LINFINITY Application Note AN-6 Power Solutions for Flexible Motherboards Copyright © 1998 Rev. 2.1 07/98 AN-6 Power Solutions for Flexible Motherboards Application Note INTRODUCTION As Intel, Cyrix, AMD, IBM and others introduce higher performance microprocessors, power requirements are continuously evolving. Power supplies are migrating to lower voltages, while currents increase. The move to lower voltages is not happening simultaneously, so while the processor manufacturer can dictate core voltages, the I/O section of the processor needs to communicate at the current standard voltage (3.3V). Split plane voltages, currently at 3.3V (I/O) and 2 - 2.8V (CPU core) will remain in a continuous state of change as Moore’s Law predicts the rate of change of the processor. Supplying power to the processor, virtually an afterthought in previous times, now becomes a critical element in the design and manufacture of the motherboard. Facing an increasingly competitive market, designers must create motherboards that are flexible enough to accept a number of processor variants at the lowest cost. Eliminating the need for board re-designs or production line changes to suit different processor power requirements reduces costs. This application note presents a discussion of the power supply options facing designers, showing low cost flexible motherboard power supply designs, based on Linfinity’s LX166x Advanced PWM Controller IC family. POWER REQUIREMENTS Most microprocessors require several power supplies: • CPU core supply (VCORE) – typically 1.8 – 3.3V at 5 - 15A. It is likely that the core supply will drop below 1.8V in the near future. Supplying VCORE is the major challenge, due to the high power levels and stringent transient load change voltage specifications. • I/O Buffer supply (VI/O) – typically 3.3V. With demand for 3.3V power from other circuits such as memory, AGP controllers, chipsets, etc., total demand for current at 3.3V can be 8 – 10A or higher. • GTL+ Bus supply (VTT) – typically 1.5V at 3 – 5A in Pentium Pro and Pentium II processor systems. • Clock supply (VCLOCK) – typically 2.5V at 500mA or less in Pentium® II processor systems. (and 3.3V, if available) rail can be converted to the required voltage using either a linear regulator, such as Low Dropout (LDO) regulators or a switching regulator. LINEAR REGULATORS Three-terminal linear low dropout regulators, such as Linfinity’s popular LX8384 series were commonly used for powering 3.3V processors, such as Pentium® processors below 200MHz, due to their ease of use, quality of performance and low system cost. However, there are several limitations of linear regulators as power levels increase. • The heat generated in a linear regulator is always (VIN VOUT) * IOUT where VIN is input voltage, VOUT is output voltage and IOUT is load current. As the current increases or the output voltage falls, heat dissipated by an LDO increases! This heat must be removed from the IC to avoid over-heating and circuit failure – heatsink requirements can become both expensive and bulky at high power levels. • The minimum differential (dropout) voltage limits linear regulators. For most low dropout linear (LDO) regulators, this is around 1.3V. Therefore, standard LDO’s cannot be used to convert 3.3 - 2.8V, where a dropout voltage under 0.5V is needed. For applications such as this, a BiCMOS LDO, such as the LX8610/30, can be used instead. Linear regulators are still the optimal power source for