advertisement No Design Switching Regulator 5V, 5A Buck (Step-Down) Regulator – Design Note 48 Ron Vinsant Introduction This simple, no design regulator, is a step-down DC/DC converter designed to convert an 8V to 40V input to a regulated 5V output. The 5V output is capable of sourcing up to 5A of output current. This converter is based on the Linear Technology LT®1074 switching regulator IC. This device needs only a few external parts to make up a complete regulator including thermal protection and current limit. This design uses off-the-shelf parts for low cost and easy availability of components. Specifications for the circuit are in Table 1. Circuit Description Figure 1 shows the schematic of the circuit. For the purpose of this explanation assume that the output is at a constant +5V DC and that the input voltage is greater than +8V DC. At intervals of ≈10μs (100kHz) the control portion of the LT1074 turns on the switch transistor between the VIN and VSW pins impressing a voltage across the inductor, L1. This causes current to build up in the inductor while also supplying current to the load and capacitor C1. The control circuit determines when to turn off the switch during the 10μs interval to keep the output voltage at +5V DC. When the switch transistor turns off, the magnetic field in the inductor collapses and the polarity of the voltage across the inductor changes to try and maintain the current in the inductor. This current in the inductor is now directed (due to the change in voltage polarity across the inductor) by the diode, D1, to the load. The current will flow from the inductor until the switch turns on again, (continuous operation) or until the inductor runs out of energy (discontinuous operation). Referring back to Figure 1, the divider circuit of R1 and R2 is used to set the output voltage of the supply against an internal voltage reference of 2.21V DC. R3 and C2 make up the frequency compensation network used to stabilize the feedback loop. Conclusion This Design Note demonstrates a fully characterized step-down converter circuit that is both simple and low cost. This design can be taken and reliably used in a production environment without the need for any custom components. A PC board layout and FAB drawing are available from Linear Technology. 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. Table 1. Performance Summary (Operating Temperature Range 0°C to 50°C) Input Voltage Range Output + 8.0V to +40.0V DC Output Voltage (± 0.15V DC) + 5.00V DC Max Output Current VIN = 8.0V to 40.0V 08/91/48_conv 5.0A DC Typical Output Ripple at IOUT = 4.0A DC at Switching Frequency With Optional Filter (L2 and C4) Without Optional Filter (L2 and C4) 5mVp-p 50mVp-p Load Regulation VIN = 8V At IOUT = 0.5A DC to IOUT = 5.0A DC 0.5% Line Regulation IOUT = 5A At VIN = + 8.0V DC to VIN = +40.0V DC 0.5% A Part of EDN Magazine HS1 +VIN +8V TO +40V 5 L1 50μH VIN VSW +5.00V 4 LT1074CT FRONT VIEW 5 VIN 4 VSW 3 GND 2 VC 1 FB GND 3 L3 5μH VFB 1 VC 2 L2 R3 2k + T PACKAGE 5-LEAD PLASTIC TO-220 LT1074CT +VOUT R1 2.80k C3 470μF 50V D1 MBR745 C2 0.1μF +VOUT* R2 2.21k + HS2 C1 330μF 50V + C4* 39μF 35V RTN RTN * OPTIONAL +VOUT = 2.21 1+ R1 R2 [ %$t4$) ] Figure 1. Package and Schematic Diagrams Table 2. Parts List REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR PCB 1 001A PCB FAB, Buck Switching Regulator LTC D1 1 MBR745 Diode, Schottky, 7A, 45V Motorola HS2 1 6038B-TT Heat Sink Thermalloy L2 1 2664000101 Shield Bead Fair-Rite VR1 1 LT1074CT Switching Regulator, 100kHz LTC HS1 1 7020B-MT Heat Sink Thermalloy C1 1 SXE50VB331M12X20LL Cap, Alum Elect, 330μF, 50V United Chemicon C2 1 CKO6BX104K Cap, Ceramic, 0.1μF, 50V AVX C3 1 UPL1H471MRH Cap, Alum Elect, 470μF, 50V Nichicon C4 1 UPL1V390MAH Cap, Alum Elect, 39μF, 35V Nichicon L1 1 CTX50-5-MP Inductor, 50μH, 5A Coiltronics L3 1 CTX5-5-FR Inductor, 5μH, 5A Coiltronics R1 1 MF 1/8W 2.80kΩ Res, MF, 1/8W, 1%, 2.80k R2 1 MF 1/8W 2.21kΩ Res, MF, 1/8W, 1%, 2.21k R3 1 CF 1/4W 2kΩ Res, CF, 1/4W, 5%, 2k Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn48f_conv BA/GP 0891 180K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1991