Embedded DC-DC Converters Using ISL6526 and ISL6527 PWM Controller ICs ® Application Note PRELIMINARY Introduction The ISL6526 and the ISL6527 provide power control and protection for applications requiring low voltage and high power. Both ICs can be biased from voltages between 3.3V and 5V. The ISL6526 and ISL6527 contain a high performance error amplifier, a high accuracy reference, a fixed 300kHz or 600kHz internal oscillator, over-current protection circuitry, and two MOSFET drivers for use in synchronous-rectified buck converters. Both the ISL6526 and the ISL6527 are capable of regulating the output voltage while the DC-DC converter is sinking current. The ISL6527 also allows for an external reference to be used. All these features are packaged in a small 14-lead SOIC or a 16-lead 5x5[mm] MLFP. More complete descriptions of the ISL6526 and the ISL6527 can be found in their respective datasheets [1, 2]. This application note details the ISL6526 and ISL6527 in DC-DC converters for applications requiring a tightly regulated, fixed output voltage. Low-cost applications requiring a DC-DC converter can benefit from one of the designs presented in this application note The ISL6526/27 evaluation board highlights the operation of the ISL6526 and ISL6527 ICs in an embedded application. There are five evaluation boards from which to choose. IC Package 300kHz and 600kHz ISL6526CB ISL6526ACB 14 ld SOIC ISL6526EVAL2 300kHz ISL6526CR 16 ld MLFP ISL6527EVAL1 300kHz and 600kHz ISL6527CB ISL6527ACB 14 ld SOIC 600kHz ISL6527ACR 16 ld MLFP ISL6526EVAL1 ISL6527EVAL2 Quick Start Evaluation The evaluation board is shipped ‘ready to use’ right from the box. The board accepts a range of input voltages from 3.3V to 5V from a standard power supply. The output can be exercised through an external load. There are posts available on the board for introducing power to the board and also for drawing current from the regulated output. Three probe points are also available for use. These probe points provide Kelvin connections to the CPVOUT(TP3) and CT1(TP1) pins on both the ISL6526 and ISL6527. The ENABLE pin on the ISL6526 and the REF_IN pin on the ISL6527 can be probed from TP2. To test the functionality of the ISL6526 or ISL6527, the following equipment is recommended: - An adjustable 0 - 5V, 5A capable bench power supply - Four channel oscilloscope with probes - Precision digital multimeter Power and Load Connections There are 2 sets of terminals that are used to supply the input voltage and load the output. INPUT VOLTAGE The ISL6526EVAL1 board is built with the ISL6526CB and shipped with an ISL6526ACB sample piece. Evaluation of the ISL6526CB can be performed immediately. If evaluation of the ISL6526ACB is desired, then it is a simple matter of replacing the controller. The ISL6527EVAL1 is shipped with the ISL6527CB and shipped with an ISL6527ACB sample piece. As with the ISL6526CB and ISL6526ACB, the parts are interchangeable. The ISL6526EVAL2 and ISL6527EVAL2 are ready for evaluation without any modifications. 1 All evaluation boards have the same output filter, compensation components and MOSFETs. They are configured for an output of 2.5V with a maximum load of 5A. The ISL6526 evaluation boards have a fixed output while the ISL6527 evaluation boards allow the use of a potentiometer to adjust the output voltage from 0.75V to 3.0V. Unless otherwise noted, this application note assumes that the output of the ISL6527 evaluation board is 2.5V. - An electronic load Table 1 - Evaluation Boards Switching Frequency AN1021 Recommended Test Equipment ISL6526/27 Reference Designs Board Name June 2002 Connect the positive lead of the adjustable bench power supply to the 3.3V post (J1). Connect the ground lead of the supply to GND post (J2). OUTPUT LOADING - SOURCING CURRENT Connect the positive terminal of the load to the VOUT post (J3). Connect the return terminal of the same load to the GND post (J4). OUTPUT LOADING - SINKING CURRENT CAUTION: The return terminal of the load must float for this to work properly. 