RT8272 3A, 24V, 1.2MHz Step-Down Converter General Description Features The RT8272 is a 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. Current Mode operation provides fast transient response and eases loop stabilization. z Wide Operating Input Range : 4.75V to 24V z Adjustable Output Voltage Range : 0.92V to 15V Output Current up to 3A 25μ μA Low Shutdown Current Power MOSFET : 0.1Ω Ω 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 RT8272 is available in a SOP-8 (Exposed Pad) surface mount package. z z z z z Ordering Information High Efficiency up to 95% 1.2MHz Fixed Switching Frequency Stable with Low ESR Output Ceramic Capacitors Thermal Shutdown Protection Cycle-By-Cycle Over Current Protection RoHS Compliant and Halogen Free Applications RT8272 Package Type SP : SOP-8 (Exposed Pad-Option 1) z Lead Plating System G : Green (Halogen Free and Pb Free) z z z Distributive Power Systems Battery Charger DSL Modems Pre-regulator for Linear Regulators Note : Pin Configurations Richtek products are : ` RoHS compliant and compatible with the current require- (TOP VIEW) ments of IPC/JEDEC J-STD-020. ` BOOT Suitable for use in SnPb or Pb-free soldering processes. VIN 2 SW 3 GND 4 GND 9 8 SS 7 EN 6 COMP 5 FB SOP-8 (Exposed Pad) Typical Application Circuit VIN 4.75V to 24V Chip Enable 2 CIN 10µF VIN BOOT 1 RT8272 7 EN 8 SS CSS 4, 0.1µF 9 (Exposed Pad) GND SW 3 CBOOT L 10nF 4.7µH D1 B330 R1 25.8k FB 5 COMP 6 CC 2.2nF RC 39k VOUT 3.3V/3A COUT 47µF R2 10k CP NC DS8272-02 March 2011 www.richtek.com 1 RT8272 Table 1. Recommended Component Selection VOUT (V) R1 (kΩ) R2 (kΩ) R C (kΩ) C C (nF) C OUT (μF) L (μH) 15 153 10 82 0.82 47 10 10 100 10 62 1.2 47 10 8 77 10 62 1.5 47 6.8 5 43 10 41 2.2 47 6.8 3.3 25.8 10 39 2.2 47 4.7 2.5 17 10 21 2.2 47 4.7 1.8 9.1 10 15 2.2 47 2.2 1.2 3 10 15 2.2 47 2.2 Functional Pin Description Pin No. Pin Name 1 BOOT 2 VIN 3 SW 4, 9 (Exposed Pad) GND 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 BOOT to power the high side switch. Power Input. V IN 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 BOOT 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 0.92V 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. DS8272-02 March 2011 RT8272 Function Block Diagram VIN VCC Internal Regulator Oscillator 1.2MHz/440kHz 1µA EN 10k 3V VA VCC Foldback Control + 1V Shutdown Comparator VCC + UV Comparator + Current Comparator z z z z z z z z z z z GND COMP (Note 1) Supply Voltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 26V Switching Voltage, SW ------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) BOOT Voltage ------------------------------------------------------------------------------------------------ (VSW − 0.3V) to (VSW + 6V) All Other Pins 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 SW + +EA Gm = 680µA/V Absolute Maximum Ratings z Logic 0.92V FB z VA BOOT 0.5V 10µA SS Current Sense Slope Comp Amplifier + - (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 DS8272-02 March 2011 www.richtek.com 3 RT8272 Electrical Characteristics (VIN = 12V, TA = 25°C unless otherwise specified) Parameter Symbol Test Conditions 4.75V ≤ VIN ≤ 24V Min Typ Max Unit 0.902 0.92 0.938 V Feedback Reference Voltage VFB High Side Switch-On Resistance RDS(ON)1 -- 0.1 0.16 Ω Low Side Switch-On Resistance Switch Leakage RDS(ON)2 VEN = 0V, VSW = 0V --- 10 -- -10 Ω μA Current Limit ILIM Duty = 80%; VBOOT−SW = 4.8V -- 4.1 -- A Current Sense Transconductance GCS Output Current to VCOMP -- 4 -- A/V Error Amplifier Tansconductance Gm ΔIC = ±10μA 500 680 900 μA/V Oscillator Frequency fSW -- 1.2 -- MHz --- 440 80 --- kHz % -- 90 -- ns 3.8 4.2 4.5 V -- 300 -- mV -- -- 0.4 V En input High Voltage 1.4 -- -- V Enable Pull Up Current 0.15 1 2.65 μA --- 25 0.8 -1 μA mA -- 10 -- μA -- 10 -- ms -- 150 -- °C Short Circuit Oscillation Frequency Maximum Duty Cycle DMAX Minimum On-Time Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis En input Low Voltage tON Shutdown Current Quiescent Current ISHDN IQ Soft-Start Current ISS Soft-Start Period Thermal Shutdown VFB = 0V VFB = 0.8V VEN = 0V VEN = 2V, VFB = 1V CSS = 0.1μF T SD 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. www.richtek.com 4 DS8272-02 March 2011 RT8272 Typical Operating Characteristics Reference Voltage vs. Input Voltage Efficiency vs. Output Current 0.925 100 VIN = 12V 90 0.924 VIN = 24V 70 Efficiency (%) Reference Voltage (V) 80 60 50 40 30 20 10 0.923 0.922 0.921 0.920 0.919 0.918 0.917 0.916 VOUT = 3.3V 0.915 0 0 0.5 1 1.5 2 2.5 4 3 6 8 10 Reference Voltage vs. Temperature 16 18 20 22 24 Output Voltage vs. Output Current 