DEMO MANUAL DC293 NO-DESIGN SWITCHER DESCRIPTIO LTC1771 Ultralow Supply Current, High Efficiency Step-Down Regulator U Demo Board DC293 is a step-down (buck) regulator using the LTC(R)1771. Exclusive use of surface mount components and the LTC1771's tiny MS8 package results in a very efficient application in a small board space. Featuring outstanding light load efficiency and requiring as little as 10A supply current to regulate the output at no load, it is ideal for cell phones and other portable electronics that have long standby times and need ultralow supply current to maximize battery life. DC293 is capable of providing 2A at various output voltages programmable from 1.8V to 5V via a jumper. This demo board highlights the capabilities of the LTC1771, which uses a current mode, constant off-time architecture to control an external P-channel power MOSFET. This results in a high performance power supply that has low output voltage ripple and fast transient response. At low output currents, the LTC1771 automatically switches to Burst ModeTM operation to maintain high operating efficiencies and to minimize supply current. The part can be shut down to further reduce the supply current to 2A. Its wide supply range allows operation from 2.8V to 18V. A MODE pin is provided to disable Burst Mode operation for noise-sensitive applications and soft-start is provided by an external capacitor that can also be used to properly sequence supplies. In dropout, the P-channel MOSFET is turned on continuously (100% duty cycle), providing low dropout operation with VOUT VIN. Gerber files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a trademark of Linear Technology Corporation. W U WW PERFOR A CE SU ARY SYMBOL PARAMETER VIN Input Voltage Range CONDITIONS (SEE NOTE) BOARD SUFFIX VALUE VOUT Output Voltage IOUT = 1A A A, B A, B B 1.8V 0.036V 2.5V 0.050V 3.3V 0.066V 5V 0.10V IOUT Maximum Output Current RSENSE = 0.05 A, B 2A A B 2.8V to 12V 4.5V to 18V W U U TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO LTC1771 Efficiency 100 VIN = 5V EFFICIENCY (%) 90 80 VIN = 10V VIN = 15V 70 60 50 VOUT = 3.3V RSENSE = 0.05 40 10 0.1 1 100 1000 LOAD CURRENT (mA) 10000 DC293 TPC01 1 DEMO MANUAL DC293 NO-DESIGN SWITCHER W U WW PERFOR A CE SU ARY SYMBOL PARAMETER CONDITIONS BOARD SUFFIX VALUE IQ Typical Supply Current VIN = 10V, ILOAD = 0, VOUT = 1.8V VIN = 10V, VRUN = 0 A A, B 10A 2A VRIPPLE Typical Output Ripple IOUT = 1A IOUT = 100mA, Burst Mode Operation Enabled IOUT = 100mA, Burst Mode Operation Disabled A, B A, B A, B 40mV 50mV 20mV VOUT Typical Line Regulation 2.8V < VIN < 12V, ILOAD = 1A 4.5V < VIN < 18V, ILOAD = 1A A B 3mV 4mV Typical Load Regulation 0 < ILOAD < 2A, VIN = 10V 0 < ILOAD < 2A, VIN = 10V A B 5mV 7mV Note: VOUT is voltage associated with the center position of JP3, unless otherwise specified. U W W PACKAGE A D SCHE ATIC DIAGRA S C6 0.01F 1 2 3 JP2 U1 LTC1771EMS8 RUN 1 RUN/SS MODE 2 C2 330pF R1 10k 1 2 3 SHDN C10 (OPT) 3 4 ITH SENSE VFB VIN GND PGATE C1 1000pF 8 7 1.8V R7 2.67M JP3 2.5V ENABLE 6 5 + C3 10F 25V Q1 Si3443DV VIN 2.8V TO 12V C9 33F 16V (OPT) 2 5 6 DRAIN VOUT 2A L1 10H R5 C4 10pF 1.69M R6 634k DISABLE R2 0.050 1 R4 1M JP1 D1 UPS5817 + C5 150F 6.3V C8 1F GND DC293 F01 R8 (OPT) 3.3V TOP VIEW RUN/SS ITH VFB GND Figure 1. LTC1771 Demo Board Version A Schematic C6 0.01F 1 2 3 JP2 U1 LTC1771EMS8 RUN 1 2 R1 C2 10k 330pF C10 (OPT) 1 2 3 SHDN 3 4 RUN/SS MODE ITH SENSE VFB VIN GND PGATE C1 1000pF 8 7 DISABLE LTC1771EMS8 ENABLE R2 0.050 C3 10F 25V Q1 Si6447DQ + C9 15F 35V (OPT) 1 2 5 6 DRAIN R4 1M R5 C4 10pF 3.09M R6 1.54M 2.5V R7 3.74M JP3 3.3V R8 (OPT) 5V + C5 150F 6.3V Figure 2. LTC1771 Demo Board Version B Schematic 2 VIN 4.5V TO 18V VOUT 2A L1 15H D1 UPS5817 8 7 6 5 MODE SENSE VIN PGATE MS8 PACKAGE 8-LEAD PLASTIC MSOP JP1 6 5 1 2 3 4 C8 1F GND DC293 F02 DEMO MANUAL DC293 NO-DESIGN SWITCHER PARTS LIST REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE C1 1 06035C102KAT1A 1000pF 50V 10% X7R Capacitor AVX (843) 946-0362 C2 1 06035C331KAT1A 330pF 50V 10% X7R Capacitor AVX (843) 946-0362 C3 1 TMK432BJ106M 10F 25V X5R Ceramic Capacitor Taiyo Yuden (800) 348-2496 C4 1 06033A100KAT2A 10pF 25V 10% NPO Capacitor