RT8256 2A, 22V, 400kHz Step-Down Converter General Description Features The RT8256 is an as high voltage buck converter that can support the input voltage range from 4.75V to 22V and the output current can be up to 2A. Current Mode operation provides fast transient response and eases loop stabilization. z Wide Operating Input Range : 4.75V to 22V z Adjustable Output Voltage Range : 1.22V to 16V Output Current up to 2A 25uA Low Shutdown Current Power MOSFET : 0.18Ω 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 25uA of supply current. The RT8256 is available in a SOP-8 surface mount package. z z z z z Ordering Information High Efficiency up to 95% 400kHz 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 RT8256 Package Type S : SOP-8 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. ` NC Suitable for use in SnPb or Pb-free soldering processes. 8 EN BOOT 2 7 COMP VIN 3 6 FB SW 4 5 GND SOP-8 Typical Application Circuit VIN 4.75V to 22V Chip Enable 3 VIN CIN 10µF BOOT 2 RT8256 SW 4 8 EN FB 6 5 GND COMP 7 CB 10nF L1 15µH D1 B230 RC CC 10k 1.5nF R1 17k VOUT 3.3V/2A COUT 22µF R2 10k CP NC DS8256-01 March 2011 www.richtek.com 1 RT8256 Table 1. Recommended Component Selection VOUT (V) R1 (kΩ) R2 (kΩ) RC (kΩ) CC (nF) L1 (μH) C OUT (μF) 12 5 88.7 30 10 10 62 20 0.82 2.2 33 22 22 22 3.3 2.5 1.8 17 10.45 4.75 10 10 10 10 7.5 6 1.5 1.5 1.5 15 10 10 22 22 22 1.222 0 10 6 3.9 6.8 22 Functional Pin Description Pin No. Pin Name Pin Function 1 NC No Internal Connection. 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. 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 BOOT to power the high side switch. 2 BOOT 3 VIN 4 SW 5 GND Ground. 6 FB Feedback Input. FB senses the output voltage to regulate said voltage. The feedback reference voltage is 1.222V typically. 7 COMP 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. 8 EN 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. Function Block Diagram VIN Internal Regulator Current Sense Slope Comp Amplifier + Oscillator 400kHz/120kHz 1µA EN VA 10k 1V 3V VA Foldback Control + 0.6V Shutdown Comparator + + UV Comparator 1.222V BOOT + - Current Comparator S Q R Q SW GND EA Gm = 780µA/V FB www.richtek.com 2 COMP DS8256-01 March 2011 RT8256 Absolute Maximum Ratings z z z z z z z z z z (Note 1) Supply Voltage, VIN ----------------------------------------------------------------------------------------- 23V Switching Voltage, SW ------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V) BOOT Voltage ------------------------------------------------------------------------------------------------ (VSW − 0.3V) to (VSW + 6V) The Other Pins ----------------------------------------------------------------------------------------------- −0.3V to 6V Power Dissipation, PD @ TA = 25°C SOP-8 ---------------------------------------------------------------------------------------------------------- 0.833W Package Thermal Resistance (Note 2) SOP-8, θJA ---------------------------------------------------------------------------------------------------- 120°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 ----------------------------------------------------------------------------------------- 22V 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 Feedback Reference Voltage VFB High Side Switch-On Resistance RDS(ON)1 Low Side Switch-On Resistance Switch Leakage RDS(ON)2 Current Limit Test Conditions 4.75V ≤ VIN ≤ 22V Min 1.184 Typ Max Unit 1.222 1.258 V -- 0.18 -- Ω VEN = 0V, VSW = 0V --- 10 -- -10 Ω μA ILIM Duty = 90%; VBOOT−SW = 4.8V -- 2.7 -- A Current Sense