UltraFast Linear Regulator Eliminates All Bulk Tantalum and Electrolytic Output Capacitors Design Note 157 Anthony Bonte New LTC Regulator Controllers The LT1575/LT1577 family of controller ICs drives discrete N-channel MOSFETs and produces low dropout, UltraFast™ transient response regulators. These ICs feature 1% typical performance over all DC tolerances. Superior transient load performance eliminates all bulk output capacitors. An LT1577 based P55C Pentium processor power supply operates with only twenty-four high frequency decoupling, 1μF ceramic capacitors required for the microprocessor core. To improve profi t margins, some manufacturers reduce output capacitance and ignore the true regulation requirements. Many power supplies are deemed reliable if Windows95 boots up more than once. Most motherboards are only warranted for 90 days. LTC believes that many system crashes (blamed on software) are attributable to poor power supply regulation. To address these issues, Linear Technology introduces the LT ®1575/LT1577. Adjustable and fixed voltage versions accommodate any microprocessor voltage. MOSFET RDS(ON) selection allows custom dropout voltage performance. The controllers also provide current limiting, on/off control and overvoltage protection or thermal shutdown. The single LT1575 package is an 8-pin SO or PDIP and the dual LT1577 package is a 16-pin narrow body SO. 12V RESET Introduction Powering 200+MHz microprocessors requires high current, tight tolerance, fast transient response power supplies. Fast load transients mandate bulk output capacitance to maintain regulation and thus, cost increases. Surface mount tantalum capacitors are expensive and require voltage derating for reliable performance. Electrolytic capacitors are physically large and exhibit increased ESR with age. Therefore, transient response and regulation performance degrade. 1 2 C2A 0.1μF R9 = TRACE RESISTOR 1/2 LT1577 SHDN1 IPOS1 VIN1 INEG1 16 15 3 14 GND1 GATE1 4 OUT 13 COMP1 – 3.3 C3 0.22μF R5 3.9Ω 12V RESET C4 10pF 1/2 LT1577 5 C2B 0.1μF 6 7 8 SHDN2 IPOS2 VIN2 GND2 FB INEG2 GATE2 COMP2 R9 0.004Ω + Q2 IRFZ14 R7 3.9k C8 1500pF C9-C11 1200μF 10V ×3 5V C1 0.1μF VCC2DET’s signal determines circuit operation. In a P54C circuit, VCC2DET is open and the core and I/O VIO C20-C31 1μF X7R CERAMIC 0805 CASE Figure 1 illustrates an LT1577 application with a fixed 3.3V and an adjustable voltage regulator for a P54C/ P55C Pentium processor autoselect circuit. The P54C Pentium processor core and I/O circuitry operate from 3.5V. The P55C Pentium processor I/O operates from 3.3V and the core operates from 2.8V. 3.3V 3.5V L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and UltraFast is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. IF VCC2DET = OPEN, THEN VIO = VCORE = 3.5V IF VCC2DET = GND, THEN VIO = 3.3V AND VCORE = 2.8V 12 C5 0.1μF 11 R6 10 3.9Ω 9 C6 10pF 50mV/DIV Q3 IRFZ34 R8 7.5k C7 1000pF C32-C55 1μF X7R CERAMIC 0805 CASE R2 1.21k VCORE R1 2.8V 1.58k 3.5V R4 10k 2A/DIV R3 2.67k Q1 2N7002 TO CPU VCC2DET DN157 F01 Figure 1. LT1577 P54C/P55C Pentium Processor Autoselect Circuit 06/97/157_conv 0 I = 0.2A TO 5A 200μs/DIV DN157 F02 Figure 2. Transient Response for 0.2A to 5A Output Load Step 1 2 12V 3 4 C6 0.1μF IPOS SHDN 8 7 U1 INEG LT1575 6 GND GATE VIN FB COMP Q1 IRLZ44 5 C21, 10pF 12V + C11 150μF 16V + C12 150μF 16V + C13 150μF 16V + C14 150μF 16V EXTVCC 1 C8, 68pF C9 1500pF R5 16.5k ITH 4 5 6 INTVCC 11 BG SGND S+ 8 VOS S– 7 10 R1 2.1k, 1% C16 1μF R2 1.21k 1% L1 4μH C3, 0.1μF D1, CMDSH-3 Q3 3.3V VCORE C1, 470pF 1μF X7R CERAMIC 0805 CASE ×40 Q2 12 SFB PGND C15 1μF 16 14 U2 SW LTC1435 15 RUN/SS BOOST 3 C10, 1000pF TG COSC 2 C7, 0.1μF C17 1μF 13 VIN 9 R9 2k C22, 1000pF R6 0.0075 R3 100 R4 100 + C18 1000μF 10V C20 1000μF 10V + + C5 0.1μF D2 MBRS330T3 C23 1μF C19 1000μF 10V C2, 1000pF C4, 4.7μF + R7 35.7k DN157 F03 R8 15K L1 = COILTRONICS CTX02-13199 Q2, Q3 = SILICONIX SUD50N03-10 supply planes connect together. Q1 turns on and the Q3 (IRFZ34) regulator controls its output to 3.5V. The Q2 (IRFZ14) regulator attempts to control its output to 3.3V, but its feedback pin (Pin 4) senses 3.5V and turns Q2 off. Q3 supplies all core and I/O power. In a P55C circuit, VCC2DET is grounded and the core and I/O supply planes are separate. Q2 controls the I/O voltage to 3.3V and Q3 controls the core voltage to 2.8V. The I/O circuitry’s lower current requirement permits a lower cost MOSFET for Q2 and reduced output capacitance. The current limit sense resistor is made of “free” PCB trace. Q2’s and Q3’s common-drain connection permits common heat sink mounting. The COMP pin components adjust frequency compensation for each regulator relative to the MOSFET and output capacitors used. Figure 2 shows the core regulator transient response for a 4.8A load current step in a P55C setup. Compensation limits overshoot/undershoot to 50mV. The ±100mV tolerance for a VRE processor is easily met. The autoselect concept is easily extended to the multiplicity of voltages required by various processors. Consult LTC for details. Figure 3 shows a 3.3V, 14A logic supply that uses an LT1575 as a post-regulator on an LTC®1435 synchronous buck regulator, generating 3.3V from 12V with an overall Data Sheet Download www.linear.com Linear Technology Corporation 50mV/DIV Figure 3. 12V to 3.3V/9A (14A Peak) Hybrid Regulator 200μs/DIV DN157 F04 Figure 4. Transient Response for Figure 3's Circuit to a 10A Load Step efficiency of 72%. The LT1575 uses an IRLZ44 as the pass transistor, allowing < 550mV dropout voltage. The switching regulator’s output is set to 4V. Figure 4 shows the transient response for a 10A, 50ns rise/fall time load step. The only output capacitors are 40, 1μF surface mount ceramic capacitors. The circuit eliminates about a dozen low ESR tantalum capacitors, which would be required without the linear regulator. Conclusion The LT1575/LT1577 combine the benefits of low dropout voltage, precision performance, UltraFast transient response and significant output capacitance cost savings. The LT1575/LT1577 controller ICs step to the next performance level required by motherboard designers. For applications help, call (408) 432-1900 dn157f_conv LT/TP 0697 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1997