0.94 3.350 0.935 3.345 Output Voltage (V) Reference Voltage (V) 14 Input Voltage (V) Output Current (A) 0.93 0.925 0.92 0.915 3.340 3.335 3.330 3.325 3.320 3.315 0.91 3.310 0.905 3.305 VIN = 12V 0.9 VIN = 12V 3.300 -50 -25 0 25 50 75 100 0 125 0.5 Temperature (°C) 1 1.5 2 2.5 3 Output Current (A) Frequency vs. Temperature Frequency vs. Input Voltage 1.23 1.30 1.22 1.25 Frequency (MHz)1 Frequency (MHz)1 12 1.21 1.20 1.19 1.20 1.15 1.10 1.05 1.18 VOUT = 3.3V, IOUT = 0.3A VIN = 12V, VOUT = 3.3V, IOUT = 0.3A 1.00 1.17 4 6 8 10 12 14 16 18 Input Voltage (V) DS8272-02 March 2011 20 22 24 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8272 Quiescent Current vs. Temperature 0.90 5.8 0.85 Quiescent Current (mA) Current Limit (A) Current Limit vs. Duty Cycle 6.2 5.4 5.0 4.6 4.2 3.8 0.80 0.75 0.70 0.65 3.4 0.60 3.0 0.55 VIN = 12V, VOUT = 3.3V 10 20 30 40 50 60 70 80 -50 -25 0 Duty Cycle (%) 50 75 100 125 Temperature (°C) UVLO vs. Temperature Enable Threshold vs. Temperature 1.40 4.8 1.30 4.6 1.20 Enable Threshold (V) 5.0 4.4 UVLO (V) 25 4.2 Rising 4.0 3.8 Falling 3.6 3.4 3.2 Rising 1.10 1.00 Falling 0.90 0.80 0.70 0.60 0.50 3.0 0.40 -50 -25 0 25 50 75 100 125 -50 0 25 50 75 100 Temperature (°C) Load Transient Response Load Transient Response VIN = 12V, VOUT = 3.3V IOUT = 1.5A to 3A VOUT (100mV/Div) IOUT (2A/Div) IOUT (2A/Div) Time (100μs/Div) 125 VIN = 12V, VOUT = 3.3V IOUT = 0A to 3A VOUT (50mV/Div) www.richtek.com 6 -25 Temperature (°C) Time (100μs/Div) DS8272-02 March 2011 RT8272 Power On from EN Pin Power Off from EN Pin VIN = 12V, VOUT = 3.3V IOUT = 3A VEN (5V/Div) VEN (5V/Div) VOUT (1V/Div) VOUT (1V/Div) I IN (500mA/Div) VIN = 12V, VOUT = 3.3V IOUT = 3A Time (2.5ms/Div) I IN (500mA/Div) Time (100μs/Div) Output Ripple VOUT (5mV/Div) VLX (10V/Div) IL (2A/Div) VIN = 12V, VOUT = 3.3V IOUT = 3A Time (500ns/Div) DS8272-02 March 2011 www.richtek.com 7 RT8272 Application Information The RT8272 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 RT8272 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 10μA 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 RT8272 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 (0.92V 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 the BOOT pin for efficiency improvement when input voltage is lower than 5.5V or duty ratio is higher than 65%. The bootstrap diode can be a low cost one such as 1N4148 or BAT54. The external 5V can be a 5V fixed input from system or a 5V output of the RT8272. 5V BOOT RT8272 10nF SW 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 × ΔIL(MAX) ⎦ ⎣ VIN(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. Figure 2. External Bootstrap Diode 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 8 DS8272-02 March 2011 RT8272 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. Do not allow the core to saturate! 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, 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 : IRMS = IOUT(MAX) VOUT VIN VIN −1 VOUT This formula has a maximum at VIN = 2VOUT, where I RMS = I OUT /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 DS8272-02 March 2011 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. 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 : 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. www.richtek.com 9 RT8272 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 RT8272 Packages Layout Consideration Follow the PCB layout guidelines for optimal performance of RT8272. 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. 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 RT8272. ` 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 RT8272, where 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 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 T J(MAX) and thermal resistance θJA. For RT8272 packages, the Figure 3 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. www.richtek.com 10 DS8272-02 March 2011 RT8272 GND VIN SW CSS CB 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 CIN Input capacitor must be placed as close to the IC as possible. BOOT D1 C OUT The output capacitor must be VOUT placed near the RT8272. L 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. PCB Layout Guide Table 2. 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 8x8x4 GOTERND GSSR2 4.7 18 5.7 10 x 10 x 3.8 Table 3. 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 4. 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 DS8272-02 March 2011 www.richtek.com 11 RT8272 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. www.richtek.com 12 DS8272-02 March 2011