Transconductance GCS Output Current to V COMP -- 2.5 -- A/V Error Amplifier Tansconductance Gm ΔIC = ±10μA -- 780 -- μA/V Oscillator Frequency fSW 340 400 460 kHz VFB = 0V -- 120 -- kHz VFB = 0.8V -- 90 -- % -- 100 -- ns 4 4.2 4.5 V -- 300 -- mV En input Low Voltage -- -- 0.4 V En input High Voltage 1.4 -- -- V -- 1 -- μA Short Circuit Oscillation Frequency Maximum Duty Cycle DMAX Minimum On-Time tON Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis Enable Pull Up Current To be continued DS8256-01 March 2011 www.richtek.com 3 RT8256 Parameter Symbol Test Conditions Min Typ Max Unit Shutdown Current ISHDN V EN = 0V -- 25 50 μA Quiescent Current IQ V EN = 2V, VFB = 1.5V -- 0.7 1 mA Thermal Shutdown TSD -- 150 -- °C 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 51-3 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 DS8256-01 March 2011 RT8256 Typical Operating Characteristics Efficiency vs. Output Current 100 Output Voltage vs. Output Current 3.300 3.297 VIN = 5V Efficiency (%) 80 Output Voltage (V) 90 VIN = 12V 70 60 VIN = 22V 50 40 30 3.294 3.291 3.288 3.285 3.282 3.279 VIN = 22V 3.276 3.273 VIN = 12V 3.270 3.267 20 10 3.264 3.261 VOUT = 3.3V 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 VOUT = 3.3V 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Output Current (A) Output Current (A) Reference Voltage vs. Input Voltage Output Voltage vs. Temperature 3.400 1.226 3.375 Output Voltage (V) Reference Voltage (V) 1.225 1.224 1.223 1.222 1.221 3.350 3.325 3.300 3.275 3.250 3.225 VIN = 4.75V to 22V, VOUT = 3.3V VIN = 12V, VOUT = 3.3V 3.200 1.220 4 6 8 10 12 14 16 18 20 -50 22 -25 0 50 75 100 125 Frequency vs. Temperature Frequency vs. Input Voltage 450 450 440 440 430 430 Frequency (kHz)1 Frequency (kHz) 1 25 Temperature (°C) Input Voltage (V) 420 410 400 390 380 420 410 VIN = 12V 400 390 VIN = 22V 380 370 370 360 360 VOUT = 3.3V, IOUT = 0.3A VOUT = 3.3V, VIN = 4.75V to 22V, IOUT = 0.3A 350 350 4 6 8 10 12 14 16 Input Voltage (V) DS8256-01 March 2011 18 20 22 -50 -25 0 25 50 75 100 125 Temperature (°C) www.richtek.com 5 RT8256 Quiescent Current vs. Temperature Shutdown Current vs. Temperature 800 60 750 Shutdown Current (µA)1 Quiescent Current (µA) 775 725 700 VIN = 22V 675 650 VIN = 12V 625 600 575 550 525 50 40 30 20 10 VIN = 12V VOUT = 3.3V 500 0 -50 -25 0 25 50 75 100 125 -50 -25 0 Temperature (°C) 50 75 100 125 Current Limit vs. Temperature Current Limit vs. Input Voltage 4.50 4.50 4.25 4.25 4.00 4.00 Current Limit (A) Current Limit (A) 25 Temperature (°C) 3.75 3.50 3.25 3.00 VIN = 22V 3.75 VIN = 12V 3.50 3.25 3.00 2.75 2.75 VIN = 4.75V to 22V, VOUT = 3.3V 2.50 VOUT = 3.3V 2.50 4 6 8 10 12 14 16 18 20 22 -50 Input Voltage (V) VOUT (200mV/Div) IOUT (1A/Div) IOUT (1A/Div) www.richtek.com 6 25 50 75 100 125 Load Transient Response VOUT (200mV/Div) Time (100μs/Div) 0 Temperature (°C) Load Transient Response VIN = 12V, VOUT = 3.3V, IOUT = 0.1A to 2A -25 VIN = 12V, VOUT = 3.3V, IOUT = 1A to 2A Time (100μs/Div) DS8256-01 March 2011 RT8256 Switching Power On from VIN VSW (10V/Div) VIN (5V/Div) VOUT (10mV/Div) VOUT (2V/Div) ISW (1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A I IN (1A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A Time (1μs/Div) Time (5ms/Div) Power On From EN Power Off From EN VEN (2V/Div) VEN (2V/Div) VOUT (2V/Div) VOUT (2V/Div) IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, IOUT = 2A Time (1ms/Div) DS8256-01 March 2011 IOUT (2A/Div) VIN = 12V, VOUT = 3.3V, I OUT = 2A Time (25ms/Div) www.richtek.com 7 RT8256 Application Information The RT8256 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75V to 22V and the output current can be up to 2A. Output Voltage Setting The resistive divider allows the FB pin to sense the output voltage as shown in Figure 1. VOUT R1 FB RT8256 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 (1.222V 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 RT8256. 