AVX (843) 946-0362 C5 1 6TPB150M 150F 6.3V 20% POSCAP Capacitor Sanyo (619) 661-6835 C6 1 06035C103KAT1A 0.01F 50V 10% X7R Capacitor AVX (843) 946-0362 C8 1 0603ZG105KAT1A 1F 10V 80% Y5V Capacitor AVX (843) 946-0362 C9 1 TPSC336M016R0300 TPSC156M035R0450 33F 16V 20% Tantalum Capacitor, Board A 15F 35V 20% Tantalum Capacitor, Board B AVX (207) 282-5111 D1 1 UPS5817 2A Schottky Diode Microsemi (617) 926-0404 TP1 to TP4 4 2502-02 Terminal Turret Mill Max (516) 922-6000 JP1, JP2 2 2802S-03-G2 2mm Pin Header Comm Con (626) 301-4200 JP3 1 2802S-02-G2 2mm Pin Jumper Comm Con (626) 301-4200 L1 1 CR75-100MC CR75-150MC 10H Inductor, Board A 15H Inductor, Board B Sumida (847) 956-0667 Q1 1 Si3443DV Si6447DQ Sublogic Threshold 12V P-Ch MOSFET, Board A Logic Threshold 20V P-Ch MOSFET, Board B Siliconix (800) 554-5565 R1 1 CR16-103JM 10k 5% 0.1W 0603 Resistor AAC (800) 508-1521 R2 1 LR2010-01-050-F 0.05 1% 0.5W 2010 Resistor IRC (361) 992-7900 R4 1 CR16-1004FM 1M 1% 0.1W 0603 Resistor AAC (800) 508-1521 R5 1 WCR0805-1694-F WCR0805-3094-F 1.69M 1% 1/16W 0805 Resistor, Board A 3.09M 1% 1/16W 0805 Resistor, Board B AAC (800) 508-1521 R6 1 WCR0805-6343-F WCR0805-1544-F 634k 1% 1/16W 0805 Resistor, Board A 1.54M 1% 1/16W 0805 Resistor, Board B AAC (714) 255-9186 R7 1 WCR0805-2674-F WCR0805-3744-F 2.67M 1% 1/16W 0805 Resistor, Board A 3.74M 1% 1/16W 0805 Resistor, Board B AAC (714) 255-9186 U1 1 LTC1771EMS8 Switching Regulator Controller IC LTC (408) 432-1900 QUICK START GUIDE Demonstration Board DC293 is easy to set up for evaluation of the LTC1771. Please follow the procedure below for proper operation. * To shut down the circuit, move the jumper JP2 to the SHDN position. For normal operation, JP2 should be in the RUN position. * Move jumper JP3 to the appropriate position for the required output voltage. For voltages other than the preset value, make sure you install the calculated resistor at the pads (see Output Voltage Setup). * Connect the input power supply to the VIN and GND terminals. * For Burst Mode operation at low load currents, move jumper JP1 to the Enable position. To disable Burst Mode operation, move the jumper to the Disable position. * Connect the load between the VOUT and GND terminals. Refer to Figure 4 for proper measurement equipment setup. 3 DEMO MANUAL DC293 NO-DESIGN SWITCHER U OPERATIO INTRODUCTION The demonstration circuit is intended for the evaluation of the LTC1771 switching regulator IC and is not designed for any other purpose. The circuits shown in Figures 1 and 2 highlight the capabilities of the LTC1771. Two versions are available for two different input supply ranges due to the limited voltage ranges of the power MOSFETs. Version A is optimized for lower voltage operation (2.8V to 12V) and provides output voltages of 1.8V, 2.5V or 3.3V, selectable by the appropriate jumper position. Version B is optimized for higher voltage operation (4.5V to 18V) and provides output voltages of 2.5V, 3.3V or 5V. The LTC1771 uses the current mode, constant off-time architecture shown in Figure 3. Current mode operation provides the well known advantages of clean start-up and excellent line and load regulation. Constant off-time adds to this list simplicity (neither an oscillator nor ramp compensation are required) and inherent 100% duty cycle in dropout. The LTC1771 is a current mode switching regulator controller that drives an external P-channel power MOSFET using a constant off-time architecture. Burst Mode operation and ultralow quiescent current provide outstanding light-load efficiency, no-load supply current and enable high efficiencies for over four decades of load current range. 