or Pb-free soldering processes. VIN 2 SW 3 GND 8 SS 7 EN GND 6 9 4 5 COMP FB SOP-8 (Exposed Pad) Typical Application Circuit VIN 4.75V to 24V Chip Enable 2 VIN C1 10µF/25V BOOT 1 RT8265 7 EN 8 SS CS 4, 0.1µF 9 (Exposed Pad) GND SW 3 CB 10nF L1 4.7µH D1 B330A R1 49.9k FB 5 COMP 6 CC RC 0.56nF 82k R2 16k VOUT 3.3V C2 47µF 6.3V CP NC DS8265-02 March 2011 www.richtek.com 1 RT8265 Table 1. Recommended Component Selection VOUT R1 (kΩ) R2(kΩ) R C (kΩ) CC (nF) Cp (pF) L1 (μH) 15V 280 16 360 0.1 Open 4.7 10V 180 16 240 0.15 Open 4.7 8V 150 16 200 0.22 Open 4.7 5V 84.5 16 140 0.35 Open 4.7 3.3V 49.9 16 82 0.56 Open 4.7 2.5V 33 16 68 0.68 Open 4.7 1.8V 20 16 49.9 1 Open 4.7 1.2V 8.06 16 24 1.5 33 4.7 1.1V 6.04 16 24 1.5 33 4.7 Functional Pin Description Pin No. Pin Name 1 BOOT 2 VIN 3 SW 4, GND 9 (Exposed Pad) 5 FB 6 COMP 7 EN 8 SS www.richtek.com 2 Pin Function High Side Gate Drive Boost Input. BOOT supplies the drive for the high side N-MOSFET switch. Connect a 10nF or greater capacitor from SW to BS to power the high side switch. Power Input. VIN supplies the power to the IC, as well as the step-down converter switches. Bypass VIN to GND with a suitable large capacitor to eliminate noise on the input to the IC. Power Switching Output. 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 BS to power the high side switch. 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 said voltage. The feedback reference voltage is 0.8V typically. 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 it off. If the EN pin is open, it will be pulled to high by internal circuit. Soft-Start Control Input. SS controls the soft start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.1μF capacitor sets the soft-start period to 10ms. DS8265-02 March 2011 RT8265 Function Block Diagram VIN Internal Regulator 1µA EN Current Sense Amplifier Comparator + + OC Limit VIN_track VA VCC 10k Oscillator 1MHz/150kHz 1V 3V + Shutdown Comparator 0.64V BOOT S Q R Q SW + UV Comparator VCC VA + - Control Logic GND 15µA 0.8V SS FB DS8265-02 March 2011 + +EA - COMP www.richtek.com 3 RT8265 Absolute Maximum Ratings z z z z z z z z z z (Note 1) Supply Voltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 26V Input Voltage, SW ------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) BOOT Voltage ------------------------------------------------------------------------------------------------ (VSW − 0.3V) to (VSW + 6V) All Other Voltage --------------------------------------------------------------------------------------------- −0.3V to 6V Power Dissipation, PD @ TA = 25°C SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------- 1.333W Package Thermal Resistance (Note 2) SOP-8 (Exposed Pad), θJA -------------------------------------------------------------------------------- 75°C/W Junction Temperature --------------------------------------------------------------------------------------- 150°C Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------- 260°C Storage Temperature Range ------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Mode) --------------------------------------------------------------------------------- 2kV MM (Machine Mode) ---------------------------------------------------------------------------------------- 200V Recommended Operating Conditions z z z z (Note 4) Supply Voltage, VIN ----------------------------------------------------------------------------------------- 4.75V to 24V Enable Voltage, VEN ----------------------------------------------------------------------------------------- 0V to 5.5V 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 4.75V ≤ V IN ≤ 24V Min Typ Max Unit 0.784 0.8 0.816 V Feedback Reference Voltage VFB High Side Switch-On Resistance RDS(ON)1 -- 0.11 -- Ω Low Side Switch-On Resistance Switch Leakage RDS(ON)2 --- 10 -- -10 Ω μA Current Limit ILIM 3.8 5 -- A Oscillator Frequency fSW -- 1 -- MHz -- 100 -- kHz -- 85 -- % -- 100 -- ns 3.8 4.2 4.65 V Under Voltage Lockout Threshold Hysteresis -- 200 -- mV En input Low Voltage -- -- 0.4 V En input High Voltage 1.4 -- -- V -- 1 3 μA V EN = 0V, V SW = 0V Short Circuit Oscillation Frequency Maximum Duty Cycle Minimum On-Time Under Voltage Lockout Threshold tON Rising Enable Pull Up Current Shutdown Current ISHDN V EN = 0V -- 25 -- μA Quiescent Current Soft-Start Period IQ V EN = 2V, VFB = 1V C SS = 0.1μF --- 0.8 10 1 -- mA ms Thermal Shutdown TSD -- 150 -- °C www.richtek.com 4 DS8265-02 March 2011 RT8265 Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for stress ratings. 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 for extended periods may remain possibility to affect device reliability. Note 2. θJA is measured in the natural convection at TA = 25°C on a high effective four layers thermal conductivity test board of JEDEC 51-7 thermal measurement standard. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. DS8265-02 March 2011 www.richtek.com 5 RT8265 Typical Operating Characteristics Efficiency Load Current Current Efficiency vs. Load Efficiency Load Current Current Efficiency vs. Load 100 100 95 95 90 Efficiency (%) Efficiency (%) 85 80 VIN = 12V 90 VIN = 12V VIN = 24V 75 70 65 60 85 VIN = 24V 80 75 70 65 60 55 55 VOUT = 5V VOUT = 3.3V 50 50 0 0.6 1.2 1.8 2.4 3 0 0.6 1.2 Load Current (A) 1.8 2.4 3 Load Current (A) Output Voltage vs. Temperature Reference Voltage vs. Input Voltage 0.820 3.40 0.810 3.34 Output Voltage (V) Reference Voltage (V) 0.815 0.805 0.800 0.795 0.790 0.785 0.780 3.28 3.22 3.16 0.775 VOUT = 3.3V, IOUT = 0A VIN = 12V, VOUT = 3.3V, IOUT = 0A 0.770 3.10 4 6.5 9 11.5 14 16.5 19 21.5 24 -50 -25 0 Input Voltage (V) 25 50 75 100 125 Temperature (°C) Shutdown Current vs. Temperature Output Voltage vs. Output Current 50 3.40 45 3.30 Shutdown Current (µA)1 Output Voltage (V) 3.35 VIN = 24V VIN = 12V 3.25 3.20 3.15 40 35 30 25 20 15 10 5 VIN = 12V, VOUT = 3.3V VOUT = 3.3V 0 3.10 0 0.5 1 1.5 2 Output Current (A) www.richtek.com 6 2.5 3 -50 -25 0 25 50 75 100 125 Temperature (°C) DS8265-02 March 2011 RT8265 Frequency vs. Temperature Frequency vs. Input Voltage 1.00 1.00 0.96 Frequency (MHz)1 Frequency (MHz) 0.98 0.96 0.94 0.92 VIN = 24V 0.92 VIN = 12V 0.88 0.84 0.90 VOUT = 3.3V, IOUT = 0.3A VOUT = 3.3V, IOUT = 0.3A 0.80 0.88 4 6.5 9 11.5 14 16.5 19 21.5 -50 24 -25 0 50 75 100 125 Temperature (°C) Input Voltage (V) Current Limit vs. Temperature Current Limit vs. Input Voltage 6.00 6.0 5.50 5.5 Current Limit (A) Current Limit (A) 25 5.00 4.50 4.00 5.0 VIN = 12V 4.5 VIN = 24V 4.0 3.5 3.50 Peak Current, VOUT = 3.3V Peak Current, VOUT = 3.3V 3.0 3.00 4 6.5 9 11.5 14 16.5 19 21.5 -50 24 -25 0 25 50 75 100 Input Voltage (V) Temperature (°C) Quiescent Current vs. Temperature Load Transient Response 125 Quiescent Current (mA) 1.0 0.9 VIN = 12V VIN = 24V 0.8 VOUT (50mV/Div) 0.7 VIN = 5.5V 0.6 0.5 IOUT (2A/Div) 0.4 VOUT = 3.3V VIN = 12V, IOUT = 1.5A to 3A 0.3 -50 -25 0 25 50 75 100 125 Time (50μs/Div) Temperature (°C) DS8265-02 March 2011 www.richtek.com 7 RT8265 Load Transient Response Switching VLX (10V/Div) VOUT (50mV/Div) VOUT (10mV/Div) IOUT (2A/Div) VIN = 24V, IOUT = 1.5A to 3A ILX (2A/Div) VIN = 12V, IOUT = 3A Time (50μs/Div) Time (500ns/Div) Power On from EN Power Off from EN VIN = 12V, IOUT = 3A VEN (5V/Div) VEN (5V/Div) VOUT (2V/Div) VOUT (2V/Div) I IN (500mA/Div) I IN (500mA/Div) VIN = 12V, IOUT = 3A Time (2.5ms/Div) www.richtek.com 8 Time (250μs/Div) DS8265-02 March 2011 RT8265 Application Information The RT8265 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75V to 24V and the output current can be up to 3A. Output Voltage Setting The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1. Soft-Start The RT8265 contains an external soft-start clamp that gradually raises the output voltage. The soft-start timming can be programed by the external capacitor between SS pin and GND. The