advertisement Dual Monolithic Step-Down Switching Regulator Provides 1.6A Outputs with Reduced EMI and VOUT as Low as 0.8V Design Note 404 Hua (Walker) Bai Introduction Electronic devices are becoming smaller while the power requirements are increasing to satisfy ever more functionality. To preserve power and manage heat, switching regulators are desirable because of their high efficiency compared to linear regulators. The LT3506 and the LT3506A are examples of how to squeeze power out of a supply without overheating the end product. These are dual 1.6A step-down monolithic regulators that simplify the lives of system engineers. They eliminate the need for external power switches, thereby reducing solution size and BOM cost. Their outputs can be as low as 0.8V, satisfying the needs of the latest DSPs. Integrated dual output channels reduce the part count, while anti-phase switching of the two channels maximizes efficiency and reduces input current ripple and EMI. Both 4.7µH CDRH4D22/HP-4R7 NSR0230M2T5G 22µF 10V X5R 52.3k 10k U1 LT3506AEDHD 1 DFLS230L 2 3 4 4.7µF 25V X5R 5 6 DFLS230L 7 8 FB1 BOOST1 SW1 VC1 VIN1 PG1 RUN/SS1 VIN1 VIN2 RUN/SS2 VIN2 PG2 SW2 VC2 BOOST2 FB2 16 220pF 15 NSR0230M2T5G The circuit in Figure 1 generates a 3.3V and a 5V output. The overall efficiency of the circuit (both channels) is shown in Figure 2. The circuit shown in Figure 3 generates a 1.2V and a 1.8V from a 3.6V to 21V input. The wide input voltage of both the LT3506 and the LT3506A (3.6V to 25V) can accept a variety of power sources, from leadacid batteries and 5V rails to unregulated wall adapters and distributed power supplies. , LT, LTC, LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 100k 39k 14 PG1 13 RUN/SS1 90 12 RUN/SS2 6.8V INPUT 11 10 220pF PG2 28k 1nF 1nF 9 GND 0.1µF Typical LT3506A and LT3506 Applications The primary difference between the LT3506A and the LT3506 is the switching frequency. The LT3506A’s switching frequency is 1.1MHz while the LT3506’s is 575kHz. Higher switching frequency allows smaller components. For lower output voltages, i.e., less than 3.3V, the LT3506 is recommended if VOUT/VIN(MAX) is less than 15%. The lower switching frequency option usually results in higher efficiency due to the lower switching and inductor core losses. 100k DN404 F01 85 EFFICIENCY (%) 0.1µF VIN 6.8V TO 25V VOUT1 5V AT 1.6A the LT3506 and the LT3506A have 0.8V high accuracy voltage references. 12V INPUT 80 10k 75 3.5µH CDRH4D22/HP-3R5 31.6k 22µF 10V X5R VOUT2 3.3V AT 1.6A 70 0 500 1000 1500 CURRENT (mA) DN404 F02 Figure 1. LT3506A Application Circuit Provides Dual Outputs of 5V at 1.6A and 3.3V at 1.6A 12/06/404 Figure 2. Overall Circuit Efficiency (Both Outputs Loaded Identically) The LT3506’s high switching frequencies allow the use of small, low profile surface mount inductors and ceramic capacitors, resulting in smaller solution size and lower assembly cost. Furthermore, the low ESR of ceramic capacitors and high switching frequency results in very low, predictable output voltage ripple. The internal supplies of both the LT3506 and LT3506A are powered from VIN1 pins. It is possible to supply power via the VIN2 pin from a different source, or it can be simply tied to VOUT1, provided VOUT1 can supply sufficient current. The efficiency of VOUT2 can be improved when supplying VIN2 from a separate, lower supply voltage. Cascading VIN2 to VOUT1 also enables some low duty cycle applications for VOUT2. The thermally enhanced 16-lead DFN or TSSOP packages have an exposed ground pad on the bottom. This ground must be soldered to a ground pad on the PCB. Adding a dozen thermal vias to this pad improves thermal performance. A higher temperature grade part, I-grade, is also available. 4.7µH TOKO D53LC, TYPE A NSR0230M2T5G 12.4k VIN 10k VOUT1 1.8V AT 1.6A U1 LT3506EDHD 0.1µF 1 DFLS230L 2 3 4 VIN 3.6V TO 21V 22µF 6.3V X5R 4.7µF 25V X5R 5 6 DFLS230L 7 8 FB1 BOOST1 SW1 VC1 VIN1 PG1 VIN1 RUN/SS1 RUN/SS2 VIN2 VIN2 PG2 SW2 VC2 BOOST2 FB2 16 100k 470pF 20k 15 14 PG1 13 RUN/SS1 12 RUN/SS2 11 10 330pF Power Sequencing without Adding Components Supply sequencing is critical in many systems in order to prevent possible latch-up and to improve system reliability. Independent PG (Power Good) indicators, RUN/SS pins and VC pins of the LT3506 and the LT3506A simplify supply sequencing. The PG pin remains low until the FB pin is within 10% of the final regulated output voltage. The easiest way to sequence outputs is to tie the PG1 pin to the VC2 pin. Then remove the pull-up resistor on PG1. This enables VOUT1 to come up before VOUT2 as shown in Figure 4. 2-Phase Switching Eases EMI Concerns A step-down switching regulator draws current from its input supply and its input capacitor, resulting in a large AC current that can generate EMI. The LT3506’s two regulators are synchronized to a single oscillator and switch out of phase by 180 degrees. Compared with two completely independent regulators, the input current ripple of the LT3506 is substantially reduced and its effective frequency doubled, thereby lowering EMI and allowing the use of a smaller input capacitor without reducing efficiency. Conclusion Both the LT3506 and LT3506A step down the output voltage to as low as 0.8V and provide 1.6A of current capability per channel. 2-channel anti-phase switching substantially reduces the input current ripple and eases EMI concerns. The PG, VC, and RUN/SS pins make supply sequencing simple and straightforward. Two package options (leadless and leaded) and two switching frequency options allow optimal solutions for most applications. PG2 24.3k 1nF 1nF VIN 5V/DIV 9 GND 0.1µF VOUT1 2V/DIV 100k DN404 F03 10k VIN NSR0230M2T5G 3.3µH TOKO D53LC, TYPE A VOUT2 2V/DIV 4.99k 47µF 6.3V X5R VOUT2 1.2V AT 1.6A VC2 2V/DIV 20μs/DIV Figure 3. LT3506 Application Circuit Provides Dual Outputs of 1.8V at 1.6A and 1.2V at 1.6A DN404 F04 Figure 4. Start-Up Waveforms of the Circuit in Figure 1 with Power Sequencing Data Sheet Download For applications help, call (408) 432-1900, Ext. 2759 www.linear.com Linear Technology Corporation dn404f LT/TP 1206 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 ●