DESIGN FEATURES Fixed Frequency, 500kHz, 4.5A Step-Down Converter in an SO-8 Operates from a 5V Input by Karl Edwards D2 1N914 Introduction 5V to 3.3V Buck Converter The circuit in Figure 1 is a step-down converter suitable for use as a local regulator to supply 3.3V logic from a 5V power bus. The high efficiency, shown in Figure 2, removes the need for bulky heat sinks or separate power devices, allowing the circuit to be placed in confined locations. Since the boost circuit only needs 3V to operate, the boost diode can still be connected to the output, improving efficiency. Figure 1’s circuit shows the shutdown pin option. If this pin is 16 C2 0.68µF INPUT 5V C3 10µF TO 50µF CERAMIC BOOST VIN + OPEN OR HIGH = ON L1 5µH OUTPUT 3.3V 4A VSW LT1506-3.3 SHDN GND SENSE VC CC 1.5nF + D1 MBRS330T3 C1 100µF, 10V SOLID TANTALUM 1506 TA01 Figure 1. 5V to 3.3V step-down converter pulled to a logic low, the output is disabled and the part goes into shutdown mode, reducing supply current to 20µ A. An internal pull-up ensures correct operation when the pin is left open. The SYNC pin, an option for the DD package, can be used to synchronize the internal oscillator to a system clock. A logic-level clock signal applied to the SYNC pin can synchronize the switching frequency in the range of 580kHz to 1MHz. to the energy that needs to be stored in the core. Three 4A inductors store less energy (1/2Li2 ) than a single 12A coil, so they are much smaller. In addition, synchronizing three converters 120° out of phase with each other reduces input and output ripple currents. This reduces the ripple rating, size and cost of the filter capacitors. Current Sharing Multiphase Supply Current sharing is accomplished by connecting the VC pins to a common compensation capacitor. The output of the error amplifier is a gm stage, so any number of devices can be connected together. The effective gm of the composite error amplifier is the product of the individual devices. In Figure 3, the compensation capacitor, C4, has been increased by 3×. Tolerances in the reference voltages cause small offset currents to flow between the VC pins. The overall effect is that the loop regulates the output at a voltage somewhere between the minimum and maximum references of the devices used. Switch-current matching between devices will be typically better than 300mA over the full current range. The negative temperature coefficient of the VC-toswitch-current transconductance prevents current hogging. The circuit in Figure 3 uses multiple LT1506s to produce a 5V, 12A power supply. There are several advantages to using a multiple switcher approach compared to a single larger switcher. The inductor size is considerably reduced. Inductor size is proportional 90 85 EFFICIENCY (%) The LT1506 is a 500kHz monolithic buck mode switching regulator, functionally identical to the LT1374 but optimized for lower input voltage applications. Its high 4.5A switch rating makes this device suitable for use as the primary regulator in small to medium power systems. The small SO-8 footprint and input operating range of 4V to 15V is ideal for local onboard regulators operating from 5V or 12V system supplies. The 4.5A switch is included on the die, along with the necessary oscillator, control and logic circuitry to simplify design. The part’s high switching frequency allows a considerable reduction in the size of external components, providing a compact overall solution. The LT1506 is available in standard 7-pin DD and fused-lead SO-8 packages. It maintains high efficiency over a wide output current range by keeping quiescent supply current to 4mA and by using a supply-boost capacitor to saturate the power switch. The topology is current mode for fast transient response and good loop stability. Full cycle-by-cycle short-circuit protection and thermal shutdown are provided. Both fixed 3.3V and adjustable output voltage parts are available. 80 75 70 0 1 2 3 LOAD CURRENT (A) 4 Figure 2. Efficiency vs load current for Figure 1’s circuit Current Sharing/ Split Input Supplies Linear Technology Magazine • August 1998 DESIGN FEATURES C1, C3: MARCON THCS50E1E106Z D1: ROHM RB051L-40 D2: 1N914 L1: DO3316P-682 3-BIT RING COUNTER 1.8MHz INPUT 6V TO 15V LT1506-SYNC LT1506-SYNC LT1506-SYNC VC SYNC SW GND VIN BOOST FB VC SYNC SW GND VIN BOOST FB VC SYNC SW GND VIN BOOST FB R1 5.36k 1% + + + C3A 10µF 25V D1A C4 68nF 25V + L1B 6.8µH C2B 330nF 10V + D2A C1 10µF 25V D1C + C2A 330nF 10V R2 4.99k 1% C3C 10µF 25V D1B + L1A 6.8µH + C3B 10µF 25V 5V 12A D2B C2C 330nF 10V L1C 6.8µH D2C 1506 F15 Figure 3. Current-sharing 5V/12A supply CURRENT PHASE 2 TIME CURRENT PHASE 3 TIME CURRENT TOTAL TIME Figure 4. Input current Linear Technology Magazine • August 1998 PHASE 1 CURRENT A ring counter generates three synchronization signals at 600kHz, 33% duty cycle, phased 120° apart. The sync input will operate over a wide range of duty cycles, so no further pulse conditioning is needed. At full load, each device’s input ripple current is a 4A trapezoidal