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 registered trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2002. All Rights Reserved AN1021 To observe the output while the regulator sinks current, connect the positive terminal of the load to the 3.3V post (J1). Connect the return terminal of the same load to the VOUT post (J3). Startup There are two distinct start up methods for both the ISL6526 and ISL6527 regulators. The first method is invoked through the application of power to the IC. The soft-start feature allows for a controlled rise of the output once the Power On Reset (POR) threshold of the input voltage has been reached. Figure 1 shows the start up profile of the regulator in relation to the start up of the 3.3V input supply and the bias supply generated by the charge pump. Both the ISL6526 and ISL6527, whether switching at 300 or 600kHz, will have the same start up profile as shown in Figure 1. Shutdown As discussed in the previous section, if the OCSET/SD pin or ENABLE pin is pulled down and held below 0.8V, the regulator will be turned off. The MOSFET gates are immediately pulled low. Figure 3 shows the shutdown profile of the regulator under full load. VOUT 1V/DIV VENABLE 1V/DIV VCPVOUT 1V/DIV 20µs/DIV FIGURE 3. SHUTDOWN WITH FULL LOAD (ISL6526) VVCC 1V/DIV ISL6527 Input Reference VOUT 1V/DIV 2ms/DIV FIGURE 1. START UP FROM POR The second method of start up is through the use of the Enable/Shutdown feature. Holding the ENABLE pin on the ISL6526 or the OCSET/SD pin on the ISL6527 below 0.8V will disable the regulator by forcing both the upper and lower MOSFETs off. Releasing the respective pin allows the regulator to start up. Figure 2 shows the start up profile with this method. A distinguishing feature of the ISL6527 is that the reference used to regulate the output is provided from an external source. This allows the converter to track the output of another regulator at any desired ratio or amplify a small signal. Figure 4 illustrates the tracking and power amplification ability of the ISL6527 with a sinusoidal reference. Figure 4 also illustrates the startup profile. VREF_IN 200mV/DIV VOUT 200mV/DIV VENABLE 1V/DIV 2ms/DIV FIGURE 4. SMALL SIGNAL POWER AMPLIFICATION VOUT 1V/DIV 1ms/DIV FIGURE 2. START UP FROM SHUT DOWN 2 Ripple Voltage Figure 5 shows the ripple voltage on the output of the regulator. The ripple voltages shown are for both the ISL6526 and ISL6527 ICs running at either 300 or 600kHz. AN1021 VOUT at 300kHz 10mV/DIV VOUT at 600kHz 10mV/DIV 5µs/DIV FIGURE 5. OUTPUT RIPPLE VOLTAGE Transient Performance Figures 6 and 7 show the response of the output when subjected to sourcing and sinking transient loading, respectively. VOUT at 300kHz 10mV/DIV VOUT at 600kHz 10mV/DIV Load Current 2A/DIV 200µs/DIV Lossless Output Voltage Droop Droop is an intentional sag in the output voltage that is proportional to the output current. Although not necessary for proper circuit operation, utilizing droop allows the dynamic regulation to be improved by taking advantage of static regulation requirements and expanding the available headroom for transient edge output excursions. In practical applications that are compared to a non-droop implementation, the droop implementation requires