chip provides a 15μA charge current for the external capacitor. If 0.1μF capacitor is used to set the soft-start and it's period will be 10ms(typ.). VOUT Inductor Selection R1 FB RT8265 R2 GND Figure 1. Output Voltage Setting The output voltage is set by an external resistive 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 and BOOT pin for efficiency improvement. The external 5V can be a 5V fixed input from system or a 5V output of the RT8265. The bootstrap diode can be a low cost one such as IN4148 or BAT54. This diode is also recommended for high duty cycle V operation (when OUT > 65%) applications. VIN 5V BOOT RT8265 SW Figure 2 DS8265-02 March 2011 10nF 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. V V ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥ VIN ⎦ ⎣ f ×L ⎦ ⎣ Having a lower ripple current reduces not only the ESR losses in the output capacitors but also the output voltage ripple. High frequency with small ripple current can achieve highest efficiency operation. However, it requires a large inductor to achieve this goal. For the ripple current selection, the value of ΔIL = 0.4(IMAX) will be 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 : ⎡ VOUT ⎤ ⎡ VOUT ⎤ L =⎢ × ⎢1− ⎥ ⎥ f I V × Δ L(MAX) ⎦ ⎣ IN(MAX) ⎦ ⎣ Inductor Core Selection The inductor type must be selected once the value for L is known. Generally speaking, high efficiency converters can not afford the core loss found in low cost powdered iron cores. So, the more expensive ferrite or mollypermalloy cores will be a better choice. The selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. As the inductance increases, core losses decrease. Unfortunately, increase of the inductance requires more turns of wire and therefore the copper losses will increase. Ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. So, design goals can focus on the reduction of copper loss and the saturation prevention. www.richtek.com 9 RT8265 Ferrite core material saturates “hard”, which means that inductance collapses abruptly when the peak design current is exceeded. The previous situation results in an abrupt increase in inductor ripple current and consequent output voltage ripple. refer to table 2 for more detail. The input capacitor has to connect another 10μF ceramic capacitor between the input and ground when the input voltage is lower than 6.5V. Do not allow the core to saturate! Moreover, the amount of bulk capacitance is also a key for COUT selection to ensure that the control loop is stable. Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy. However, they are usually more expensive than the similar powdered iron inductors. The rule for inductor choice mainly depends on the price vs. size requirement and any radiated field/EMI requirements. Diode Selection 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 minimum voltage drop and recovery times. Schottky diode is recommended and it should be able to handle those current. The reverse voltage rating of the diode should be greater than the maximum input voltage, and current rating should be greater than the maximum load current. For more detail please refer to Table 3. CIN and COUT Selection The input capacitance, CIN, 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 : IRMS = IOUT(MAX) VOUT VIN VIN −1 VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. 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, a 10μF low ESR ceramic capacitor is recommended. For the recommended capacitor, please www.richtek.com 10 The selection of COUT is determined by the required ESR to minimize voltage ripple. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, ΔVOUT , is determined by : 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. Higher values, lower 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 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. DS8265-02 March 2011 RT8265 Checking Transient Response 1.6 Maximum Power Dissipation (W) 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) also begins to charge or discharge COUT 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 that would indicate a stability problem. 