advertisement No Design Switching Regulator 5V Buck-Boost (Positive-to-Negative) Regulator – Design Note 49 Ron Vinsant Introduction This simple, no design regulator, operates with an input between 4.5V DC and 40V DC. It provides a –5V output at a maximum output current of 1A to 3A depending on input voltage. 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 +4.5V 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. 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). C2 is a low ESR type electrolytic capacitor that is used in conjunction with L1 as the output filter. C5 and L2 form a post filter that reduces output ripple further. Referring back to Figure 1, the divider circuit of R1, R2, R3 and R4 is used to set the output voltage of the supply against an internal voltage reference of 2.21V DC. R3, R4, C3 and C4 make up the frequency compensation network used to stabilize the feedback loop. Conclusion This Design Note demonstrates a fully characterized positive to negative 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 magnetics. A P.C. 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 09/91/49_conv +4.5V to +40.0V DC Output Voltage (± 0.15V DC) – 5.00V DC Max Output Current At VIN = 4.5V DC 1.0A DC Max Output Current At VIN = 40.0V DC 3.5A DC Typical Output Ripple at IOUT = 2.5A DC at Switching Frequency With Optional Filter (L2 and C5) Without Optional Filter (L2 and C5) 50mVp-p 300mVp-p Load Regulation VIN = 4.5V DC At IOUT = 0.1A DC to 1.0A DC 0.6% Line Regulation ILOAD = 1A At VIN = 4.5V DC to 40.0V DC 0.2% A Part of EDN Magazine –VIN COMMON + $ 220μF 50V 5 +VIN - ) )4 VIN V48 R3 L )4 VC 3 –VOUT* R4 L FB C5* 39μF 35V 4 73-5$5 GND + 3 2.74k % MBR745 2 C4 0.047μF C3 ' R2 L L2* ) + C2 ' 7 –VOUT * OPTIONAL ] [ 333 o7 OUT R2 %$t4$) FRONT VIEW 5 VIN 4 VSW 3 GND 2 VC 1 FB T PACKAGE 5-LEAD PLASTIC TO-220 Figure 1. Package and Schematic Diagrams Table 2. Parts List REFERENCE DESIGNATOR QUANTITY PCB 1 PART NUMBER DESCRIPTION VENDOR 003A PCB FAB, Buck-Boost Converter LTC D1 1 MBR745 Diode, Schottky, 7A, 45V Motorola HS2 1 6038B-TT Heat Sink Thermalloy VR1 1 LT1074CT Switching Regulator, 100kHz LTC HS1 1 7020B-MT Heat Sink Thermalloy C1 1 UPL1H221MPH Cap, Alum Elect, Low ESR, 220μF, 50V Nichicon C2 1 LXF10VB272M12X30LL Cap, Alum Elect, Low ESR, 1000μF, 10V United Chemicon C3 1 CKO6BX103K Cap, Ceramic, 0.01μF, 100V AVX C4 1 CKO5BX473K Cap, Ceramic, 0.047μF, 100V AVX C5 1 UPL1V390MAH Cap, Alum Elect, Low ESR, 39μF, 35V Nichicon L1 1 CTX 25-5-52 Inductor, 25μH, 5A Coiltronics L2 1 CTX5-5-FR Inductor, 5μH, 5A Coiltronics R1 1 MF 1/8W 2.74kΩ Res, MF, 1/8W, 1%, 2.74kΩ R2 1 MF 1/8W 1.82kΩ Res, MF, 1/8W, 1%, 1.82kΩ R3 1 CF 1/4W 5.1kΩ Res, CF, 1/4W, 5%, 5.1kΩ R4 1 CF 1/4W 10kΩ Res, CF, 1/4W, 5%, 10kΩ Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn49f_conv BA/GP 0991 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