advertisement 5V to 3.3V Regulator with Fail-Safe Switchover – Design Note 82 Mitchell Lee Newer microprocessors designed for replacing existing 5V units operate from lower voltage supplies. In the past a processor swap was simply a matter of removing one IC and replacing it with an updated version. But now the upgrade path involves switching from a 5V chip to one that requires 3.xxV. One means of changing supply voltage from 5V to 3.xxV is to clip a jumper that bypasses a local 3.xxV regulator. This is not a good solution since it leaves too much to chance. Failure to remove the jumper can result in the instant destruction of the new microprocessor upon application of power. A means of automatically sensing the presence of a 3.xxV or 5V processor is necessary. Intel microprocessors include a special pin called “VOLDET” which can be used to determine whether or not a particular chip needs 3.xxV or 5V. Figure 1 shows a simple circuit that takes advantage of this pin to automatically “jumper out” a 3.xxV regulator whenever a 5V processor is inserted into the socket. VOLDET is pulled low on 3.xx processors; it is buffered by transistor Q2 which grounds the gate of a bypassing switch (Q1). Q1 is turned off leaving the LT®1085 to regulate the microprocessor’s VCC line. For 5V microprocessors the VOLDET pin is high; Q2 is turned off allowing Q1’s gate to pull up to 12V, turning itself on. With the LT1085 shorted from input to output by the MOSFET, 5V flows directly to the microprocessor. No service intervention is required to ensure correct VCC potential. The circuit in Figure 1 is fine for cases where 12V is available to enhance the MOSFET switch. However, in portable applications,12V is frequently not available or available only on an intermittent basis. Figure 2 shows a second solution using a high-side gate driver to control the MOSFET. A VOLDET pull-up resistor is required in both figures because in some cases VOLDET is an open circuit or a shorting link, and in other cases it is an open-drain output. 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. 5V 12V 5V R5 100k R1 20k R4 100k R4 100k VS G1 Q2 2N2222 R1 3.3M Q1 MTB30N06EL IN1 1/2 LTC1157 Q1 MTB30N06EL VOLDET (H = 5V, L = 3.45V) VOLDET (H = 5V, L = 3.45V) MICROPROCESSOR 3.45V, 5V 5V + IN LT1085 OUT C1 10μF ADJ + R2 115Ω 1% C2 22μF TA VCC * MICROPROCESSOR 3.45V, 5V 5V DN82 • F01 + IN LT1085 OUT C1 10μF ADJ R3 200Ω *FOR BEST PERFORMANCE, PLACE 4 EACH 47μF, 9 EACH 0.1μF AND 9 EACH 0.01μF BYPASS CAPACITORS ON THE VCC PINS OF THE 1% + R2 115Ω 1% C2 22μF TA VCC * DN82 • F02 R3 200Ω 1% INTEL 486 MICROPROCESSOR. ESR OF THE 47μF < 0.1Ω Figure 1. Bypass Circuit for 3.xxV and 5V Microprocessor Swaps Using Transistor Buffer 05/94/82_conv Figure 2. Bypass Circuit for 3.xxV and 5V Microprocessor Swaps Using High-Side Gate Driver (No 12V Supply Required) VOLDET is pulled up to 5V in both circuits. This could pose a problem for 3.xxV processors with open-drain VOLDET pins, but for 3.xxV devices VOLDET is always pulled low and 5V never reaches it. The 5V reaches VOLDET only on 5V devices. For certain families of microprocessors, 3.3V is required. The circuits shown in Figures 1 and 2 are fully compatible with 3.3V applications by simply substituting a fixed 3.3V version of the regulator (use an LT1085-3.3). Higher current operation is also possible. The LT1085 is suitable for 3A applications; use an LT1084 and an MTB50N06EL for up to 5A. Table 1 shows the wide range of linear regulators available at currents of up to 10A. In some applications the complexity of a high efficiency switching regulator may be justified for reasons of battery life. Figure 3 shows a switcher that not only converts 5V to 3.45V but also acts as its own bypass switch for applications where a 5V output is required. An open drain or collector pulling the VFB pin low causes the top side P-channel MOSFET to turn on 100%, effectively shorting the output to the 5V input. If the open collector is turned off, the LTC1148 operates as a high efficiency buck mode power converter, delivering a regulated 3.45V to the load. For 3.3V applications a fixed 3.3V version of the LTC1148 is available. Table 1. Linear Regulators for 5V to 3.3V Conversion LOAD CURRENT DEVICE FEATURES 150mA 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 2 × LT1087 Parallel, Kelvin Sensed The topic of powering low voltage microprocessors in a 5V environment is covered extensively in Application Note 58, available on request. Both linear and switching solutions are discussed. 5V INPUT D1 MBRS140T3 Q1 Si9430 + C1: Ta C3: SANYO (OS-CON) 20SA100M, ESR = 0.037Ω, IRMS = 2.25A C7: SANYO (OS-CON) 10SA220M, ESR = 0.035Ω, IRMS = 2.36A Q1: SILICONIX PMOS BVDSS = 20V, DCRON = 0.100Ω, Qg = 50nC Q2: SILICONIX NMOS BVDSS = 30V, DCRON = 0.050Ω, Qg = 30nC D1: MOTOROLA SCHOTTKY VBR = 30V R2: KRL NP-2A-C1-0R020J, PD = 1W L1: COILTRONICS CTX10-4 C3 100μF 20V w2 Q2 Si9410 C1 1μF 2 + 3 4 C5 200pF NPO 5 6 C4 3300pF X7R R1 470Ω 7 P-DRIVE NC VIN N-DRIVE NC LTC1148 P-GND CT INT VCC ITH SENSE – S-GND SHDN VFB SENSE + 14 13 L1 10μH 12 11 10 5V R4 20k 1% SHUTDOWN 9 8 C6 0.01μF R2 0.033Ω + 1 C2 0.1μF C7 220μF 6.3V w2 5V Q3 2N2222 VOLDET 3.45V R3 35.7k 1% DN82 • F03 100k 3.45V 3A Figure 3 Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn82f_conv LT/GP 0594 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 1994