60V, 4A Synchronous Monolithic Step-Down Regulator Has Rail-to Rail Operation Design Note 550 Victor Khasiev Introduction The LTC3649 is a high efficiency synchronous monolithic step-down regulator, which integrates top and bottom N-channel MOSFETs on the die. This regulator features a wide input voltage range, 3.1V to 60V, and a wide output voltage range from 0V to (VIN − 0.5V). This extremely wide range of operating voltages makes the LTC3649 especially attractive for industrial, medical and transportation applications, where the rail voltage can be less than 5V or above 40V during voltage transient events. The input voltage range of the LTC3649 also covers mass produced and widely accepted 12V and 24V solar panels, where it is used as a downstream converter in alternative energy systems. In contrast to the majority of DC/DC controllers and regulators, the output voltage of LTC3649 can be programmed by either a single resistor or the reference of its internal error amplifier. It also features a current monitoring output, IMON. The LTC3649’s unique comR2 2.1k 4.7n bination of features opens up interesting applications in power adapters and portable computer systems. Circuit Description and Functionality A 5V output power supply is shown in Figure 1. This circuit is centered on the extremely small footprint high voltage step-down DC/DC LTC3649. The input voltage extends up to 60V and the output is set to 5V at 4A. Only a few additional components are required for the complete solution, including inductor L1 and a few passive components. The feedback loop can be closed by external components off the ITH pin or by internal compensation. The ITH pin should be connected to INTVCC through optional resistor RITH if its internal loop compensation is used. The output voltage is set by resistor RST. Despite the small size of the converter and integrated switching MOSFETs, efficiency reaches 95%, as shown in Figure 2. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. RITH OPT 10pF 2.2µF 10nF INTVCC ITH RST 100k VINREG ISET 10k LTC3649 IMON GND 470pF SGND INTVCC 100k 357k 1M 137k VIN MODE/SYNC RUN PGOOD GND RT PGDFB GND GND CIN1 SUN ELECT. 63CE10KX CIN2 MURATA GRM32ER71J106KA12L L1 COILCRAFT XAL5050-562MEB INTVCC 10k VIN VIN SW SW SW SW SW SW SW BOOST VOUT EXTVCC VIN 5.5V-60V CIN2 10µF L1 5.6µH C6 0.1µF + CIN1 10µF/63V VOUT 5V AT 4A COUT2 10µF + COUT1 150µF 16V 100Ω 10Ω dn550 F04 Figure 1. LTC3649 Step-Down Converter Delivers 5V at 4A from Inputs Up to 60V 05/16/550 to the output current. Increases in the load current proportionally change the voltage on the V_TERM output, but the load voltage at the end of the cable remains unchanged, as shown in Figure 4. A similar approach can be used for remote sensing. See the LTC3649 data sheet for details. 95 93 89 87 85 83 81 79 75 0.0 4.5 12VIN 24VIN 48VIN 77 0.5 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (A) 3.5 4.0 OUTPUT VOLTAGE (V) EFFICIENCY (%) 91 dn550 F02 Figure 2. Efficiency vs Load and Input Voltage for Circuit in Figure 1 4.0 3.5 3.0 V_TERM V_LOAD Figure 3 shows the LTC3649 in an application with cable drop due to a remote load. This approach can be used in systems which do not have additional remote sense wires in long power cables. The circuit takes advantage of the current monitoring terminal IMON and the ability set the output voltage with an external resistor. 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (A) 3.5 4.0 dn550 F04 Figure 4. Cable Drop Operation Load Curves Conclusion LTC3649 is a highly integrated, high performance step-down regulator, with the input and output ranges that enable it to satisfy the requirements of industrial test and measurement, and transportation applications, as well as regulating solar panels and portable devices. In this example, two 0.1Ω resistors simulate the impedance of the power wires in a long cable. Resistors RC1 and RC2 set the voltage reference and are fed by two current sources: from pin ISET, a constant 50µA, and from pin IMON, a current proportional RITH 0Ω 2.2µF 10nF RC2 8.06k RC1 59k 470pF ITH INTVCC ISET VINREG VIN LTC3649 VIN IMON GND 100k INTVCC 200k 1M 137k VIN RUN PGOOD GND PGDFB GND GND 10k VIN CIN2 10µF SW SW SW SW SW SW SW SGND MODE/SYNC RT INTVCC BOOST VOUT EXTVCC CIN1 SUN ELECT. 63CE10KX CIN2 MURATA GRM32ER71J106KA12L L1 COILCRAFT XAL5050-562MEB + CIN1 10µF/63V CABLE L1 5.6µH C6 0.1µF 0.1Ω COUT2 10µF + COUT1 150µF/16V 0.1Ω 100Ω 0.1Ω dn550 F03 Figure 3. The LTC3649 Can Provide Cable Drop Compensation for Long PCB Traces Data Sheet Download www.linear.com/LTC3649 Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3161 DN550 LT/AP 0516 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2016