fewer output capacitors or better regulation with the same type and number of output capacitors. By moving the regulation point ahead of the output inductor (at the PHASE node), droop becomes equal to the average voltage drop across the output inductor’s DC resistance as well as any distributed resistance. The droop circuitry is simply an RC low pass filter placed across the output inductor. This filter must have the same time constant that the output inductor and it’s corresponding DCR have. The design must be careful to include any parasitic impedances of the PC board if the DCR of the inductor is very low. The ISL6526/27 evaluation board does not have any component footprints that would allow an implementation of droop. This section is included in this application note as a guideline in the event that droop is necessary in a design utilizing the ISL6526 or ISL6527. Figure 8 shows a schematic of the power stage and Type III compensation network of an ISL6526/27 regulator with droop. The droop ISL6526 OR ISL6527 UGATE VIN QU LOUT DCR FIGURE 6. SOURCING TRANSIENT QL LGATE Rparasitic VOUT + RD COUT CD VOUT at 300kHz RFB 10mV/DIV FB VOUT at 600kHz COMP ROS 10mV/DIV Load Current 2A/DIV 200µs/DIV FIGURE 7. SINKING TRANSIENT 3 FIGURE 8. DROOP IMPLEMENTATION circuitry is represented by resistor RD and capacitor CD. The output of this low pass filter is fed directly into the feedback compensation network of the regulator. To insure symmetric output voltage excursions about the nominal voltage in response to load transients, the output voltage should be programmed to be above the nominal level by half the calculated droop. AN1021 L out τ = ----------------------------------------------- = ( R D || R FB ) ⋅ C D DCR + R parasitic Note that the impedance portion of the time constant calculation for the RC filter includes the parallel combination of the filter resistor, RD, and the feedback resistor, RFB. Since the regulation point is now located at the phase node, and a resistor, RD, is being added to the DC path for regulation, then the offset resistor, ROS, must be adjusted. V REF R OS = ---------------------------------------------------------------------------------------------------------- ⋅ ( R FB + R D ) 1 V NOMINAL + --- ⋅ I O ( max ) ⋅ DCR – V REF 2 Where: VREF = 0.8V for ISL6526 VREF = External Reference for ISL6527 Efficiency The ISL6526 and ISL6527 regulators allow for highly efficient systems. The efficiency of the evaluation board in is shown in Figure 10. The efficiencies of the converter for both sinking and sourcing current are identical. 95 94 Efficiency (%) A properly designed droop implementation will have the time constant of the RC filter equal to or very close to the time constant of the output inductor. 93 92 91 90 0 1 2 3 4 5 Load Current (A) Figure 9 shows the output voltage of the converter with the droop circuitry added. FIGURE 10. EFFICIENCY - SINKING AND SOURCING CURRENT Conclusion VMAX WITHOUT DROOP VNOMINAL VMIN WITH DROOP OUTPUT VOLTAGE The ISL6526 and ISL6527 are versatile PWM controllers. Compact and highly efficient regulators can be implemented with either IC. Both ICs offer small footprints and features which make them an ideal for many low voltage power solutions. References 0A OUTPUT CURRENT IO(max) For Intersil documents available on the web, see http://www.intersil.com/ FIGURE 9. OUTPUT VOLTAGE DROOP With the proper selection of the components used in the RC filter across the inductor, the frequency response of the system is only minimally affected and the compensation network does not need to be recalculated. 