low to medium current 5 – 3.3V or 3.3 – 1.5V conversion for applications such as VCLOCK and GTL+ Bus supplies. They can be used on motherboard or accessory cards where point-of-use power distribution is required. Linfinity’s family of low dropout regulators ranges from 500mA to 10A current capability, and is available in a range of surface mount and through-hole packages and in fixed and adjustable voltage versions. The only external components required with LDO’s are input and output capacitors; resistors (to set the output voltage in adjustable LDO’s) and a heatsink (this can be PCB copper for surface mount packages at low power levels). For full datasheets, application notes and selection guides, see http://www.linfinity.com. Available Power Sources SWITCHING REGULATORS The silver box power supply on a typical motherboard supplies only 5V and 12V (at high power levels), although 3.3V is also available in ATX and NLX power supplies. The 5V Switching regulators usually have higher efficiency than linear regulators, reducing or eliminating the cost and size of heatsink. Switchers do, however, involve a higher de- Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note Page 2 AN-6 Power Solutions for Flexible Motherboards gree of complexity in design and layout. Some controllers are very susceptible to noise problems due to poor layout. Switchers are generally more expensive than linear regulators at lower currents, but the benefits of higher efficiency make switching regulators the solution of choice for higher power levels. Many switching regulators have been specifically designed for microprocessor applications, and are optimized for fast transient response, performance and low cost. SWITCHING REGULATOR SOLUTIONS The LX166x family of Advanced PWM Controllers was designed by Linfinity to satisfy the power requirements of microprocessors with a low cost, high performance design requiring a minimum of external components. The controllers use voltage-mode, constant off-time control to achieve fast response and simplify loop compensation. In this technique, the off-time is modulated by the output voltage to provide a virtually fixed frequency of operation. Other key features include: • Adaptive transient response – at low load, the DC set point is higher than the designated set point by about 40mV. At high loads, the output will be in the lower part of the allowable tolerance window. This technique results in added margin to handle transient load changes. See Figure 1. • Flexible and accurate current sensing and current limiting to protect against over-current situations. The controllers detect a voltage drop across a resistive current sense element to trigger current limiting. The current sense element can be a sense resistor; a PCB trace resistance or the parasitic resistance of the main inductor. This method eliminates the errors caused by the variation of RDS(ON) and peak current levels, used in some alternative current sense methods. The LX1668 also offers the additional protection feature of hiccupmode current limiting. • Programmable output voltage for VCORE by means of 5-bit Voltage Identification (VID) code. The output voltage can be adjusted from 1.30 to 2.05V in 50mV increments, and from 2.1 to 2.5V in 100mV increments. The VID codes correspond to Intel’s specifications. • Integral linear regulator and/or linear regulator driver in many devices. The LX1664/65 and LX1668 have a linear regulator output to drive a MOSFET