LM2688 Non-synchronous Step-down Controller for Pentium Microprocessors General Description Features The LM2688 is a low-cost non-synchronous voltage-mode PWM controller which maintains an accurate and adjustable output voltage for high current, low voltage DC/DC applications such as the Pentium microprocessor family. By using different combinations of feedback resistors, the regulator output voltage can be precisely adjusted to match different requirements. Coming in an 8-lead surface mount package, the LM2688 can be used to control a buck regulator with an load current up to 10A. An on-chip precision trimmed reference allows tight voltage regulation and an internal 150 kHz fixed-frequency oscillator eliminates extra components to set the switching frequency. The LM2688 also provides an ON/ OFF pin which enables the CPU to shutdown the regulator when necessary. Self protection features include a two stage current limit for the output switch and an over temperature shutdown for complete protection under fault conditions. The controller also provides internal loop gain compensation, minimizing the number of external components. n n n n n n n n Adjustable output voltage Available in 8-pin surface mount Guaranteed 0.5A gate drive current Supply voltage up to 40V 150 kHz fixed frequency internal oscillator TTL shutdown capability Low power standby mode, IQ typically < 85 µA Thermal shutdown and current limit protection Applications n Controller for voltage regulators for Pentium, Pentium MMX, K5, K6 MMX, Alpha and PowerPC processors n Efficient pre-regulator for linear regulators Typical Application Pentium Processor Power Supply DS100050-1 For more information about the above application, please refer to the Application Hints section. © 1997 National Semiconductor Corporation DS100050 www.national.com LM2688 Non-synchronous Step-down Controller for Pentium Microprocessors July 1997 Absolute Maximum Ratings Lead Temperature (Note 1) M8 Package If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Supply Volatge 45V ON/OFF Pin Input Voltage −0.3V ≤ V ≤ +25V Feedback Pin Voltage −0.3V ≤ V ≤ +25V Vapor Phase (60 seconds) +215˚C Infrared (15 seconds) +220˚C Maximum Junction Temperature Operating Ratings Output Voltage to Ground −40˚C ≤ TJ ≤ +125˚C Temperature Range (Steady State) −1V Power Dissipation Internally Limited Storage Temperature Range −65˚C to +150˚C ESD Susceptibility Human Body Model (Note 2) +150˚C Supply Voltage 4.5V to 40V 2 kV LM2688-ADJ Electrical Characteristics Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V, IOUT = 100 mA. Symbol Parameter Conditions LM2688-ADJ Typ (Note 3) Limit (Note 4) Units (Limits) SYSTEM PARAMETERS (Note 5) VFB Feedback Voltage Converter Input = 5V VCORE Programmed for 3V η Efficiency Converter Input = 5V, VCORE = 2.8V, ICORE = 10A 1.230 V 86 % DEVICE PARAMETERS Ib Feedback Bias Current VFB = 1.3V 10 fO Oscillator Frequency (Note 6) 150 VSAT DC ICL Saturation Voltage IOUT = 0.5A (Notes 7, 8) 50/100 127/110 kHz(min) 173/173 kHz(max) 1.1/1.2 V(max) 0.9 V Max Duty Cycle (ON) (Note 8) 100 % Min Duty Cycle (OFF) (Note 9) 0 % Current Limit Peak Current, (Notes 7, 8) 0.8 A 0.65/0.58 IL IQ ISTBY θJA nA kHz Output Leakage Current Quiescent Current Standby Quiescent Current Thermal Resistance (Notes 7, 9, 10) Output = 0V Output = −1V (Note 9) 1.3/1.4 A(max) 50 µA(max) 15 mA(max) 10 mA(max) 200/250 µA(max) 2 mA 5 ON/OFF Pin = 5V (OFF) (Note 10) Junction to Ambient (Note 11) A(min) mA 85 µA 150 ˚C/W ON/OFF CONTROL ON/OFF Pin Logic Input VIH Threshold Voltage VIL IH 1.3 Low (Regulator ON) High (Regulator OFF) ON/OFF Pin VLOGIC = 2.5V (Regulator OFF) Input Current www.national.com 2 V 0.6 V(min) 2.0 V(max) 15 µA(max) 5 µA LM2688-ADJ Electrical Characteristics (Continued) Specifications with standard typeface are for TJ = 25˚C, and those with boldface type apply over full Operating Temperature Range. Unless otherwise specified, VIN = 12V, IOUT = 100 mA. Symbol Parameter Conditions LM2688-ADJ Typ (Note 3) Limit (Note 4) Units (Limits) ON/OFF CONTROL VLOGIC = 0.5V (Regulator ON) IL 0.02 µA 5 µA(max) Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin. Note 3: Typical numbers are at 25˚C and represent the most likely norm. Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator system performance. When the LM2688 is used as shown in the typical application, system performance will be shown in system parameters section Electrical Characteristics. Note 6: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current overload. Note 7: No diode, inductor or capacitor connected to output pin. Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON. Note 9: Feedback pin removed from output and connected to 12V to force the output transistor switch OFF. Note 10: VIN = 40V. Note 11: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will lower thermal resistance further. Typical Performance Characteristics Switch Current Limit Minimum Operating Supply Voltage Switching Frequency DS100050-4 DS100050-2 DS100050-3 Quiescent Current Standby Quiescent Current Feedback Pin Bias Current DS100050-5 DS100050-6 3 DS100050-7 www.national.com Typical Performance Characteristics (Continued) ON/OFF Threshold Voltage ON/OFF Pin Current (Sinking) DS100050-8 DS100050-9 Connection Diagram and Ordering Information 8-Lead Surface Mount (M) DS100050-10 *No internal connection, but should be soldered to PC board for best heat transfer. Top View Order Number LM2688M-ADJ See NS Package Number M08A LM2688 Block Diagram DS100050-11 www.national.com 4 Output Filter Application Hints The output filter plays an extremely important role in meeting the load transient requirement. From the viewpoint of load transient response, it is desirable to have a low inductance and a high output capacitance. However, too low an inductance causes huge ripple current through the MOSFETs which translates into lower efficiency. Also too big a bank of output capacitors may not be economical in terms of both cost of the regulator and motherboard real estate. It may also require a larger input capacitor bank to stay within input di/dt specification. The output voltage ripple is determined by the amount of ripple current in the inductor and the ESR of the output capacitor bank. For the typical situation where Pentium processor is the load, a 2.5µH inductor and a bank of three 1500µF electrolytic capacitors are recommended. Sanyo’s ultra low-ESR electrolytic capacitor, the 6MV1500GX, is recommended. GENERAL This is a cost effective non-synchronous buck solution for powering Pentium and similar microprocessors (AMD’s K5, Motorola’s PowerPC, etc.) that draws large current at a low voltage. Fairly high efficiency of 86% or better at 10A load can be achieved by properly selecting the components. PIN FUNCTIONS +VIN—This is the positive input supply for the controller to work. A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the current needed to drive the external MOSFET. Ground—Circuit ground. Output—Sources pulsed current up to 1.4A maximum to drive external MOSFET. To minimize turn-on delay of the external MOSFET, copper trace between this pin and the gate of the MOSFET should be kept as short as possible. Feedback—Senses the regulator output voltage so that a feedback control loop is achieved. A resistor divider can be used to choose the desired output voltage. ON/OFF—Allows the controller to be shut down by a logic level signal. If the shutdown feature is not needed, the pin can be either grounded or left open, both of which enable the controller. MOSFET Selection It is desirable to have the on-resistance of the MOSFET as low as possible so that its conduction loss is minimized and high efficiency can be maintained. A good idea is to use two low-Rdson MOSFETs in parallel so that the total conduction loss is halved compared with using one MOSFET. In the typical application circuit, two Fairchild low Rdson (10 mΩ) MOSFETs, the NDS8410A, are used in parallel. Diode Selection Since current flows through the freewheeling diode when the MOSFETs are off, efficiency is affected greatly by the forward voltage drop of the diode. Generally a Schottky diode is used here. Motorola MBR2515L Schottky diode has a forward voltage drop of less than 0.4V at 20A which fits very well in this application. COMPONENTS SELECTION Input Filter Depending on the system requirement, an inductor may or may not be necessary. When there is no explicit input di/dt limitation, the filter can be composed of capacitors only. The inductor plays an important role in limiting the input di/dt so that the input power rail appears a quiet supply for other loads. It also limits the inrush current during power on. The best inductance value is too complicated to calculate but can be easily determined by experiment. For a typical Pentium motherboard application, 2µH is recommended. The bulk capacitors in the input filter should be of low ESR type or otherwise the input di/dt requirement may not be met. A low cost low ESR electrolytic capacitor manufactured by United Chemicon, LXV16VB102M, is recommended here. Output Voltage Adjustment Different output voltages can be obtained by using different combinations of feedback resistors. The formula to calculate output voltage is VCORE=(R2/R3+1)X1.235V. In the “typical application” circuit, two jumpers are used to adjust the value of R2 so that four output voltages can be obtained. When necessary, C9 and C10 can be used to further compensate the converter to result in a more stable circuit, the penalty is a slower transient recovery speed. Bill of Material Label Value Type Part Number Manufacturer C1 1000µ Capacitor LXV16VB102M United Chemi-Con C2 1000µ Capacitor LXV16VB102M United Chemi-Con C3 1000µ Capacitor LXV16VB102M United Chemi-Con C4 10µ Capacitor, Tantalum C5 0.1µ Capacitor, Ceramic C6 1500µ Capacitor 6MV1500GX Sanyo C7 1500µ Capacitor 6MV1500GX Sanyo C8 1500µ Capacitor 6MV1500GX Sanyo C9 1n Capacitor, Ceramic C10 15n Capacitor, Ceramic L1 2.5µ Inductor R1 300, 0.5W Resistor R2 3.65k 1% Resistor R3 2k 1% Resistor 5 www.national.com Bill of Material Label R6 (Continued) Value 10 Type Part Number Manufacturer Resistor Q1 BJT, PNP 3906 National U1 IC, PWM LM2688M-ADJ National D1 Diode, Schottky MBR2515L Motorola D2 Diode 1N4148 National M1 MOSFET, 10 mΩ Rdson NDS8410A Fairchild M2 MOSFET, 10 mΩ Rdson NDS8410A Fairchild www.national.com 6 7 LM2688 Non-synchronous Step-down Controller for Pentium Microprocessors Physical Dimensions inches (millimeters) unless otherwise noted Order Number LM2688M-ADJ See NS Package Number M08A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. 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