advertisement 3A, 2MHz Monolithic Synchronous Step-Down Regulator Provides a Compact Solution for DDR Memory Termination – Design Note 309 Joey M. Esteves Introduction The LTC ®3413 is a monolithic synchronous, step-down switching regulator that is capable of generating a bus termination voltage for DDR/DDR2 memory applications. While sourcing and sinking up to 3A of current, the LTC3413 allows switching frequencies as high as 2MHz. Increasing the switching frequency makes compact solutions possible by allowing the use of smaller inductors and capacitors. The internal power switches have a mere 85mΩ of on-resistance making it possible to achieve efficiencies as high as 90% while generating an output voltage as low as 0.6V. For improved thermal handling, the LTC3413 is offered in a 16-Lead TSSOP package with an exposed pad. The LTC3413 utilizes a constant frequency, current mode architecture that operates from an input voltage range of 2.25V to 5.5V and provides a regulated output voltage equal to VREF/2. The switching frequency can be set between 300kHz to 2MHz by a single external VIN 2.5V 1 PGOOD RITH 5.11k CITH CC 2200pF 100pF X7R 2 3 RSS 4.7M CSS 330pF X7R PGOOD SW ITH SW VFB PGND CIN1** 100μF 16 L, LT, LTC, LTM, Linear Technology, the Linear logo and OPTI-LOOP are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 15 14 100 4 13 L1* 0.47μH 6 7 8 RT VREF RUN/SS SGND PGND SW SW PVIN 12 11 COUT** 100μF w2 10 VIN = 2.5V f = 1MHz 80 70 60 50 40 30 20 9 GND CIN2** 100μF DN309 F01 *VISHAY DALE IHLP-2525CZ-01 0.47μH **TDK C4532X5R0J107M Figure 1. 2.5V to 1.25V, ±3A DDR Memory Termination Supply 04/03/309_conv 90 VOUT 1.25V ±3A LTC3413 5 ROSC 309k PVIN 3A, 2.5V to 1.25V Step-Down DC/DC Converter Figure 1 illustrates a design solution for a 2.5V to 1.25V step-down DC/DC converter that is capable of sourcing and sinking up to 3A of output current. Efficiency for this circuit is as high as 90% as shown in Figure 2. Because of their low cost and low ESR, ceramic capacitors were selected for the input and output capacitors. Although many switching regulators have difficulty operating with ceramic capacitors and rely on the zero that is generated by the larger ESR of tantalum capacitors, OPTI-LOOP® compensation allows the LTC3413 to operate successfully with ceramic input and output capacitors. The frequency for this circuit is set at 1MHz EFFICIENCY (%) RPG 100k SVIN resistor. Output voltage ripple is inversely proportional to the switching frequency and the inductor value. Having the ability to increase the switching frequency as high as 2MHz allows lower inductor values to be used while still maintaining low output voltage ripple. Because smaller case sizes are usually offered for lower inductor values, the overall solution footprint can be reduced. An internal voltage divider halves the reference voltage, eliminating the need for external resistors to perform this task. 10 0 0.01 0.1 1 LOAD CURRENT (A) 10 DN309 F02 Figure 2. Efficiency vs Load Current, VIN = 2.5V by a single external resistor. Operating at frequencies this high allows the use of smaller external components such as the inductor and capacitors shown in Figure 1. Many DDR termination applications require the bus termination voltage to be stepped down from a higher system voltage while tracking one-half of a reference voltage. This option is allowable in most systems since a reference voltage is typically available. Figure 3 shows a design solution for a 3.3V to 1.25V, ±3A DDR memory termination supply with a 2.5V external reference. VIN 3.3V RPG 100k PGOOD RITH 5.11k CITH CC 2200pF 100pF X7R 2 3 4 6 2.5V RSS 4.7M 7 CSS 330pF X7R PVIN PGOOD SW ITH SW VFB PGND CIN1** 100μF 16 15 100 90 14 8 RT VREF RUN/SS SGND PGND SW SW PVIN VIN = 3.3V f = 1MHz 80 13 L1* 0.47μH VOUT 1.25V ±3A LTC3413 5 ROSC 309k SVIN Conclusion The LTC3413 is a monolithic, synchronous step-down DC/DC converter that is well suited for DDR memory termination applications requiring up to ±3A of output current. Its high switching frequency and internal low RDS(ON) power switches allow the LTC3413 to offer compact, high efficiency design solutions. 12 11 EFFICIENCY (%) 1 Efficiency for this circuit is as high as 90% as shown in Figure 4. Figure 5 shows another design solution for a 3.3V to 0.9V, ±3A termination supply with a 1.8V external reference. Stepping down from a higher system voltage has the advantage of reducing the resistive losses due to the internal power switches, thus improving efficiency. COUT** 100μF w2 70 60 50 40 30 20 10 10 0 0.01 9 0.1 1 LOAD CURRENT (A) GND CIN2** 100μF 10 DN309 F04 DN309 F03 Figure 4. Efficiency vs Load Current, VIN = 3.3V *VISHAY DALE IHLP-2525CZ-01 0.47μH **TDK C4532X5R0J107M Figure 3. 1.25V, ±3A DDR Memory Termination Supply from 3.3V VIN 3.3V 1 RPG 100k PGOOD RITH 10k CITH CC 2200pF 100pF X7R 2 3 4 ROSC 309k 6 RSS 4.7M 7 CSS 330pF X7R PVIN PGOOD SW ITH SW VFB PGND CIN1** 100μF 16 *VISHAY DALE IHLP-2525CZ-01 0.47μH **TDK C4532X5R0J107M †TAIYO YUDEN JMK325BJ226MM †† SANYO POSCAP 4TPD470M 15 14 13 L1* 0.47μH VOUT 0.9V ±3A LTC3413 5 1.8V SVIN 8 RT VREF RUN/SS SGND PGND SW SW PVIN 12 COUT† 22μF 11 + 10 COUT†† 470μF 9 GND CIN2** 100μF DN309 F05 Figure 5. 0.9V, ±3A HSTL Memory Termination Supply Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn309f_conv LT/TP 0403 316.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003