wave at 600kHz, as shown in Figure 4. Summing these waveforms gives the effective input ripple for the complete system. The resultant waveform, shown at the bottom of Figure 4, remains at 4A but its frequency has increased to 1.8MHz. The higher frequency eases the requirements on the value of input filter without the 3× increase in ripple current rating that would normally occur. Although only a single input capacitor is required, practical layout restrictions usually dictate an individual capacitor at each device. Figure 5 shows the output ripple current waveforms. The resultant 1.8MHz triangular waveform has a maximum amplitude of 350mA at an input voltage of 10V. This is significantly lower than would be expected for a 12A output. Interestingly, at inputs of 7.6V and 15V, the TIME PHASE 2 CURRENT TIME Synchronized Ripple Currents theoretical summed output ripple current cancels completely. To reduce board space and ripple voltage, C1 and C3 are ceramic capacitors. Loop compensation capacitor C4 must be adjusted when using ceramic output capacitors, due to the lack of effective series resistance (ESR). The typical TIME PHASE 3 CURRENT CURRENT PHASE 1 the backplane, copper losses, connectors and so on. The common VC signal ensures that loading is still shared between the devices. TIME TOTAL CURRENT A common VC voltage forces each LT1506 to operate at the same switch current, not at the same duty cycle. Each device operates at the duty cycle defined by its input voltage. This is a useful feature in a distributed power system. The input voltage to each device could vary due to drops across TIME Figure 5. Output current 17 DESIGN FEATURES tantalum compensation value of 1.5nF is increased to 22nF (×3) for the ceramic output capacitor. If synchronization is not used and the internal oscillators free run, the circuit will operate correctly, but ripple cancellation will not occur. Input and output capacitors must be ripple rated for the individual output currents. Redundant Operation The circuit shown in Figure 3 is fault tolerant when operating at less than 8A of output current. If one power stage fails open circuit, the output will remain in regulation. The feedback loop will compensate by raising the voltage on the VC pin, increasing the switch current of the two remaining devices. 5V to 3.3V at 2.5A on 0.25in2 of board space, 0.125in High higher ripple current can be tolerated, allowing the use of small, low value, high current inductors. A ceramic output capacitor also reduces board area and improves voltage ripple. Using Figure 1’s circuit with the SO-8 LT1506 and the component changes in Table 1, a very small, low profile, step-down converter can be implemented. In many space-sensitive applications, the component that dominates both board area and overall height is the inductor. One of the factors affecting inductor value choice is maximum ripple current. Using the high current switch rating of the LT1506, Conclusion Table 1. Component changes for a low profile version of Figure 1’s circuit Part Value C1, C3 22µF, 10V CC 22nF L1 2.2µH The LT1506 is a compact, easy to use, monolithic switcher. The internal 4.5A switch covers a wide range of medium power applications. Its input operating range of 4V to 15V and availability in SO-8 or DD packages make it ideal for very space-efficient, local onboard DC/DC converters. Vendor/ Part# Tokin 1210ZG226Z Sumida CD43 2R2 Authors can be contacted at (408) 432-1900 RF 4.7Ω 1 VIN = 20V VOUT = 2.5V INTVCC EFFICIENCY (%) 95 CSS LTC1625 0.1µF 2 3 4 90 CC1 1nF LTC1435 RC1 5 10k 6 CC2 220pF 85 7 8 LTC1625 16 EXTVCC VIN SYNC TK RUN/SS SW FCB TG ITH BOOST SGND INTVCC VOSENSE VPROG BG PGND 15 13 12 1 2 3 LOAD CURRENT (A) 4 **DB 11 10 CB O.1µF VIN 12V–28V CIN 22µF 35V ×2 L1* 39µH D1 MBRS140T3 CVCC 4.7µF R2 35.7k 1% + R1 3.92k 1% 9 80 0 M1 Si4412DY 14 + 100 + CF 0.1µF VOUT 12V/2.2A COUT 100µF 16V 0.030Ω ESR M2 Si4412DY 5 DI_1068_02a. EPS * L1 = SUMIDA CDRH127-390MC ** DB = CMDSH-3 Figure 4. Efficiency vs load current Figure 5. 12V/2.2A adjustable-output supply LTC1625, continued from page 4 Conclusion A circuit demonstrating the wide output range of the LTC1625 is shown in Figure 5. This application uses Si4412DY MOSFETs to deliver a 12V output at up to 2.2A. Note that the SYNC pin is tied high for 225kHz operation in order to reduce the inductor size and ripple current. The LTC1625 step-down DC/DC controller offers true current mode control without the expense and difficulty of using a sense resistor. Popular features from Linear Technology’s other controllers, such as fixed frequency operation, N-channel MOSFET drive, Burst Mode operation, soft-start and output voltage programming make 18 this controller useful in a variety of applications. By eliminating the power loss in the sense resistor, even higher efficiencies can be achieved than were previously possible, making the LTC1625 an excellent choice for DC/DC converter designs requiring the highest performance. Linear Technology Magazine • August 1998