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Thermal Considerations For continuous operation, do not exceed the maximum operation junction temperature 125°C. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : Four Layer PCB 1.4 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curves for RT8265 Packages Layout Consideration Follow the PCB layout guidelines for optimal performance of RT8265. PD(MAX) = ( TJ(MAX) - TA ) / θJA ` Where T J(MAX) is the maximum operation junction temperature 125°C, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. Keep the traces of the main current paths as short and wide as possible. ` Put the input capacitor as close as possible to the device pins (VIN and GND). ` LX node is with high frequency voltage swing and should be kept at small area. Keep analog components away from the LX node to prevent stray capacitive noise pickup. ` Connect feedback network behind the output capacitors. Keep the loop area small. Place the feedback components near the RT8265. ` Connect all analog grounds to a command node and then connect the command node to the power ground behind the output capacitors. ` An example of PCB layout guide is shown in Figure 4 for reference. For recommended operating conditions specification of RT8265, where T J(MAX) is the maximum junction temperature of the die (125°C) and TA is the maximum ambient temperature. 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 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) / (75°C/W) = 1.333W for SOP-8 (Exposed Pad) packages The maximum power dissipation depends on operating ambient temperature for fixed TJ(MAX) and thermal resistance θJA. For RT8265 packages, the Figure 3 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. DS8265-02 March 2011 www.richtek.com 11 RT8265 GND SW VIN CS CB D2 CIN Input capacitor must be placed as close to the IC as possible. VIN 2 SW 3 6 The parallel distance between COMP and FB traces must be SS as short as possible. EN CC COMP GND 4 5 FB BOOT D1 COUT The output capacitor must be VOUT placed near the RT8265. L1 8 GND 7 CP RC GND SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. VOUT The resistor divider must be connected as close to the device as possible. Figure 4 Table 1. Suggested Inductors for Typical Application Circuit Component Supplier Series Inductance (µH) DCR (mΩ) Current Rating (A) Dimensions (mm) TDK RLF7030 4.7 31 3.5 7.3 x 6.8 x 3.2 TAIYO YUDEN NR8040 4.7 18 4.7 8x 8 x 4 GOTERND GSSR2 4.7 18 5.7 10 x 10 x 3.8 Table 2. Suggested Capacitors for CIN and COUT Component Supplier Part No. Capacitance (µF) Case Size MURATA GRM31CR61E106K 10 1206 TDK C3225X5R1E106K 10 1206 TAIYO YUDEN TMK316BJ106ML 10 1206 MURATA GRM31CR60J476M 47 1206 TDK C3225X5R0J476M 47 1210 TAIYO YUDEN EMK325BJ476MM 47 1210 Table 3. Suggested Diode Component Supplier Series VRRM (V) IOUT (A) Package DIODES B330A 30 3 DO-214AC DIODES B340 40 3 DO-214AB PANJIT SK33 30 3 DO-214AB PANJIT SK34 40 3 DO-214AB www.richtek.com 12 DS8265-02 March 2011 RT8265 Outline Dimension H A M EXPOSED THERMAL PAD (Bottom of Package) Y J X B F C I D Dimensions In Millimeters Dimensions In Inches Symbol 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 Richtek Technology Corporation Headquarter Taipei Office (Marketing) 5F, No. 20, Taiyuen Street, Chupei City 5F, No. 95, Minchiuan Road, Hsintien City Hsinchu, Taiwan, R.O.C. Taipei County, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611 Tel: (8862)86672399 Fax: (8862)86672377 Email: [email protected] Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design, specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek. DS8265-02 March 2011 www.richtek.com 13