100% duty cycle provides low dropout operation, extending operating time in battery-operated systems. MAIN CONTROL LOOP During normal operation, the P-channel MOSFET is turned on at the beginning of each cycle and turned off when the current comparator, C, triggers the one-shot timer. The external MOSFET switch stays off for the 3.5s one-shot duration and then turns back on again to begin a new cycle. VIN VIN 6 1A CSS READY RUN/SS 1 MODE 8 + VIN CIN 1.23V REFERENCE 22k RSENSE (BURST ENABLE) 10% CURRENT SOFT-START ON EA ON C SENSE + + - 1.23V - 7 10% CURRENT VOUT SLEEP * 250k ITH 2 RC CC 2V READY 1V - GND 4 1V + BLANKING VIN PGATE B 5 MODE ON TRIGGER L 1-SHOT 3.5s *OPTIONAL FOR FOLDBACK CURRENT LIMITING STRETCH VOUT VFB 3 + COUT DC293 F03 Figure 3. LTC1771 Block Diagram 4 DEMO MANUAL DC293 NO-DESIGN SWITCHER U OPERATIO The peak inductor current at which C triggers the one-shot is controlled by the voltage on Pin 2 (ITH), the output of the error amplifier, EA. An external resistive divider connected between VOUT and ground allows EA to receive an output feedback voltage, VFB. When the load current increases, it causes a slight decrease in VFB relative to the 1.23V reference, which, in turn, causes the ITH voltage to increase until the average inductor current matches the new load current. The main control loop is shut down by pulling Pin 1 (RUN/ SS) low. Releasing RUN/SS allows an internal 1A current source to charge the soft-start capacitor, CSS. When CSS reaches 1V, the main control loop is enabled with the ITH voltage clamped at approximately 40% of its maximum value. As CSS continues to charge, ITH is gradually released, allowing normal operation to resume. CSS can also be used for power supply sequencing by setting a turn-on delay equal to approximately CSS /IRUN/SS seconds. Burst Mode OPERATION The LTC1771 provides outstanding low current efficiency and ultralow no-load supply current by using Burst Mode operation when Pin 8 (MODE) is pulled above 2V. Burst Mode operation commences when the load, detected by a comparator monitoring the ITH voltage, falls below about 20% to 30% of the maximum load. During Burst Mode operation, short burst cycles of normal switching to charge the output capacitor are followed by a longer sleep period with the switch off and the load current supplied by the output capacitor. During this sleep period, only the minimum required circuitry--the reference voltage and the error amplifier--are left on. Supply current is further reduced with innovative new circuitry that allows the error amplifier to run on 10% of its normal operating current during sleep mode with no degradation in the transient response, reducing the total supply current to only 9A. At light loads, the regulator spends most of the time in this low quiescent current sleep mode, thus minimizing the losses that would normally dominate (DC supply current losses and switching losses due to the MOSFET switch gate charge). Burst Mode operation can be disabled by pulling the MODE pin to ground. Disabling Burst Mode operation allows the loads to decrease by another decade, to about 1% to 2% of the maximum load, before the regulator must skip cycles to maintain regulation. Although less efficient, disabling Burst Mode operation is useful for reducing both audio and RF interference by reducing voltage and current ripple and keeping frequency constant to lower output currents. SHORT-CIRCUIT PROTECTION When the output is shorted to ground, the off-time is increased in inverse proportion to VOUT, to a maximum of 70s at VOUT = 0V. This increased off-time allows the inductor current to discharge, preventing runaway. Foldback current limiting can be implemented by adding two diodes in series between the output and the ITH pin, as shown in Figure 3, to minimize heat dissipation in the catch diode during the short-circuit condition. OUTPUT VOLTAGE SETUP In this demonstration circuit, output voltages of 1.8V (version A only), 2.5V, 3.3V and 5V (version B only) can be obtained by moving the jumper JP3 to the appropriate position, as indicated on the demo board. If an output voltage other than those provided is desired, one of the feedback resistors R4, R5, R6, R7 or R8 can be removed and replaced with a new value to set the desired voltage according to the following equation: VOUT = 1.23(R4 + R5RX)/R4 where RX is the resistor R6, R7 or R8 associated with the position of jumper JP3. Note also that the output capacitor is rated at 6.3V; if the output voltage approaches this limit, the capacitor must be replaced with a capacitor with the proper rating (preferably twice the output voltage). CHECKING TRANSIENT RESPONSE Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to (ILOAD)(ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT until the regulator loop adapts to the current change and returns VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing, which 5 DEMO MANUAL DC293 NO-DESIGN SWITCHER U OPERATIO would indicate a stability problem. The external components shown in Figure 1's circuit will prove adequate for most applications. HOW TO MEASURE VOLTAGE REGULATION When measuring voltage regulation, remember that all measurements must be taken at the point of regulation. This point is where the LTC1771's control loop looks for the information to keep the output voltage constant. On this demonstration board, it is located between Pin 5 of the LTC1771, the signal ground and the output side of R5. These points correspond to the output terminals of the demonstration board. Test leads should be attached to these terminals. Measurements should not be taken at the end of test leads at the load. This applies to line regulation (input-to-output voltage regulation) as well as load regulation tests. In doing line regulation tests, always look at the input voltage across the input terminals. Refer to Figure 4 for proper monitoring equipment configuration. For the purposes of these tests, the demonstration circuit should be fed from a regulated DC bench supply so additional variation on the DC input does not add an error to the regulation measurements. For measurement of no-load supply current and measurement of efficiency at loads below a milliamp, the input impedance of the voltmeters may have a significant impact on these measurements. For example, for voltmeters with 10M input impedance connected to the input and output, the no-load supply current at VIN = 15V will increase from 10A with no voltmeters connected, to 11.5A with them connected. Likewise, with VIN = 15V and ILOAD = 100A, + VOUT + V GND DC293 F04 Figure 4. Correct Measurement Setup 6 Optimizing the Inductor When the optimal inductance value for L1 is used, the regulator has the highest efficiency and the smoothest transition between Burst Mode operation and continuous mode. The optimal inductor value is, however, dependent upon output voltage. Since the demo boards provide a selection of three output voltages, the inductor provided can only be optimized for one of the three, which is the output voltage with the jumper in the center position, i.e., 2.5V for version A and 3.3V for version B. The optimal inductance for the other output voltages can be calculated with the following equation: LOPT = 75H(VOUT + VD)RSENSE IMAX = 0.1/RSENSE A + V GND This demo board is designed for easy modification. It can accommodate a variety of different MOSFET footprints: TSOP-6, TSSOP-8, SO-8 (on bottom) and SOT-23, a larger catch diode and extra input/output capacitors. Component selection can be very critical in power supply applications. Be sure to refer to the LTC1771 data sheet for guidelines in selecting the external components surrounding the IC. This section highlights a few of the effects to consider when changing components to optimize or change the specifications of the demo board. The demo board is equipped with a 0.05 resistor to set the maximum current to 2A according to the equation: VIN + + COMPONENT CONSIDERATIONS Setting the Maximum Load LTC1771 A the efficiency decreases from 55% to 53% when the voltmeters are connected. Therefore, for the most accurate measurements at light loads, first record the voltmeter readings, then disconnect the voltmeters before making the input supply current measurement. LOAD This resistor can be increased or decreased as necessary to program the regulator for the desired current. If the current is increased, make sure that the increased current does not exceed the ratings of the input capacitor (ripple current), the power MOSFET, Schottky diode or the inductor. DEMO MANUAL DC293 NO-DESIGN SWITCHER U OPERATIO Minimizing No-Load Supply Current Optional Input Capacitor The no-load supply current of the regulator originates from three sources: the LTC1771's 9A sleep mode quiescent current, Schottky diode reverse leakage and feedback resistor leakage. The LTC1771's IQ is