advertisement 2-Phase Switching Regulator Fits in Tight Places Design Note 210 Steve Hobrecht The third generation switching regulator controllers from Linear Technology address these new requirements. The LTC ®1628, a 2-phase, constant frequency, dual synchronous buck controller, has been specifically designed to minimize both the size and total cost of the power system while delivering the highest efficiency in the industry. A 2-phase switching technique minimizes the peak and resultant RMS input current while doubling the effective frequency of the input current pulses. The input capacitor ESR requirement is reduced by 40% and the capacitance requirement is reduced by 75% when compared to a single phase controller solution! Ceramic input capacitors provide an ideal choice for the resultant requirements. Figure 1 contrasts the power loss due to the input RMS ripple current for a single and 2-phase solution over the 5V to 25V input voltage range for a 5V/3A and 3.3V/3A power supply. The input path resistive losses include the ESR of the input capacitor, internal battery resistance, PC trace/ connector losses, fuse resistance and diode or MOSFET resistance that may be used in the power path for switching power sources. The third generation controllers have also been designed to minimize the output capacitor requirements. External OPTI-LOOP® compensation allows optimal loop compensation for the wide range of characteristics associated with switching power supply output capaci08/99/210_conv 8 INPUT PATH POWER LOSS (W/Ω) Almost every week manufacturers present yet another component that has been reduced in size by 30% to 50% while maintaining similar performance characteristics. The size and weight requirements of the notebook and sub-notebook computer market has resulted in the development of increasingly smaller components. These consumer products have placed very stringent requirements on size, weight and run time. Because of component density, heat dissipation has also become one of the highest concerns for notebook computers and other portable equipment. 7 SINGLE PHASE 6 VOUT1 = 5V/3A VOUT2 = 3.3V/3A 5 POWER SAVED 4 3 2 2-PHASE 1 0 0 5 10 15 INPUT VOLTAGE (V) 20 25 DN210 F01 Figure 1. Input Path Power Loss tors. A unique Burst Mode® architecture keeps the low current output voltage ripple down to a value close to the normal value for the continuous mode of operation, even when using output capacitors in the 47μF range. The output capacitor ESR requirement is determined by the expected load step and the capacitance value is determined by output current and speed of the feedback loop. A higher capacitance value cannot make up for high ESR and very low ESR cannot make up for insufficient capacitance. A third key element in reducing solution size is an internal current foldback circuit that operates when the output voltage drops below 70% of the nominal value. This feature enables the use of MOSFETs properly sized for the output load current without additional power dissipation margin required for short-circuit conditions. The application shown in Figure 2 occupies less than two square inches of PC board area (components on one side only) and delivers 5V/3A, 3.3V/4A, and 12V/150mA—the three main system power supplies for a notebook computer. The application in this Design Note addresses very common requirements of small L, LT, LTC, LTM, Linear Technology, the Linear logo OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. R20 66.5k 1% C13 27pF R5 20k 1% + R9 1M 1 2 R3 105k, 1% C11 1000pF 3 4 C20 0.01μF 5 C21 0.01μF 6 3.3V STANDBY 8 C9 27pF 9 C10 27pF 10 R7 6.8k C8 470pF 11 12 R7 6.8k C14 27pF 13 R4 63.4k 1% C12 1000pF 14 SENSE1 + TG1 SENSE1 – SW1 VOSENSE1 BOOST1 FREQSET VIN STBYMD BG1 FCB EXTVCC LTC1628 ITH1 INTVCC SGND PGND 3.3VOUT BG2 BOOST2 ITH2 VOSENSE2 SW2 SENSE2 – TG2 SENSE2 + RUN/SS2 D5 10MQ100N 28 T1 1:1.8 C23 4.7μF 10V R1 0.015Ω 27 VOUT1 5V 3A 26 C3 0.1μF 25 M1A M1B D1 MBRM 140T3 24 23 R11 10Ω 22 D3 C18Ω 0.1μF 21 20 C1 47μF 6.3V SP CAP 22μF 50V CER C15 1μF D4 C16 4.7μF C2 56μF 4V SP CAP 19 17 16 ON/OFF R7 221k 1% R18 100k 1% + VOUT3 12V 150mA C22 1μF 25V M2A M2B L1 8μH VIN 7V TO 28V D2 MBRM 140T3 18 C4 0.1μF LT1121 + R7 15k FLTCPL + C7 220pF 7 RUN/SS1 + C5 0.1μF R10 1M R19 1M 1% R2 0.015Ω 15 VOUT2 3.3V 3A C6 0.1μF DN210 F02 5V STANDBY VIN: 7V TO 28V VOUT: 5V 3A, 3.3V 3A, 12V 150mA SWITCHING FREQUENCY = 220kHz D3, D4: CENTRAL CMDSH-3 M1, M2: FAIRCHILD FDS8936A L1: SUMIDA CEP123-8R0MC T1: 9μH 1:1.8 — BH ELECTRONICS 501-0657 Figure 2. Two Square Inch 5V/3.3V/12V Portable Power Supply physical size and dynamic 1A step load changes. Single, low ESR capacitors were used for the input and each output. Dual-packaged MOSFETs are used to further minimize PC board space. The inductors were selected based upon the maximum inductance for a reasonable case size while meeting peak current requirements. The photo in Figure 3 shows normal output ripple voltage and transient response to a 1.5A to 2.5A load step in continuous mode (5V output). The output ripple voltage is 25mVP-P and the 1A load steps result in 60mV output voltage transients. The output ripple voltage in Burst Mode operation is 70mVP-P. The efficiency of the power supply with a 15V input voltage and 5V/3A, 3.3V/3A and 12V/120mA output voltages measures 91%. Other features of the LTC1628 include: • A 0.8V reference and 1% output voltage accuracy over line, load and temperature variations • 7.5% overvoltage “soft-latch” • Defeatable overcurrent latch-off timer • Adjustable soft-start • 25μA shutdown current 2.5A • 5V and 3.3V standby linear voltage regulators 1.5A DN210 F03 Figure 3. Upper Trace: 5V Output, Transient Load Response (50mV/DIV) Bottom Trace: Load Current (1A/DIV) Data Sheet Download www.linear.com Linear Technology Corporation • Loop bandwidth normally limited only by external inductor, output capacitor, switching frequency and the quality of the printed circuit board design For applications help, call (408) 432-1900 dn210f_conv LT/TP 0899 340K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1999