advertisement Techniques for Deriving 3.3V from 5V Supplies – Design Note 74 Mitchell Lee Microprocessor chip sets and logic families that operate from 3.3V supplies are gaining acceptance in both desktop and portable computers. Computing rates, and in most cases, energy consumed by these circuits show a strong improvement over 5V technology. The main power supply in most systems is still 5V, necessitating a local 5V to 3.3V regulator. Linear regulators are viable solutions at lower (IO ≤ 1A) currents, but they must have a low dropout voltage in order to maintain regulation with a worst-case input 5 4.5V TO 5.5V IN OUT 1 + LT1129-3.3 4 SHUTDOWN SENSE 2 GND 3.3V OUT 700mA 3.3μF SOLID TANTALUM of only 4.5V. Figure 1 shows a circuit that converts a 4.5V minimum input to 3.3V with an output tolerance of only 3% (100mV). The LT®1129-3.3 can handle up to 700mA in surface mount configurations, and includes both 16μA shutdown and 50μA standby currents for system sleep modes. Unlike other linear regulators, the LT1129-3.3 combines both low dropout and low voltage operation. Small input and output capacitors facilitate compact, surface mount designs. For the LT1129-3.3, dissipation amounts to a little under 1.5W at full output current. The 5-lead surface mount DD package handles this without the aid of a heat sink, provided the device is mounted over at least 2500mm2 of ground or power supply plane. Efficiency is around 62%. Dissipation in linear regulators becomes prohibitive at higher current levels where they are supplanted by high efficiency switching regulators. A 2A, 5V to 3.3V switching regulator is shown in Figure 2. This synchronous buck converter is implemented with %/t' 3 Figure 1. Low Dropout Regulators Delivers 3.3V from 5V Logic Supply L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 4.5V TO 5.5V INPUT + C1 1μF + C2 100nF 3 VIN 10 SHUTDOWN P DRIVE 1 Q1 Si9430DY L1 50μH LT1148-3.3 6 ITH R1 1k SENSE + 8 SENSE – 7 R2 0.05Ω 3.3V 2A C6 10nF + 4 C4 3.3nF X7R C3 22μF w 3 25V C5 470pF NPO CT N DRIVE SGND PGND 11 12 14 Q2 Si9410DY D1 MBRS140T3 C7 220μF w 2 10V %/t' Figure 2. 94% Efficiency Synchronous Buck Regulator Pumps Out 2A at 3.3V from 5V Logic Supply 09/93/74_conv an LTC1148-3.3 converter. The LTC1148 uses both Burst Mode® operation and continuous, constant offtime control to regulate the output voltage, and maintain high efficiency across a wide range of output loading conditions. Efficiency as a function of output current is plotted in Figure 3. 100 A 5V to 3.3V converter circuit collection is presented in Application Note 55, covering the entire range of currents listed in Tables 1 and 2. Table 1. Linear Regulators for 5V to 3.3V Conversion VIN = 5V LOAD CURRENT 90 EFFICIENCY (%) Table 2 summarizes the practical current range of a number of switching regulators for 5V to 3.3V applications, along with their typical efficiencies. 80 70 LT1121-3.3 Shutdown, Small Capacitors 700mA LT1129-3.3 Shutdown, Small Capacitors 800mA LT1117-3.3 SOT-223 1.5A LT1086 DD Package 3A to 7.5A LT1083 LT1084 LT1085 High Current, Low Quiescent Current at High Loads 10A 10mA 100mA FEATURES 150mA 60 50 1mA DEVICE 2 × LT1087 Parallel, Kelvin Sensed 1A 2A OUTPUT CURRENT %/t' Figure 3. LTC1148-3.3: Measured Efficiency All of the components used in the Figure 2 switching regulator are surface mount types, including the inductor and shunt resistor, which are traditionally associated with through hole assembly techniques. Depending on the application, a variety of linear and switching regulator circuits are available for output currents ranging from 150mA to 20A. Choices in linear regulators are summarized in Table 1. There are some cases, such as in minicomputers and workstations, where higher dissipations may be an acceptable compromise against the circuit complexity and cost of a switching regulator, hence the >1A entries. Heat sinks are required. Data Sheet Download www.linear.com Linear Technology Corporation Table 2. Switching Regulators for 5V to 3.3V Conversion LOAD CURRENT DEVICE EFFICIENCY FEATURES 200mA to 400mA LTC1174-3.3 90% Internal P-Channel Switch, 1μA Shutdown 0.5A to 2A LTC1147-3.3 92% 8-Pin SO, High Efficiency Converter 1A to 5A LTC1148-3.3 94% Ultra-High Efficiency Synchronous Converter 5A to 20A LT1158 91% Ultra-High Current Synchronous Converter For applications help, call (408) 432-1900 dn74f_conv LT/GP 0993 190K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1993