drawn directly from the supply, whereas the Schottky and feedback resistor leakage are drawn from the output; thus their effect on the supply varies with duty ratio: from about 10A at low duty ratios to about 15A at higher duty ratios. The demo board is equipped with an extra input capacitor, C9, that may not be needed in the final application but is provided for evaluating the demo board over the full input supply range. The 10F ceramic capacitor C3 is usually sufficient for duty ratios less than about 80% but above this ratio the optional capacitor C9 is recommended. Also, when evaluating the demo board connected to a lab bench supply with typical long leads, disconnecting and reconnecting the supply may cause transients, due to the resonance of the high lead inductance with the high Q ceramic input capacitor, which may exceed the absolute maximum supply voltage of the LTC1771. The lower Q tantalum capacitor in parallel with the ceramic greatly reduces the amplitude of this resonance, eliminating this potential problem. The feedback resistor leakage can be minimized by simply using large valued resistors in the megaohm range. Unfortunately, 1% resistors above 1M are currently not available in sizes smaller than 0805. Selecting Schottky diodes with low reverse leakage current is critical, since the leakage can often approach the magnitude of the LTC1771 supply current. Selecting a low leakage Schottky diode, however, is complicated by the fact that diodes with lower reverse leakage tend to have higher forward drops. Low forward drop is critical for high current efficiency, since loss is proportional to forward drop. Thus a trade-off must be made between low no-load supply current and high efficiency. The UPS5817 used on the demo board provides a good trade-off for a 2A application. Component Manufacturers Besides those components that are used on the demonstration board, other components may also be used. Table 1 is a partial list of the manufacturers whose components you can use for the switching regulator. Using components other than the ones on the demo board requires recharacterizing the circuit for efficiency. Table 1 MANUFACTURER AVX AVX Central Semiconductor Coilcraft Cooper Electronic Technology International Rectifier Microsemi ON Semiconductor Murata-Erie Sanyo Vishay Vishay Siliconix Sprague Sumida TDK Zetex DEVICE Capacitors Resistors Diodes Inductors Inductors MOSFETs, Diodes Diodes MOSFETs, Diodes Capacitors Capacitors Inductors MOSFETs Capacitors Inductors Inductors Diodes TELEPHONE (843) 448-9411 (843) 946-0524 (631) 435-1110 (847) 639-6400 (561) 752-5000 (310) 322-3331 (617) 926-0404 (602) 244-6600 (770) 436-1300 (619) 661-6835 (605) 665-9301 (408) 988-8000 (207) 324-4140 (847) 956-0667 (847) 803-6100 (631) 543-7100 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. FAX (843) 448-1943 (843) 448-6042 (631) 453-1824 (847) 639-1469 (561) 742-0134 (310) 322-3332 (617) 924-1235 (602) 244-3345 (814) 238-0490 (619) 661-1055 (605) 665-0817 (408) 567-8977 (207) 324-7223 (847) 956-0702 (847) 803-6294 (631) 864-7630 7 DEMO MANUAL DC293 NO-DESIGN SWITCHER U W PCB LAYOUT A D FIL Component Side Silkscreen Component Side Component Side Solder Mask Component Side Paste Mask Solder Side Solder Side Solder Mask U PC FAB DRAWI G 2.000 B A D E 2.000 D SYMBOL DIAMETER NUMBER OF HOLES A 0.094 4 PLTD B 0.070 2 NPLTD C 0.035 6 PLTD D 0.026 6 PLTD E 0.010 47 PLTD TOTAL HOLES 65 E A A C NOTES: UNLESS OTHERWISE SPECIFIED 1. MATERIAL: 2 LAYERS, 0.062" THK. FR-4 GLASS EPOXY 2 0Z COPPER CLAD 2. ALL HOLES SHALL BE PLATED THRU 3. PLATE THRU HOLES WITH COPPER 0.0014 MIN. THICKNESS ALL HOLE SIZES IN HOLE TABLE ARE AFTER PLATING 4. SILKSCREEN: WITH WHITE NONCONDUCTIVE EPOXY INK 5. NO SILKSCREEN ALLOWED ON PAD LANDS 6. SOLDERMASK: LPI, GREEN 7. NO BLOCK SOLDERMASKING OF PAD ROWS 8. DO NOT MAKE CHANGES ON SILKSCREEN, SUCH AS COMPANY LOGO, QC STAMPS 9. DO NOT PLATE TOOLING (3 PLCS) AND SCORING (26 PLCS) HOLES 10. SCORING: 0.02 0.017 B 8 PLTD Linear Technology Corporation dc293 LT/TP 0900 500 * PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 FAX: (408) 434-0507 www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 2000