as the pass element for a LDO, reducing costs by eliminating an LDO or shunt regulator. The LX1668 also has an internal 2.5V fixed LDO, suitable for powering VCLOCK, in addition to the LDO driver. • Available Power Good signal and Over Voltage Protection crowbar driver to protect against faults such as a short circuit in the upper MOSFET. • Small package sizes (14-pin SOIC – LX1662; 16-pin – LX1663/64; 18-pin – LX1665 and 20 pin – LX1668). Lin- Adaptive Voltage Positioning Offset VOFFSET (40mV) Output Voltage VOUT (50mV/Div) Nominal set point voltage, VSET (2.0V) Steady state voltage at high current is approximately VSET + VOFFSET - IOUT×RSENSE Dynamic voltage tolerance, VDYN(100mV for 2µs) Initial voltage drop is mainly due to the product of the load current step and ESR of the capacitors. ∆V = ∆I × ESR. (ESL effects are ignored) Output Current transient step, ∆I = 0 to 14A (5A/Div) L = 2.5µH; COUT = 6 × 1500µF Sanyo MV-GX; RSENSE = 2.5mΩ Figure 1: Adaptive Voltage Positioning Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note Page 3 AN-6 Power Solutions for Flexible Motherboards finity’s controllers have the highest integration in the industry, leading to lower cost and more space efficient designs. The LX1668 is also available in a 20-pin TSSOP package for applications where small size is important. The following application ideas present typical circuit configurations with key features and bills of material. Full datasheets and application notes can be downloaded from Linfinity’s web site at http://www.linfinity.com. Evaluation boards and electronic (Gerber) layouts are available for most devices upon request. Controllers for Microprocessor Applications Linfinity offers controllers with single, dual and triple outputs to suit the requirements of different applications. The controller families are summarized in the table below. OTHER APPLICATION NOTES CONCLUSION AN Number It is clear that as processor core voltages continue to decrease and currents increase the heatsink requirements for linear regulators are getting tougher to meet, and linear solutions become more costly. Switching regulators, such as Linfinity’s LX166x family, can provide the various voltages at higher efficiency and a lower total system cost. Their programmable outputs enable future processors to be powered with the same power supply. Title Applicable Devices AN-7 A Simple Current-Sense Technique Eliminating a Sense Resistor LX166x AN-8 Hiccup Mode Current Limiting LX1668 LX1669 AN-9 Modulated Constant Off-Time Control Mechanism LX166x AN-10 Design Procedure for Microprocessor Buck Regulators LX166x Download from http://www.linfinity.com Current Sense Threshold (mV) Production • • 100 Now • • 100 Now • • 60 Now • • • 100 Now • • • 60 Now • • • • 100 Now SO-16 • • • • 60 Now LX1665 SO-18 • • • • • • 100 Now LX1665A SO-18 • • • • • • 60 Now LX1668 SO-20 TSSOP •(TTL) • • • • • • 60 8/98 LX1669 SO-16 •(TTL) • • • • • 60 8/98 Package SO-16 LX1661 SO-16 LX1662 SO-14 • LX1662A SO-14 • LX1663 SO-16 • • LX1663A SO-16 • • LX1664 SO-16 LX1664A • Internal LDO Now LX1660 External LDO Synchronous Rectification 100 Device OVP Driver • Power Good • 5-bit VID Hiccup Mode Current Limit Adaptive Voltage Positioning SWITCHING REGULATOR SELECTION GUIDE • Pentium is a registered trademark of Intel Corporation Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note Page 4 AN-6 Power Solutions for Flexible Motherboards Controller Options for Microprocessor Power The