RS6512 2A, 20V, 400KHz DC/DC Asynchronous Step-Down Converter DESCRIPTION The RS6512 is a high‐efficiency asynchronous step‐down DC/DC converter that can deliver up to 2A output current from 4.75V to 20V input supply. The RS6512's current mode architecture and external compensation allow the transient response to be optimized over a wide range of loads and output capacitors. Cycle‐by‐cycle current limit provides protection against shorted outputs and thermal shutdown protection. The RS6512 also provides output under voltage protection and thermal shutdown protection. The low current (<30μA) shutdown mode provides output disconnection, enabling easy power management in battery‐powered systems. FEATURES • • • • • • • • • • • 2A output current Up to 93% efficiency Integrated 100mω power MOSFET switches Fixed 400khz frequency Cycle‐by‐cycle over current protection Thermal shutdown function Wide 4.75V to 20V operating input range Output adjustable from 1.23V to 18V Programmable under voltage lockout Available in an sop‐8 package RoHS compliant and 100% lead (Pb)‐free and green(halogen free with commercial standard) APPLICATIONS • • • • • • • • • • • PC motherboard, graphic card LCD monitor Set‐top boxes DVD‐video player Telecom equipment ADSL modem Printer and other peripheral equipment Microprocessor core supply Networking power supply Pre‐regulator for linear regulators Green electronics/appliances BLOCK DIAGRAM Tel: 886-66296288‧Fax: 886-29174598‧ http://www.princeton.com.tw‧2F, No. 233-1, Baociao Rd., Sindian Dist., New Taipei City 23145, Taiwan RS6512 APPLICATION CIRCUIT ORDER INFORMATION Device RS6512-XX Y Z V1.0 Device Code XX is nominal output voltage: AD: ADJ Y is package & Pin Assignments designator: S: SOP-8 Z is Lead Free designator: P: Commercial Standard, Lead (Pb) Free and Phosphorous (P) Free Package G: Green (Halogen Free with Commercial Standard) 2 March 2013 RS6512 PIN ASSIGNMENTS SOP-8 PIN DESCRIPTION Pin Name BS IN SW GND FB COMP EN NC V1.0 Description Bootstrap. This capacitor (C5) is needed to drive the power switch’s gate above the supply voltage. It is connected between the SW and BS pins to form a floating supply across the power switch driver. The voltage across C5 is about 5V and is supplied by the internal +5V supply when the SW pin voltage is low. Supply Voltage. The RS6512 operates from a 4.75V to 20V unregulated input. C1 is needed to prevent large voltage spikes from appearing at the input. 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. Feedback Input. FB senses the output voltage and regulates it. Drive FB with a resistive voltage divider from the output voltage to ground. The feedback threshold is 1.23V. See Setting the Output Voltage. Compensation Node. COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND. In some cases, an additional capacitor from COMP to GND is required. See Compensation. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator, drive it low to turn it off. For automatic startup, leave EN unconnected. No internal connection. 3 Pin No. 1 2 3 4 5 6 7 8 March 2013 RS6512 FUNCTION DESCRIPTION The RS6512 is a synchronous high voltage buck converter that can support the input voltage range from 4.75V to 20V 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. Figure 1. Output Voltage Setting The output voltage is set by an external resistive divider according to the following equation： R1 VOUT = VFB 1 + R2 Where VFB is the feedback reference voltage(1.23V 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. 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. VOUT VOUT ΔIL = × 1 − 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 = 0.2375(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 ) V1.0 4 March 2013 RS6512 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. 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. 