advertisement Cascadable 7A Point-of-Load Monolithic Buck Converter Design Note 387 Peter Guan Introduction Easy-to-use and compact point-of-load power supplies are necessary in systems with widely distributed, high current, low voltage loads. The LTC® 3415 provides a compact, simple and versatile solution. It includes a pair of integrated complementary power MOSFETs (32mΩ top and 25mΩ bottom) and requires no external sense resistor. A complete design requires an inductor and input/output capacitors, and that’s it. The result is a fast, constant frequency, 7A current mode DC/DC switching regulator. Features The overall solution is extremely compact since the LTC3415’s 5mm × 7mm QFN package footprint is small and its high operating frequency of 1.5MHz allows the use of small low-profile surface mount inductors and ceramic capacitors. For loads higher than 7A, multiple LTC3415s can be cascaded to share the load while running mutually anti-phase, which reduces overall ripple at both the input and the output. Other features include: • Spread spectrum operation to reduce system noise • Output tracking for controlled VOUT ramp-up and ramp-down • Output margining for easy system stress testing Operation The LTC3415 offers several operating modes to optimize efficiency and noise reduction: Burst Mode operation, pulse-skipping mode or forced continuous mode. The mode is set by tying the Mode pin to SVIN, SVIN/2 or SGND, respectively. Burst Mode operation offers high efficiency at light load by shutting off the internal power MOSFETs as well as most of the internal circuitry between pulses. Forced continuous mode maintains a constant switching frequency throughout the entire load range, making it easier to filter switching noise for sensitive applications. Pulse-skipping mode allows constant frequency operation until the inductor current reaches zero, at which point it goes into discontinuous operation and finally it will skip cycles. Pulse-skipping mode offers low output voltage ripple while offering efficiency levels between Burst Mode operation and forced continuous mode. , LTC, LT and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 0.1µF 47µF 6.3V 3x SGND 38 1 2 3 4 10pF 5 6 ® • Burst Mode operation to lower quiescent current and boost efficiency during light loads 7 8 9 • Low shutdown current of less than 1µA • 100% duty-cycle for low drop out operation • Phase-lock-loop to allow frequency synchronization of ±50% of nominal frequency 1Ω VIN MODE 10 11 12 37 CLKOUT RUN NC 35 PVIN 34 SVIN 33 ITHM 32 ITH NC SGND TRACK PLLLPF VFB PVIN PVIN PVIN PVIN SW SW LTC3415EUHF 5mm × 7mm QFN SW SW SW SW SW SW MODE PGND (39) PGOOD CLKIN BSEL PHMODE MGN PGND PGND PGND PGND PGND PGND PGND 13 14 0.2µH • Internal or external ITH compensation for ease of use or loop optimization, respectively 36 PVIN 15 16 17 18 31 15k 30 SVIN 29 28 27 26 22pF 30.5k 25 24 23 22 10k 21 20 19 100µF, 6.3V 2x DN387 F01 Figure 1. 3.3V to 1.8V/7A Application 04/06/387 SVIN VOUT 1.8V/7A Figure 1 shows an application of the LTC3415 in a 3.3V to 1.8V/7A step-down converter configuration. Figure 2 shows its efficiency and power loss vs load current in Burst Mode operation. Efficiency reaches as high as 92%. Figure 3 shows its fast transient response to a 5A load step. As shown, VOUT recovers in 10µs with a dip of less than 100mV. Frequency can be changed easily from its nominal 1.5MHz to 1MHz or 2MHz by simply strapping the PLLLPF pin to SGND or SVIN, respectively. Or if a particular frequency is desired, an external clock can be used to synchronize the operating frequency from 750KHz to 2.25MHz with the internal phase-lock-loop. Spread spectrum operation is available for EMI-sensitive applications by tying the CLKIN pin to SVIN. For applications that require controlled output voltage tracking between various outputs in order to prevent excessive current draw or even latch-up during turn-on and turn-off, the LTC3415 has a Track pin that allows the user to program how its output voltage ramps dur95 Greater than 7A Outputs By stacking multiple LTC3415s together, more output power is attained without increasing the number of input and output capacitors. Operating multiple LTC3415s out of phase not only allows accurate current sharing, but it also reduces the overall voltage ripple at both the input and the output, thus allowing fewer capacitors. Figure 5 shows an efficiency curve of the LTC3415 in 1-phase, 2-phase, 3-phase, 4-phase and 6-phase operation. Conclusion With its many operational features and compact total solution size, the LTC3415 is an ideal fit for today’s pointof-load power supplies. It allows for accurate, compact, efficient and scalable power supplies with advanced features, including tracking and margining. 10000 EFFICIENCY 90 85 VOUT2 = 3.3V/7A 500mV/DIV 1000 80 75 100 70 65 POWER LOSS POWER LOSS (mW) EFFICIENCY (%) ing start-up and shutdown. Figure 4 shows the output waveforms of two LTC3415s in track mode. VOUT1 = 1.8V/14A 500mV/DIV 10 60 VIN = 3.3V VOUT = 1.8V BURST MODE 55 50 10 100 1000 LOAD CURRENT (mA) DN387 F04 500µs/DIV 1 10000 Figure 4. Output Tracking of Two LTC3415s DN387 F02 Figure 2. Efficiency and Power Loss of 3.3V to 1.8V/7A Application in Figure 1 100 95 IINDUCTOR 5A/DIV EFFICIENCY (%) 90 VOUT = 1.8V 100mV/DIV AC COUPLED 85 80 1* 2* 3* 4* 6* 75 70 65 60 40µs/DIV VIN = 3.3V L = 0.2µH COUT = 2 × 100µF DN387 F03 IOUT STEP 0A TO 5A 5A/DIV Figure 3. VOUT Transient Response to a 0A to 5A Load Step of the Circuit Shown in Figure 1 55 50 * PHASE OPERATION 1 10 LOAD CURRENT (A) 100 DN387 G05 Figure 5. Efficiency vs Load Current of LTC3415s in Multiphase Operation Data Sheet Download For applications help, call (408) 432-1900, Ext. 2759 www.linear.com Linear Technology Corporation dn387f LT/TP 0406 305K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●