table below shows the major options and topology of Linfinity’s regulator controllers for microprocessor power. Topology Devices External LDO Driver (1.5V/ADJ) Triple Output Combination of programmable switching regulator with two linear regulators (one internal, one external). Block Diagram LX1668 Programmable Buck Regulator for CPU Core Internal LDO 2.5V / 0.5A External LDO Driver (1.5V/ADJ) Dual Output Combination of programmable switching regulator with a single external linear regulator driver. LX1664 LX1665 Programmable Buck Regulator for CPU Core Single Output Programmable or adjustable switching regulator. Programmable Buck Regulator for CPU Core LX1662/63 LX1669 (programmable) LX1660/61 (adjustable) Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note Page 5 AN-6 Power Solutions for Flexible Motherboards Application Idea 1: LX1668 Triple Output Regulator DESCRIPTION KEY FEATURES The LX1668 provides a programmable synchronous buck regulator controller, together with a linear regulator driver (external MOSFET) and an internal 2.5V (fixed) low dropout regulator. The main output is programmable between 1.3 and 3.5V using a 5-bit, TTL-compatible voltage identification (VID) code. n PWM Output Programmable By 5-Bit TTL-Compatible Voltage Identification (VID) Code n External Adjustable Linear Regulator Driver (Requires No Resistors For Setting 1.5V Output) n Internal 2.5V Fixed Low Dropout Regulator (500ma) n Hiccup Mode Current-Limiting For Fault Protection n “Green PC” Shutdown Mode n Soft-Start n 20-Pin SOIC or TSSOP Package n Accurate Current Sensing By PCB Trace Resistor; Loss-Less Inductor Resistance Or SMD Sense Resistor Methods n Lowest Total Cost Of Any Pentium II Processor Power Supply Solution The LX1668 is compatible with Energy Star (Green PC) specifications – the CPU core output can be disabled while keeping the internal LDO active. Advanced fault protection features include internal over-voltage protection (OVP) and hiccup-mode over-current protection. An OVP pin can be used to drive an SCR to clamp the output to ground, or to turn off an ATX power supply in case of an over-voltage condition. The modulated constant off-time architecture results in simple system design with no external compensation network required. Transient response is enhanced by 40mV adaptive voltage positioning. APPLICATIONS n Pentium® II processor supplies n AMD-K6™ and Cyrix M II™, MediaGX™ supplies TYPICAL APPLICATION 5V 12V C8 C3 1µF 1µF L2 3.3V or 5V 1µH C2 C7 1500µFx3 Q1 220µF IRL3102 L1 R SENSE CPU Core VCORE 2.5µH 2.5m Ω V OUT2 2.5V C5 220µF 1 TDRV C9 2 1µF 3 V CC12 4 3.3V V OUT3 1.5V 5 C4 6 330µF 7 Q3 IRLZ44 C6 330µF R1, 0 8 PGND 20 AGND 18 V OUT2 SS/EN 17 F FB 16 V CC3 LDRV Q2 BDRV 19 V CC5 LX1668 LFB V CORE PWRGD IRL3303 CSS Q4 0.1µF SCR 2N6504 15 14 VID0 OVP 13 9 VID1 VID4 12 10 VID2 VID3 11 R2, 50k PWRGD VID4 VID3 VID2 VID1 VID0 C1 1500µF x 6 R3, 10k Sampling 8/98 Evaluation board: LXE9014 BILL OF MATERIALS Item Description U1 C1, C2 C4, C6 C5, C7 C3, C8, C9 CSS Q1, Q2 Q3 RSENSE LX1668 Controller IC Capacitor, 1500µF, 6.3V, 44mΩ ESR R1, R2 L1 L2 Quantity Capacitor, 330µF Capacitor, 220µF Capacitor, 1µF Capacitor, 0.1µF FET (low RDS(ON)) FET (IRLZ44 or similar - RDS(ON) need not be low) Sense resistor, 2.5mΩ (PCB trace) Resistor (R1 = 0Ω; R2 = 50kΩ for VOUT2 = 1.5V) Inductor, 2 – 3µH Inductor, 1µH Total Component Count Copyright © 1998 Rev. 2.1 07/98 1 9 2 2 3 1 2 1 1 1 1 1 25 LinFinity Application Note Page 6 AN-6 Power