Using the ISL6430 in DC-DC Converters for Microprocessors in Gateway Applications TM Application Note September 2001 AN9974 Introduction Intersil ISL6430 Today’s high-performance microprocessors present many challenges to their power source. High power consumption, low bus voltages, and fast load changes are the principal characteristics which have led to the need for a switch-mode DC-DC converter local to the microprocessor. A common requirement of these and similar processors are decreasing supply voltages as the processor clock frequency increases. The Intersil ISL6430 is a voltage-mode controller with many functions needed for high-performance processors. Figure 1 shows a simple block diagram of the ISL6430. The device contains a high-performance error amplifier, a high-accuracy reference, a programmable free-running oscillator, and overcurrent protection circuitry. The ISL6430 has two MOSFET drivers for use in synchronous-rectified buck converters. A more complete description of the parts can be found in their data sheets. The Intersil ISL6430 pulse-width modulator (PWM) controllers are targeted specifically for DC-DC converters powering high-performance microprocessors with varying core voltage requirements. ISL6430EVAL1 This device provides a cost-effective solutions for point-ofuse switch-mode, DC-DC converters for many applications. This application note details the use of the ISL6430 in DC-DC converters for high-performance microprocessors with a fixed core voltage. VCC MONITOR AND PROTECTION EN BOOT OSC UGATE ISL6430 PHASE REF FB PVCC + - LGATE + 90 PGND GND COMP NOT PRESENT (PINS NC) ON HIP6007 FIGURE 1. BLOCK DIAGRAM OF ISL6430 EFFICIENCY (%) RT Efficiency Figure 2 displays the ISL6430EVAL1 efficiency versus load current for both 5V and 12V inputs with 100 linear feet per minute (LFM) of airflow. For a given output voltage and load, the efficiency is lower at higher input voltages. This is due primarily to higher MOSFET switching losses and is displayed in Figure 2. OCSET SS The ISL6430EVAL1 is a synchronous buck converter capable of providing up to 9A of current at a fixed 2.5V output voltages. Simple resistor value changes allow for outputs as low as 1.3V. The schematic and bill-of-materials for this design can be found in the appendix. VIN = 5V 85 VIN = 12V 80 75 2 4 6 LOAD CURRENT (A) 8 10 FIGURE 2. ISL6430EVAL1 EFFICIENCY vs LOAD 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 321-724-7143 | Intersil and Design is a trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2001, All Rights Reserved Application Note 9974 OC Protection 2.50V VOUT 50mV/DIV COMP 2V/DIV 0V IL 5A/DIV The ISL6430EVAL1 has lossless overcurrent (OC) protection. This is accomplished via the current-sense function, which senses converter load current by monitoring the drop across the upper MOSFET (Q1 in the schematics). By selecting the appropriate value of the OCSET resistor (R6), an overcurrent protection scheme is employed without the cost and power loss associated with an external currentsense resistor. See the Over-Current Protection section of the ISL6430 data sheet for details on the design procedure for the OCSET resistor. Customization of Reference Designs 0A TIME (10ms/DIV) FIGURE 3. ISL6430EVAL1 TRANSIENT RESPONSE WITH V IN = 12V The ISL6430EVAL1 reference design provides solutions for microprocessors with current demands of up to 9A. One basic design is employed to meet many different applications. The evaluation board can be powered from +5V or +12V and a synchronous buck topology may be employed. Employing one basic design for numerous applications involves some trade-offs. These trade-offs are discussed below to help the user optimize for a given application. Control Loop Bandwidth/Transient Response VOUT 20mV/DIV IL 1A/DIV TIME (1ms/DIV) FIGURE 4. ISL6430EVAL1 OUTPUT VOLTAGE RIPPLE Transient Response Figure 3 shows a laboratory oscillogram of the ISL6430EVAL1 in response to a 0-9A load transient application. The output voltage responds rapidly and is within 1% of its nominal value in less than 15µs. Check the Feedback Compensation section of the data sheet for details on loop compensation design. Table 1 details simulated closed-loop bandwidth and phase margin for both reference boards at both +5V and +12V input sources. Table 1 shows how the control loop characteristics vary with line voltage and topology. The line voltage determines the amount of DC gain, which directly affects the modulator (control-to-output) transfer function. The topology (standard buck or synchronous buck) is important because we have chosen to use a larger output inductor for the standard buck (HIP6005) design. This lowers the boundary of continuous conduction mode (ccm) and discontinuous conduction mode (dcm) operation. Staying in ccm at light loads can have an adverse affect on transient response of the converter. The ISL6430EVAL1 design will not go into dcm operation because the lower MOSFET conducts current even at light or zero load conditions. From Table 1, we see that the highest control loop bandwidth is with VIN = 12V. The transient response of the converter for this case is shown in Figure 3. The other cases have slower responding loops and can be improved with value changes in the compensation components. Table 2 