5V BOOT RT8256 10nF SW Figure 2. External Bootstrap Diode www.richtek.com 8 Soft-Start The RT8256 contains an internal soft-start clamp that gradually raises the output voltage. The soft-start time is designed by the internal capacitor. The typical soft-start time is 2ms. Inductor Selection 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. 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. DS8256-01 March 2011 RT8256 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 4. 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 DS8256-01 March 2011 refer to table 3 for more detail. 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 RT8256 1.0 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 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. 0.9 Power Dissipation (W) Checking Transient Response Four Layer PCB 0.8 0.7 0.6 SOP-8 0.5 0.4 0.3 0.2 0.1 0.0 Thermal Considerations 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 : 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 of RT8256, the maximum junction temperature is 125°C. The junction to ambient thermal resistance θJA for SOP-8 package is 120°C/W on the standard JEDEC 51-7 fourlayers thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : PD(MAX) = (125°C − 25°C) / (120°C/W) = 0.833W for SOP-8 packages The maximum power dissipation depends on operating ambient temperature for fixed T J(MAX) and thermal resistance θJA. For RT8256 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 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curves for RT8256 Packages Layout Consideration Follow the PCB layout guidelines for optimal performance of the RT8256. ` 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 sensitive components away from the LX node to prevent stray capacitive noise pickup. ` Place the feedback components to the FB pin as close as possible. ` The GND should be connected to a strong ground plane for heat sinking and noise protection. DS8256-01 March 2011 RT8256 GND Input capacitor must C IN be placed as close to the IC as possible. CC SW CB NC V IN 8 EN CP BOOT 2 7 COMP VIN 3 6 FB SW 4 5 GND The feedback and compensation components must be connected as close to the device as possible. RC R1 V OUT R2 D1 L1 GND C OUT VOUT SW should be connected to inductor by wide and short trace. Keep sensitive components away from this trace. Figure 4. PCB Layout Guide Table 2. Suggested Inductors for Typical Application Circuit Component Supplier Series Dimensions (mm) TDK SLF12555T 12.5x12.5x5.5 TAIYO YUDEN NR8040 8x8x4 TDK SLF12565T 12.5x12.5x6.5 Table 3. 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 GRM32ER61E226M 22 1210 COUT TDK C3225X5R0J226M 22 1210 COUT TAIYO YUDEN EMK325BJ226MM 22 1210 Table 4. Suggested Diode Component Supplier Series VRRM (V) IOUT (A) Package DIODES B330A 30 3 SMA PANJIT SK23 30 2 DO-214AA DS8256-01 March 2011 www.richtek.com 11 RT8256 Outline Dimension H A M J 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 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 0.170 0.254 0.007 0.010 I 0.050 0.254 0.002 0.010 J 5.791 6.200 0.228 0.244 M 0.400 1.270 0.016 0.050 8-Lead SOP 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 DS8256-01 March 2011