Solutions for Flexible Motherboards Application Idea 2: LX1669 Single Output Regulator DESCRIPTION KEY FEATURES The LX1669 provides a programmable synchronous buck regulator controller, programmable between 1.3 and 3.5V using a 5-bit, TTL-compatible voltage identification (VID) code. Advanced fault protection features include internal over-voltage protection (OVP) and hiccup-mode over-current protection. An OVP pin can be used to drive an SCR to clamp the output to ground, or to turn off an ATX power supply in case of an over-voltage condition. n PWM Output Programmable By 5-Bit TTL-Compatible Voltage Identification (VID) Code n Hiccup Mode Current-Limiting For Fault Protection n Disable Function n Soft-Start n 16-Pin Narrow Body SOIC n Accurate Current Sensing By PCB Trace Resistor; Loss-Less Inductor Resistance Or SMD Sense Resistor Methods The modulated constant off-time architecture results in simple system design with no external compensation network required. Transient response is enhanced by 40mV adaptive voltage positioning. n Pentium® II processor supplies n AMD-K6™ and Cyrix M II™, MediaGX™ supplies APPLICATIONS TYPICAL APPLICATION 5V 12V L2, 1µH C2 C8 1µF C3 1µF IRL3102 2 3 4 5 6 7 8 L1 2.5µH PGND VCC12 BDRV 15 Q2 VCC5 AGND 14 IRL3303 PWRGD OVP VID0 SS/EN FFB VCORE VID1 VID4 VID2 VID3 RSENSE 2.5mΩ CPU Core VCORE 16 TDRV LX1669 1 Sampling 8/98 Use evaluation board: LXE9014 1500µFx3 Q1 13 12 CSS 0.1µF 11 10 9 VID4 VID3 VID2 VID1 VID0 Q4 R3 10kΩ SCR 2N6504 C1 1500µF x 6 BILL OF MATERIALS Item Description U1 C1, C2 C3, C8 CSS Q1, Q2 RSENSE L1 L2 LX1669 Controller IC Capacitor, 1500µF, 6.3V, 44mΩ ESR Quantity Capacitor, 1µF Capacitor, 0.1µF FET (low RDS(ON)) Sense resistor, 2.5mΩ (PCB trace) Inductor, 2 – 3µH Inductor, 1µH Total Component Count Copyright © 1998 Rev. 2.1 07/98 1 9 2 1 2 1 1 1 18 LinFinity Application Note Page 7 AN-6 Power Solutions for Flexible Motherboards Application Idea 3: LX1664/65 Dual Output Regulator DESCRIPTION KEY FEATURES The LX1664/65 provides a programmable synchronous buck regulator controller together with a linear regulator driver (external MOSFET). The main output is programmable between 1.3 and 3.5V using a 5-bit, TTL-compatible voltage identification (VID) code. n PWM Output Programmable By 5-Bit VID Code n External Adjustable Linear Regulator Driver (Requires No Resistors For Setting 1.5V Output) n Soft-Start And Pulse-By-Pulse Current Limiting n 16-Pin Narrow Body SOIC (LX1664) Or 18-Pin Wide Body SOIC (LX1665) Packages n OVP Driver And Power Good Flag (LX1665 Only) n Accurate Current Sensing By PCB Trace Resistor; Loss-Less Inductor Resistance Or SMD Sense Resistor Methods Advanced fault protection features include internal over-voltage protection (OVP) and pulse-by-pulse current limiting. An OVP pin (LX1665 only) can be used to drive an SCR to clamp the output to ground, or to turn off an ATX power supply in case of an over-voltage condition. The LX1665 also has a Power Good pin. The modulated constant off-time architecture results in simple system design with no external compensation network required. Transient response is enhanced by 40mV adaptive voltage positioning. APPLICATIONS n Pentium II Supplies n Socket 7 Processor Supplies n PowerPC™ And DEC Alpha™ Supplies TYPICAL APPLICATION F1 20A 12V C3 0.1µF C5 1µF U1 LX1665A 1 2 3 4 VID0 5 VID1 6 VID2 7 VID3 8 VID4 9 In volume production. Evaluation board: LXE9010 SS INV VC1 TDRV VCC_CORE GND VID0 BDRV VID1 VCC VID2 CT VID3 OV VID4 