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 × −1 VIN 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 x 2 low ESR ceramic capacitor is recommended. For the recommended capacitor, please 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： ΔVOUT ≤ ΔIL × ESR + 1 8 fCOUT 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 V1.0 5 March 2013 RS6512 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. OUTPUT RECTIFIER DIODE The output rectifier diode supplies the current to the inductor when the high‐side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current. Choose a rectifier who’s maximum reverse voltage rating is greater than the maximum input voltage, and who’s current rating is greater than the maximum load current. V1.0 6 March 2013 RS6512 CHECKING TRANSIENT RESPONSE 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. Component Supplier Series Dimensions (mm) MAGLAYERS MSCDRI-124-150M 12 x 12 x 5.0 SUMIDA CDRH104R 10.1 x 10 x 3.0 TOKO D104C 10 x 10 x 4.3 Table 1. Suggested Inductors for Typical Application Circuit Component Supplier MURATA TDK MURATA TDK VIN (Max.) 20V V1.0 Part No. B320 SK33 SS32 Part No. Capacitance (μF) GRM31CR61E106K 10 C3225X5R1E106K 10 GRM32ER71C226M 22 C3225X5R1C226M 22 Table 2. Suggested Capacitors for CIN and COUT Case Size 1206 1206 1200 1200 2A Load Current Vendor Diodes, Inc. (www.diodes.com) Pan Jit International (www.panjit.com.tw) General Semiconductor (www.gensemi.com) Table 3. Schottky Rectifier Selection Guide 7 March 2013 RS6512 ABSOLUTE MAXIMUM RATINGS Parameter Supply Voltage SW Pin Voltage Boot Strap Voltage Feedback Voltage Enable/UVLO Voltage Comp Voltage Junction Temperature Operating Temperature Range Storage Temperature Range Lead Temperature Symbol VIN VSW VBS VFB VEN VCOMP TJ TOPR TSTG TLEAD Range ‐0.3 to +21 ‐0.3 to VIN +0.3 VSW ‐0.3 to VSW +6 ‐0.3 to +6 ‐0.3 to +6 ‐0.3 to +6 150 ‐20 to +85 ‐40 to +150 260 Units V V V V V V o C o C o C o C ELECTRICAL CHARACTERISTICS (VIN=12V, TA=25°C, unless otherwise specified) Parameter Input Voltage Feedback Voltage Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Limit (NOTE 1) Current Sense Transconductance Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance Oscillator Frequency Short Circuit Frequency Maximum Duty Cycle Minimum On Time EN Shutdown Threshold Enable Pull Up Current EN UVLO Threshold Rising EN UVLO Threshold Hysteresis Supply Current (Shutdown) Supply Current (Quiescent) Thermal Shutdown Symbol VIN VFB RDS(ON)1 RDS(ON)2 ISw ILIM Conditions 4.75V ≤ VIN ≤ 20V VEN = 0V, VSW = 0V - Min. 4.75 1.19 - Typ. 1.23 0.22 10 3.8 Max. 20 1.26 10 - Unit V V Ω Ω μA A GCS - - 1.95 - A/V AVEA GEA FS FOSC1 DMAX tON ISD IQ TSD VFB = 0V VFB = 1.0V ICC > 100uA VEN = 0V VIN Rising VIN ≤ 0.4V VEN ≥ 3V - 550 0.7 2.35 - 400 830 400 240 90 100 1.0 1.0 2.50 200 23 1.1 160 1150 1.3 2.65 36 1.3 - V/V μA /V KHz KHz % ns V μA V mV μA mA o C Notes: 1. Slope compensation changes current limit above 40% duty cycle. 2. 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. 3. Devices are ESD sensitive. Handling precaution is recommended. 4. The device is not guaranteed to function outside its operating conditions. 5. 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. V1.0 8 March 2013 RS6512 PACKAGE INFORMATION 8-PIN, SOP . Notes: 1. All units are in millimeter 2. Refer to JEDEC MS‐012 variation AA. V1.0 9 March 2013 RS6512 IMPORTANT NOTICE Princeton Technology Corporation (PTC) reserves the right to make corrections, modifications, enhancements, improvements, and other changes to its products and to discontinue any product without notice at any time. PTC cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a PTC product. No circuit patent licenses are implied. Princeton Technology Corp. 2F, 233-1, Baociao Road, Sindian Dist, New Taipei 23145, Taiwan Tel: 886-2-66296288 Fax: 886-2-29174598 http://www.princeton.com.tw V1.0 10 March 2013