LinearTechnologyChronicle A Showcase of Linear Technology's Focus Products November 1995 100 Product of the Month The LTC1430 includes a fixed frequency PWM oscillator for a predictable output noise spectrum under virtually all operating conditions. The 200kHz free-running clock frequency can be externally adjusted from 50kHz to above 400kHz. The LTC1430 features a low 350A quiescent current, allowing greater than 90% efficient converter designs. The output drivers of the LTC1430 can drive 10,000pF gates at greater than 300kHz. Figure 2 shows the efficiency of the LTC1430 when used as a 5V to 3.3V converter with 10A output current. Shutdown mode drops the LTC1430 supply current to 1A. The LTC 1430 is a high power, high efficiency switching regulator controller optimized for 5V to 3.3V applications. It includes a precision internal reference and an internal feedback system that provides 1% output voltage accuracy over line, load and temperature variations. The LTC1430 uses a synchronous switching architecture with two N-channel output devices, eliminating the need for a high power, high cost P-channel device. Additionally, it senses output current across the drain source resistance of the upper N-channel FET, providing an adjustable current limit without an external low value sense resistor. Figure 1 shows a typical LTC1430 application. 5V + PVCC2 + 4.7F PVCC1 0.1F SS 0.01F NC SHUTDOWN CC 4700pF 0.1F IMAX 2.7H/15A 1k SHDN + G2 FREQSET CIN 220F x4 M1A, M1B 2 IN PARALLEL LTC1430 IFB M2 PGND COMP RC 7.5k 0.1F 16k G1 VCC C1 220pF + 1N4148 1F 100 COUT 330F x6 3.3V 10A EFFICIENCY (%) 90 High Current, High Efficiency Step-Down Switching Regulator Controller Has 1% Accurate Output (R) Vol. 4 No. 11 TA = 25C PVCC = 5V VOUT = 3.3V 80 70 60 50 40 0.1 1 LOAD CURRENT (A) 10 LTC1430 * TA02 Figure 2. LTC1430 Efficiency Remains Over 90% from 1A to 10A The LTC1430 is an ideal regulator for powering Pentium(R) and Pentium(R) Pro microprocessors. The device is also ideal as a high power, high efficiency 5V to 3.xV regulator for logic supplies and battery voltage sources. The device is available in 16-lead plastic dual-in-line and surface mount packages. A low cost version is also available in an 8-lead plastic dual-in-line and surface mount package. The 8-lead version eliminates soft start, current limiting and the externally programmable clock. Devices are specified from 0C to 70C. Contact your local Linear Technology sales office for a data sheet and free evaluation samples of the LTC1430. Pentium is a registered trademark of Intel Corporation. GND SENSE+ SENSE - FB LTC1430 * TA01 NC M1A, M1B, M2: MOTOROLA MTD20N03HL CIN: AVX-TPSE227M010R0100 COUT: AVX-TPSE337M006R0100 Figure 1. All N-Channel 5V to 3.3V, 10A Power Supply for Desktop Computer Systems Inside This Issue: 500kHz Switching Regulator Has On-Chip 3A Power Switch, Up to 90% Efficiency in Boost Applications ............................................................................ 2 A Circuit That Smoothly Switches Between 3.3V and 5V ....................................................... 3 5A Low Dropout Linear Regulator Has Fast Transient Response for Microprocessor Applications ..................................................................................................... 4 Regulated Positive-to-Negative Voltage Converter Synchronized to System Clock ............... 4 , LTC and LT are registered trademarks of Linear Technology Corporation. Linear Technology Chronicle * November 1995 Low Power, 10kV Protected RS485 Transceiver Operates on 3.3V Supply The LTC1480 is an ultralow power differential line transceiver which provides full RS485 compliance while operating from a single 3.3V supply. This new device also meets the requirements of RS422 and features high speed operation to 2.5Mbps. The CMOS design offers significant power savings without sacrificing ruggedness against overload or ESD damage. Typical quiescent current is only 300A and shutContinued on page 2 1 LTC1480 from page 1 down current is only 1A. Figure 1 shows a 3.3V powered RS485 network using the LTC1480. The driver and receiver of the LTC1480 feature three-state outputs, with the driver outputs maintaining high impedance over the entire common mode range. Excessive power dissipation caused by bus contention or faults is prevented by a thermal shutdown circuit which forces the driver outputs into a high impedance state. The receiver of the LTC1480 has a fail-safe feature which guarantees a high output state when the inputs are left open. Also, the I/O pins are protected against multiple ESD strikes of up to 10kV. The LTC1480 is an ideal RS485 interface device for use in battery-powered RS485 networks or as a low power level translator. The LTC1480 is fully specified over the commercial and extended industrial temperature range. The LTC1480 is available in 8-pin plastic surface mount and 8-pin dual-in-line packages. Contact your local Linear Technology sales office for a data sheet and free evaluation samples of the LTC1480. 