LFB L2 1µH LDRV PWRGD 6.3V 1500µF x3 C2 Q1 18 5V IRL3102 17 16 C8 680pF 10 18-pin Wide-Body SOIC VOUT 6.3V, 1500µF x 5 C9 330µF 13 11 Supply Voltage for CPU Core C1 IRL3303 14 12 R1 0.0025 2.5µH Q2 15 L1 Q4 IRLZ44 R5 OV PWRGD Supply Voltage For I/O Chipset or GTL+ Bus C7 330µF R6 BILL OF MATERIALS Item Description Quantity U1 C1, C2 LX1665 Controller IC Capacitor, 1500µF, 6.3V, 44mΩ ESR 1 6 C7, C9 C5 C3 C8 Q1, Q2 Q4 R1 L1 L2 Capacitor, 330µF Capacitor, 1µF Capacitor, 0.1µF Capacitor, 680pF FET (low RDS(ON)) Linear regulator FET (IRLZ44 or similar – RDS(ON) need not be low) Sense resistor, 2.5mΩ (PCB trace) 2 1 1 1 3 1 1 1 1 Inductor, 2 – 3µH Inductor, 1µH (5V-input filter) Total Component Count Copyright © 1998 Rev. 2.1 07/98 19 LinFinity Application Note Page 8 AN-6 Power Solutions for Flexible Motherboards Application Idea 4: LX1662/63 Programmable Regulator… DESCRIPTION KEY FEATURES The LX1662/63 provides a programmable synchronous buck regulator controller suitable for powering the CPU core of advanced microprocessors. The output is programmable between 1.3 and 3.5V using a 5-bit, TTL-compatible voltage identification (VID) code. The LX1662 is the smallest available programmable controller for Pentium II processor power supplies, in a 14-pin narrow body SOIC package. n PWM Output Programmable By 5-Bit VID Code n Soft-Start And Pulse-By-Pulse Current Limiting n 14-Pin Narrow Body SOIC (LX1662) Or 16-Pin Narrow Body SOIC (LX1663) Packages n OVP Driver And Power Good Flag (LX1663 Only) n Accurate Current Sensing By PCB Trace Resistor; Loss-Less Inductor Resistance Or SMD Sense Resistor Methods. n Adaptive Voltage Positioning Enhances Transient Response With Fewer Capacitors Advanced fault protection features include internal over-voltage protection (OVP) and pulse-by-pulse current limiting. An OVP pin (LX1663 only) can be used to drive an SCR to clamp the output to ground, or to turn off an ATX power supply in case of an over-voltage condition. The LX1663 also has a Power Good pin. APPLICATIONS The modulated constant off-time architecture results in simple system design with no external compensation network required. Transient response is enhanced by 40mV adaptive voltage positioning. Pentium II supplies Socket 7 processor supplies PowerPC™ and DEC Alpha™ supplies DC:DC converters n n n n TYPICAL APPLICATION 12V 5V L2 1µH C3 0.1µF U1 LX1662A 1 2 3 4 VID0 VID1 VID2 VID3 C5 1µF VC1 14 INV TDRV 13 VCC_CORE GND 12 BDRV 11 10 SS VID0 5 VID1 VCC 6 VID2 CT 9 7 VID3 VID4 8 6.3V 1500µF x3 C2 Q1 IRL3102 L1, 2.5µH C8 680pF Q2 IRL3303 R1 2.5mΩ 6.3V, 1500µF x 5 Supply Voltage for CPU Core VOUT C1 VID4 14-pin, Narrow Body SOIC In volume production. Evaluation board: LXE9009 Item BILL OF MATERIALS Description Quantity U1 LX1663 controller IC Capacitor, 1500µF, 6.3V, 44mΩ ESR C1, C2 C5 Capacitor, 1µF C3 Capacitor, 0.1µF C8 Capacitor, 680pF Q1, Q2 FET Sense resistor, 2.5mΩ (PCB trace) R1 L1 Inductor, 2 – 3µH L2 Inductor, 1µH (5V-input filter) Total Component Count Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note 1 6 1 1 1 2 1 1 1 15 Page 9 AN-6 Power Solutions for Flexible Motherboards Application Idea 5: LX1660/61 PWM Controller DESCRIPTION KEY FEATURES Hiccup Mode Current-Limiting For Fault Protection Soft-Start 16-Pin Narrow Body SOIC Package Output Voltage Adjustable By Resistor Divider Soft-Start And Pulse-By-Pulse Current Limiting Accurate Current Sensing By PCB Trace Resistor, Loss-Less Inductor Resistance Or SMD Sense Resistor