details suggested changes and the improved control loop characteristics for the three applications with slower control loops. TABLE 2. MODIFICATIONS TO CONTROL LOOP ISL6430EVAL1 VIN = 5V VIN = 12V R5 30.1K no change C14 no change no change f0dB 47kHz 61kHz ϕMARGIN 53o 62o TABLE 1. CONTROL LOOP CHARACTERISTICS ISL6430EVAL1 VIN = 5V VIN = 12V f0dB 27kHz 61kHz ϕMARGIN 72o 62o 2 Application Note 9974 Ripple Voltage ∆V OUT = ∆IL • ESR VIN = 5V, RFP25N05 VIN = 5V, RFP45N06 90 EFFICIENCY (%) The amount of ripple voltage on the output of the DC-DC converter varies with input voltage, switching frequency, output inductor, and output capacitors. For a fixed switching frequency and output filter, the voltage ripple increases with higher input voltage. The ripple content of the output voltage can be estimated with the following simple equation: 85 VIN = 12V, RFP25N05 80 VIN = 12V, RFP45N06 75 where V OUT ( VIN – VOUT ) • ---------------- • Ts VIN ∆IL = ----------------------------------------------------------------------L OU T ESR = equivalent series resistance of output capacitors 2 4 6 LOAD CURRENT (A) 8 10 FIGURE 5. ISL6430EVAL1 EFFICIENCY WITH EITHER RFP25N05 MOSFETs OR RFP45N06 MOSFETs Ts = switching period (1/Fs) Conclusion LOUT = output inductance The ISL6430EVAL1 is a DC-DC converter reference design for microprocessors with fixed core voltages and current requirements of up to 9A. In addition, the design can be modified for applications with different requirements. The printed circuit board is laid out to accommodate the necessary components for operation at currents up to 15A. Therefore, for equivalent output ripple performance at VIN = 12V as at 5V, the output filter or switching frequency must change. Assuming 200kHz operation is desired, either the output inductor value should increase or the number of parallel output capacitors should increase (to decrease the effective ESR). Increased Output Power Capability The ISL6430EVAL1 printed circuit board is laid out with flexibility to increase the power level of the DC-DC converter beyond 9A. Locations for additional input capacitors and output capacitors are provided. In conjunction with higher current MOSFETs, Schottky rectifiers, and inductors, the evaluation board can be tailored for applications requiring upwards of 15A. The ISL6430 data sheet’s Component Selection Guidelines section helps the user with the design issues for these applications. Of course, the ISL6430EVAL1 can be modified for more cost-effective solutions at lower currents as well. MOSFET Selection As a supplement to the data sheets’ application information on MOSFET Selection Considerations, this section shows graphically that a larger, lower rDS(ON) MOSFET does not always improve converter efficiency. Figure 8 shows that smaller RFP25N05 MOSFETs are more efficient over most of the line and load range than larger RFP45N06 MOSFETs. The RFP25N05 (used on the ISL6430/7EVAL1) has a rDS(ON) equal to 47mΩ (maximum at 25oC) versus 28mΩ for the RFP45N06. In comparison to the RFP25N05, the RFP45N06 MOSFETs increased switching losses are greater than its decreased conduction losses at load currents up to about 7A with a 5V input and about 9A with a 12V input. 4-3 References For Intersil documents available on the web, see http://www.intersil.com/ Application Note 9974 12VCC VIN C1-3 3 x 680µF C17-18 2 x 1µF 1206 RTN C12 1µF 1206 R7 10K C19 VCC EN 6 SS 3 ENABLE CR1 4148 1000pF 14 2 OCSET MONITOR AND PROTECTION R6 3.01k PHASE TP2 10 BOOT RT 1 Q1 C13 0.1µF U1 R1 SPARE FB 5 C14 33pF C15 0.01µF C16 15K Q2 12 LGATE + + - R5 SPARE R3 1K VOUT 13 PVCC 11 PGND 4 R2 1K L1 8 PHASE ISL6430 REF C20 0.1µF 9 UGATE OSC GND JP1 COMP TP1 R4 SPARE FIGURE 6. ISL6430EVAL1 SCHEMATIC 4 C6-9 4 x 1000µF RTN 7 COMP CR2 MBR 340 Application Note 9974 Bill of Materials for ISL6430EVAL1 PART NUMBER DESCRIPTION PACKAGE QTY REF VENDOR 25MV680GX 680µF, 25V Aluminum Capacitor Radial 10 x 22 3 C1 - C3 Sanyo 6MV1000GX 1000µF, 6.3V Aluminum Capacitor Radial 8 x 20 4 C6 - C9 Sanyo 1206YZ105MAT1A 1.0µF, 16V, X7S Ceramic Capacitor 1206 3 C12, C17-C18 AVX 1000pF Ceramic 1nF, X7R Ceramic Capacitor 0805 1 C19 Various 0.1uF Ceramic 0.1µF, 25V X7R Ceramic Capacitor 0805 2 C13, C20 AVX/Panasonic 0.01uF Ceramic 0.01µF, X7R Ceramic Capacitor 0805 1 C15 Various 33pF Ceramic 33pF, X7R Ceramic Capacitor 0805 1 C14 Various Spare Spare Ceramic Capacitor 0805 1N4148 Rectifier 75V DO35 1 CR1 Various MBR340 3A, 40V, Schottky Axial 1 CR2 Motorola CTX09-13313-X1 PO343 5.3µH, 12A Inductor T50-52B Core, 10 Turns of 16 AWG Wire Wound Toroid 1 L1 Coiltronics Pulse RFP25N05 47mΩ, 50V MOSFET TO220 2 Q1, Q2 Intersil ISL6430 Synchronous Rectified Buck Controller SOIC-14 1 U1 Intersil 10kΩ 10kΩ, 5% 0.1W, Resistor 0805 1 R7 Various Spare Spare 0.1W, Resistor 0805 15kΩ 15kΩ, 5%, 0.1W, Resistor 0805 1 R5 Various 1kΩ 1kΩ, 5%, 0.1W, Resistor 0805 2 R2-R3 Various 3.01kΩ 3.01kΩ, 1%, 0.1W, Resistor 0805 1 R6 Various 576802B00000 TO-220 Clip-on Heatsink 2 1514-2 Terminal Post 6 VIN, 12VCC, VOUT, RTN Keystone 1314353-00 Scope Probe Test Point 1 VOUT Tektronics SPCJ-123-01 Test Point 3 ENABLE, TP1, TP2 Jolo 4-5 C16 R1,R4 AAVID Application Note 9974 All Intersil products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. 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