4 [1] ISL6526 Data Sheet, Intersil Corporation, File No. FN9055 [2] ISL6527 Data Sheet, Intersil Corporation, File No. FN9056 AN1021 Evaluation Board Schematic 3.3V NOTE: All evaluation boards share the same schematic. J1 6527_SD C1 GND J2 VCC 4(2) 3.3V OCSET(/SD) CT1 ISL6526 (ISL6527) C4 5(3) 10(10) 1(15) R7 U2 D1 BOOT CPGND UGATE GND R6527A R9 ENABLE(EA+) LGATE COMP C6 14(14) L1 12(12) 2.5V @ 5A J3 2(16) C8,9 Q1 FB 7(6) 8(7) R6526 R8 C5 13(13) C7 PHASE 9(8) R1 3(1) CT2 TP2 RPOT1 C3 6(4) TP3 CPVOUT R6 C2 U1 11(11) TP1 C10 R3 PB1 R6527B R2 C11 6527_SD R5 R4 C12 GND J4 Pin numbers are designated as follows: The first number is the SOIC pin number. The proceeding number in parentheses is the MLFP pin number. Table 2 - Evaluation Board Specific Bill of Material Ref Des Description Vendor Vendor P/N QTY ISL6526 300kHz Synchronous Buck PWM Controller - SOIC Intersil ISL6526CB 1 Zero Ohm Jumper Various --- 1 Unpopulated Zero Ohm Jumper Various --- 0 ISL6526A 600kHz Synchronous Buck PWM Controller - SOIC Intersil ISL6526ACB 1 ISL6526EVAL1 Evaluation Board U1 R6526 R6527A,R6527B --- The additional IC is bagged and tagged and shipped with the assembled evaluation board ISL6526EVAL2 Evaluation Board U1 R6526 R6527A,R6527B ISL6526 300kHz Synchronous Buck PWM Controller - MLFP Intersil ISL6526CR 1 Zero Ohm Jumper Various --- 1 Unpopulated Zero Ohm Jumper Various --- 0 ISL6527EVAL1 Evaluation Board ISL6527 300kHz Synchronous Buck PWM Controller - SOIC Intersil ISL6527CB 1 R6526 U1 Unpopulated Zero Ohm Jumper Various --- 0 R6527A,R6527B Unpopulated Zero Ohm Jumper Various --- 2 ISL6527A 600kHz Synchronous Buck PWM Controller - SOIC Intersil ISL6527ACB 1 --- The additional IC is bagged and tagged and shipped with the assembled evaluation board 5 AN1021 Table 2 - Evaluation Board Specific Bill of Material (Continued) Ref Des Description Vendor Vendor P/N QTY ISL6527A 600kHz Synchronous Buck PWM Controller - MLFP Intersil ISL6527ACR 1 Unpopulated Zero Ohm Jumper Various --- 0 Zero Ohm Jumper Various --- 2 ISL6527EVAL2 Evaluation Board U1 R6526 R6527A,R6527B Table 3 - Evaluation Board Common Bill of Material Ref Des C1,C7 C2 C3,C8,C9 Vendor Vendor P/N QTY 0.1µF Capacitor, 0603 Description Various --- 2 1000pF Capacitor, 0603 Various --- 1 Panasonic EEF-UE0J151R 3 150µF Capacitor C4 0.22µF Capacitor, 0805 Various --- 1 C5 10µF Capacitor Various --- 1 C6 1µF Capacitor, 0805 Various --- 1 C10 33pF Capacitor, 0603 Various --- 1 C11 5600pF Capacitor, 0603 Various --- 1 C12 8200pF Capacitor, 0603 Various --- 1 D1 Diode, 30mA, 30V Digikey MA732 1 L1 1µH Inductor Panasonic ETQP6F1R0SFA 1 Q1 Dual MOSFET, 8 Pin SOIC Fairchild ITF86110DK8 1 R1 9.76kΩ 1% Resistor, 0603 Various --- 1 R2 6.49kΩ 1% Resistor, 0603 Various --- 1 R3 2.26kΩ 1% Resistor, 0603 Various --- 1 R4 124Ω 1% Resistor, 0603 Various --- 1 R5 1.07kΩ 1% Resistor, 0603 Various --- 1 R6 1.33kΩ 1% Resistor, 0603 Various --- 1 R7 61.9kΩ 1% Resistor, 0603 Various --- 1 R8 13.3kΩ 1% Resistor, 0603 Various --- 1 R9 100kΩ 1% Resistor, 0603 Various --- 1 25 Turn Potentiometer 0Ω to 50kΩ Digikey 3299Y-503-ND 1 Pushbutton, miniature Digikey P8007S-ND 1 RPOT1 PB1 Adjustable Precision Zener Shunt Regulator National LM431CCM3/N1B 1 TP1,TP2,TP3 U2 Test Points Digikey 5002K-ND 3 J1,2,3,4 Test Points Keystone 1514-2 4 6 AN1021 Board Description - MLFP Silk Screen Top Layer Ground Layer Power Layer Bottom Layer 7 AN1021 Board Description - SOIC Silk Screen Top Layer Ground Layer Power Layer Bottom Layer All Intersil U.S. 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, software 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. For information regarding Intersil Corporation and its products, see www.intersil.com 8