Methods. n Adaptive Voltage Positioning Enhances Transient Response With Fewer Capacitors n n n n n n The LX1660/61 is an adjustable synchronous buck regulator controller suitable for powering microprocessors or other low voltage loads. Advanced fault protection features include internal over-voltage protection (OVP) and hiccup-mode overcurrent protection. The modulated constant off-time architecture results in simple system design with no external compensation network required. The LX1660/61 can be used in synchronous or non-synchronous rectifier configurations. Transient response is enhanced (in the LX1661 only) by 40mV adaptive voltage positioning. The LX1660 does not have adaptive voltage positioning and is ideal for use in applications requiring an accurate DC setpoint, at the expense of transient response voltage tolerance. APPLICATIONS n Graphics and Media Processor Power n Socket 7 Processors The LX1660/61 can also be used with the LX1670 Programmable Reference and Voltage Monitor for a complete Pentium II processor power supply. TYPICAL APPLICATION OUTEN 12V VIN 5V In volume production. OUTPUT VOLTAGE C2 C7 16V, 1000µF Sanyo MV-GX or equivalent U1 LX1661 R15 2.0k 1% 1 2 3 C4 390pF 4 5 6 7 8 C3 0.1µF EN VC1 OTADJ TDRV SGND PGND VREF BDRV INV VCC NINV SYNCEN HICCUP CT C8 390pF CS+ CS- C9 1µF 16 15 806Ω 3.3 1.30kΩ Set R15 = 2kΩ L1 5µH Toroid D1 14 R14 2.8 VOUT = 2 × (1 + R14/R15) Q1 IRL3103 R14, 1% See Table 1 R16 10k VOUT VOUT 13 R1, 5m C5 C6 16V, 1000µF Sanyo MV-GX or equivalent 12 11 R5, 1k 10 9 C1 390pF 16-pin SOIC R6, 1k BILL OF MATERIALS Item Description Quantity U1 C2, C5, C6, C7 C9 C3 C1, C4, C8 Q1 D1 R1 R5, R6 R14, R15 L1 LX1661 controller IC Capacitor, 1500µF, 6.3V, 44mΩ ESR Capacitor, 1µF Capacitor, 0.1µF Capacitor, 390pF FET Schottky diode Sense resistor, 5mΩ (PCB trace) Resistor, 1kΩ Resistors – see table Inductor, 2 – 3µH Total Component Count Copyright © 1998 Rev. 2.1 07/98 1 4 1 1 3 1 1 1 2 2 1 18 LinFinity Application Note Page 10 AN-6 Power Solutions for Flexible Motherboards Application Idea 6: High Current, Low Cost 5V to 3.3V DC:DC Converter DESCRIPTION KEY FEATURES The LX1553 is a current-mode PWM controller, and can be used for a 5V – 3.3V regulator at 10A or more for I/O, chipset and memory supplies where an ATX power supply is not available, and the current levels are too high for a low dropout regulator. The LX1553 is designed for low cost, not optimal transient response, making it ideal for this application. The output voltage is adjusted by means of the resistors R1 and R2 according to the following equation: n Low cost, high current 5V – 3.3V regulator solution n SO-8 package APPLICATIONS n Graphics and media processor power n 5V – 3.3V conversion for chipsets, I/O and memory on motherboards VOUT = VREF * (1 + R2/R1) where VREF = 2.5V TYPICAL APPLICATION 12VIN In volume production. Evaluation board: LXE9012 5VIN C1, C2 1500µF, 6.3V x2 R5, 20k R2 1k 1% C5 0.01µF R4 4.7k, 5% R1 3.1k 1% R3 1.2k C6 0.010µF U1 Q1 LX1553 COM VREF VFB VCC ISENSE RT/CT 3.3VOUT L1 5µH OUTPUT GND D1 MBR2545CT C3, C4 1500µF, 6.3V x2 C7 1µF, 25V BILL OF MATERIALS Item Description Quantity U1 PWM Controller IC, LX1553, Linfinity C1 – C4 Capacitor, 1500µF, 6.3V C7 Capacitor, 1µF C5, C6 Capacitor, 0.1µF Q1 FET D1 Schottky diode Resistor, 20kΩ R5 Resistor, 1.2kΩ R3 R1, R2 See table for details L1 Inductor, 5µH Total Component Count Copyright © 1998 Rev. 2.1 07/98 LinFinity Application Note 1 4 1 2 1 1 1 1 2 1 15 Page 11