3.3V 3.3V LTC1480 LTC1480 RO 1 RE 8 R SHIELD 2 7 B 3 6 A 8 SHIELD 120 DE 4 D1 7 2 A 6 3 120 5 D 1 RO R B 5 3.3V B A 8 7 6 DE D1 5 LTC1480 R 1 4 D RE D 2 RO RE 3 4 LTC1480 * TA01 DE D1 Figure 1. RS485 Network Runs on 3.3V Supply The LT(R)1371 is a 500kHz fixed frequency current mode switching regulator IC which contains a fully protected 3A switch. This new device provides surface mount packaging, logic compatible shutdown, easy synchronization capability up to 800kHz, operation from 2.7V to 30V, negative or positive voltage sensing and low supply current. The high frequency operation of the LT1371 allows small inductors and capacitors to be used and keeps output frequency components away from critical IF frequencies. Similar in architecture to the 1.5A LT1372 , the 3A LT1371 extends the available output power such that a 5V to 12V converter can deliver up to 800mA with a 2 5V L1* 4.7H D1 MBRS330T3 VIN OFF ON S/S 100 VOUT 12V VSW R1 53.6k 1% FB + LT1371 + C1** 22F 25V GND VC C2 0.047F R3 2k R2 6.19k 1% C4** 22F 25V x2 C3 0.0047F 80 70 60 50 0.01 LT1371 * TA01 *COILCRAFT DO3316P-472 (4.7H), DO3316P-103 (10H) OR SUMIDA CD104-100MC (10H) **AVX TPSD226M025R0200 VIN = 5V 90 EFFICIENCY (%) 500kHz Switching Regulator Has On-Chip 3A Power Switch, Up to 90% Efficiency In Boost Applications 0.1 OUTPUT CURRENT (A) 1 LT1371 * TA02 MAX IOUT L1 IOUT 4.7H 0.7A 10H 0.8A Figure 2. LT1371 Efficiency Figure 1. LT1371 5V to 12V Converter Provides 800mA Output Current minimum number of external components, as shown in Figure 1. The low loss 3A switch has a 0.25 typical on resistance which allows for up to 90% efficient boost converter designs (Figure 2). Operating quiescent current of the LT1371 is only 4mA and a logic compatible Shutdown pin reduces supply current to 12A. The LT1371 is available in surface mount 7-lead DD, 7-lead TO-220 and 20-lead SW packages enabling an all surface mount design at higher power levels than previously available. Devices are specified for operation from 0C to 125C. Contact your local Linear Technology sales office for a data sheet and free evaluation samples of the LT1371 high frequency boost regulator. Linear Technology Chronicle * November 1995 Application of the Month A Circuit That Smoothly Switches Between 3.3V and 5V Many subsystems require supply switching between 3.3V and 5V to support both low power and high speed modes. This back-and-forth voltage switching can cause havoc to the main 3.3V and 5V supply buses. If done improperly, switching the subsystem from 5V to 3.3V can cause a momentary jump on the 3.3V bus, damaging other 3.3V devices. When switching the subsystem from 3.3V to 5V, the 5V supply bus can be pulled down while charging the subsystem's capacitors and may inadvertently cause a reset. The circuit shown in Figure 1 allows smooth voltage switching between 3.3V and 5V, with added protection features to ensure safe operation. IC1 is an LTC1470 power switch matrix. This part has on-chip charge pumps running from the 5V supply to fully enhance the internal N-channel MOSFETs. The LTC1472 also has guaranteed break-before-make switching to prevent cross-conduction between buses. It also features current limiting and thermal shutdown. capacitor. The example shown in Figure 1 is for a 250mA subsystem. The discharge time constant should be about 4ms. Once the subsystem supply has dropped below the 3.3V supply, the comparator will trip, turning on the 3.3V switch. The comparator has some hysteresis to avoid instabilities. The subsystem supply will reach a low point of about 3V before the 3.3V switch is fully enhanced. When switching from 3.3V to 5V, IC1's current limiting prevents the main 5V bus from being dragged down while charging the holding capacitor and the subsystem's capacitance. Without current limiting, the inrush current to charge these capacitors could cause a droop in the main 5V supply. If done improperly, supply voltage switching leads to disastrous system consequences. The voltage switch should monitor the output voltage and have current limiting to prevent main supply transient problems. A correctly designed supply voltage switch avoids these pitfalls and results in a safe, reliable system. When switching the subsystem from 5V to 3.3V, the holding capacitor and the load capacitance are initially charged up to 5V. Connecting these capacitors directly to the main 3.3V bus causes a momentary step to 5V. This transient is so fast that the power supply cannot react in time. Switching power supplies have a particularly difficult time coping with this jump. Switching supplies source current to raise the supply voltage and require the load to sink current to lower the voltage. A switching supply will be unable to react to counter the large positive voltage step. This jump will cause damage to many low voltage devices. The circuit in Figure 1 employs a comparator (IC2) and utilizes the high impedance state of the LTC1470 to allow switching with minimal effect on the supply. When the 3.3V output is selected, IC1's output will go into a high impedance state until its output falls below the 3.3V bus. The output capacitors will slowly discharge to 3.3V, with the rate of discharge depending on the current being pulled by the subsystem and the size of the holding 5V 3.3V 1F + 1F + 5V 2 0 = 5V 1 = 3.3V 1k 3.3V VOUT 0.1F 3 2 3 + 8 IC2 LT1011 - 7 5VIN 3VIN 3VIN 51k 5V 6 7 4 IC1 LTC1470 VOUT 500mV/DIV 1 3.3V TO SUBSCRIBER 8 EN1 + EN0 4 220F TANTALUM HOLDING CAPACITOR 5 0V dI1470_1.eps Figure 1. Schematic Diagram of 3.3V and 5V Switchover Circuit Linear Technology Chronicle * November 1995 5V 5V/DIV 2ms/DIV dl1470_2eps Figure 2. Oscillograph of the Switchover Waveform Showing Smooth Transitions 3 5A Low Dropout Linear Regulator Has Fast Transient Response for Microprocessor Applications 1.5 INPUT/OUTPUT DIFFERENTIAL (V) 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 The LT1585A is a new low dropout linear regulator optimized to handle fast transients commonly found in high speed microprocessor power supply applications. The 5A output current capability of the LT1585A allows it to handle the combined power demand of a high speed microprocessor and cache RAM. The dropout voltage of the LT1585A is guaranteed to be less than 1.4V over full load and temperature, optimizing this new device for 5V to 3.3V applications. Figure 1 shows the dropout voltage vs load current characteristics of the LT1585A. Regulated Positive-toNegative Voltage Converter Synchronized to System Clock The LTC1429 is a switched capacitor voltage inverter that provides a regulated negative output voltage from a single positive supply without using inductors. This new voltage converter IC features user controlled clock synchronization for sensitive RF applications. Supply current is only 600A and a shutdown mode reduces current to 0.2A for portable applications. The 14-lead LTC1429CS operates from a single 3V to 8V supply and provides an adjustable output voltage from -1.25V to -8V. An on-chip resistor string allows the LTC1429CS to be configured for output voltages of -3.5V, - 4V, - 4.5V or -5V. The 8-lead LTC1429CS8 is optimized for 0.5 0 IFULL LOAD OUTPUT CURRENT (A) LT1585A TA02 Figure 1. LT1585A Dropout Is Less Than 1.4V at Full Load The LT1585A has lower dropout and faster transient response than its LT1083/ 84/85 predecessors. Output regulation is less than 2% over fast changing line and load conditions. These improvements make the LT1585A ideal for low voltage microprocessors requiring a regulated 2.5V to 3.6V supply. 1 5V 2 C2 0.1F 3 C1 0.1F 4 VCC SYNC/SD 10k 5V REG C1+ LTC1429-4CS8 OUT C1- GND 8 COMP 7 POWER VALID 6 VOUT -4V AT 10mA 5 C3 0.001F OPTIONAL + C4 3.3F LTC1429 * TA01 Figure 1. LTC1429 GaAs Bias with Power Valid Output applications requiring a fixed - 4V output from a 5V supply and only requires a single external 0.1F flying capacitor. Output ripple is typically as low as 5mV and an optional compensation pin is provided on both versions to reduce output ripple. Figure 1 shows the LTC1429CS8 used as a 5V to - 4V converter for GaAs power transistor bias. Other family members include the LTC1550/51 and the LTC1261. The LTC1550/51 provide a negative output voltage with 1mV output ripple and the Current limit is trimmed to ensure controlled short-circuit current. On-chip thermal limiting provides protection against any combination of overload that would create excessive junction temperatures. The fast transient response and low dropout voltage of the LT1585A make it an ideal regulator for powering Pentium, PowerPCTM and other high speed microprocessors. The device can also be used to provide regulated power for low voltage logic supplies or as a post regulator for switching power supplies. The LT1585A is available in the TO220 package with a fixed 3.3V or user adjustable output voltage. Devices are specified for operation from 0C to 125C. Contact your local Linear Technology sales office for a data sheet and free evaluation samples of the LT1585A. PowerPC is a trademark of IBM Corporation. LTC1261 contains an onboard oscillator. All these devices are ideal for use as GaAs transistor bias voltage generators commonly used in portable RF applications such as RF wireless LANs, cellular phones and CDPD data transfer applications. The LTC1429 is available in 8-lead surface mount and 14-lead surface mount packages. Devices are specified over the 0C to 70C range. Contact your local Linear Technology sales office for a data sheet and free evaluation samples of the LTC1429. Linear Technology Products Are Distributed By: Almac/Arrow Arrow/Schweber Arrow/Zeus Digi-Key Electrosonic Gerber Electronics Farnell Electronics Marshall Industries Phase 1 (c) 1995 Linear Technology Corporation/Printed in USA 4 Technology Corporation * 1630 McCarthy Blvd. * Milpitas, California 95035-7417 * (408) 432-1900 * Linear Linear FAX: (408) 434-0507 * Chronicle For Literature*Only: 1-800-4-LINEAR Technology November 1995