DEMO MANUAL DC270 NO-DESIGN SWITCHER DESCRIPTIO LTC1772 Constant Frequency Current Mode Step-Down DC/DC Converter U Demo board DC270 is a step-down (buck) regulator using the LTC1772. The exclusive use of surface mount components results in a highly efficient application in a very small board space. It is ideal for cell phones and other portable electronics operating from one or two Li-Ion cells or three to six NiCd cells. DC270 is capable of providing 1A at an output voltage of 2.5V with an input supply of 4.2V. This demo board highlights the capabilities of the LTC1772, which uses a current mode PWM architecture to drive an external P-channel power MOSFET. The result is a high performance power supply that has low output voltage ripple. Constant operating frequency makes the LTC1772 attractive for noise-sensitive applications. In addition, high efficiency over a wide load current range makes the LTC1772 ideal for battery-powered applications. In dropout, the external P-channel MOSFET is turned on continuously (100% duty cycle), providing low dropout operation with VOUT VIN. To further enhance efficiency at low load currents, the LTC1772 is configured for Burst ModeTM operation. The LTC1772 is capable of operating down to approximately 2.2V input voltage before the undervoltage lockout feature is activated. 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 CONDITIONS VIN Input Working Voltage Range VOUT = 2.5V VOUT Output Voltage IOUT = 1A VFB Feedback Voltage IQ Typical Supply Current VALUE 2.5V to 9.8V 2.5V 0.0625V 0.8V 0.02V Normal Mode Shutdown VIN = 4.2V, IOUT = 0mA VIN = 4.2V, VITH/RUN = 0V 220A 7A W U U TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO Component Side Efficiency vs Load Current 100 VIN = 3.3V EFFICIENCY (%) 90 VIN = 5V 80 VIN = 8V 70 60 50 VOUT = 2.5V RSENSE = 0.03 40 1 100 1000 10 LOAD CURRENT (mA) 10000 DC270 TA01 1 DEMO MANUAL DC270 NO-DESIGN SWITCHER W U WW PERFOR A CE SU ARY SYMBOL PARAMETER CONDITIONS VALUE IOUT Maximum Output Current VIN = 4.2V, VOUT = 2.5V VOUT Typical Load Regulation 0mA IOUT 1A, VIN = 8.5V VRIPPLE Typical Output Ripple in Burst Mode Operation IOUT = 100mA, VIN = 4.2V 1A (Min) -1% 120mVP-P W W U PACKAGE A D SCHE ATIC DIAGRA SM SHDN CC1 220pF RC1 10k 1 2 3 RF2 80.6k 1% U1 LTC1772 ITH/RUN PGATE GND VFB VIN SENSE - 6 5 CIN 10F 10V RCS 0.040 VIN+ GND 4 TOP VIEW 4 5 6 RF1 174k 1% 4 SENSE - VFB 3 L1 4.7H D1 MBRM120T3 5 VIN GND 2 M1 FDC638P 3 2 1 6 PGATE ITH/RUN 1 + CO1 47F 6V CO2 4.7F 6.3V OPTIONAL VOUT 2.5V S6 PACKAGE 6-LEAD PLASTIC SOT-23 LTC1772CS6 GND DC270 F01 Figure 1. LTC1772 Constant Frequency, Current Mode, Step-Down DC/DC Converter Schematic PARTS LIST REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR CO1 1 6TPA47M 47F 6V POSCAP Capacitor Sanyo (619) 661-6835 CO2 (Optional) CIN 1 1 JMK212BJ475MG LMK325BJ106K-T 4.7F 6.3V Capacitor 10F 10V Capacitor Taiyo Yuden Taiyo Yuden (408) 573-4150 (408) 573-4150 CC1 D1 1 1 06035A221KAT MBRM120T3 220pF 10% NPO Capacitor Schottky Diode AVX ON Semiconductor (843) 946-0362 (602) 244-6600 L1 M1 1 1 DO1608C-472 FDC638P 4.7H Inductor MOSFET Coilcraft Fairchild (847) 639-6400 (408) 822-2126 RCS RC1 1 1 LR1206-01-R040F CR16-103JM 0.040 1% 0.25W 1206 Resistor 10k 5% 1/8W 0603 Resistor IRC TAD (361) 992-7900 (800) 508-1521 RF1 RF2 1 1 CR16-1743FM CR16-8062FM 174k 1% 0.1W 0603 Resistor 80.6k 1% 0.1W 0603 Resistor TAD TAD (800) 508-1521 (800) 508-1521 U1 1 LTC1772CS6 6-Pin SOT-23 IC LTC (408) 432-1900 2 TELEPHONE DEMO MANUAL DC270 NO-DESIGN SWITCHER QUICK START GUIDE This demonstration board is easy to set up to evaluate the performance of the LTC1772. Please follow the procedure outlined below for proper operation. 2. Connect the load between the VOUT and GND terminals. Refer to Figure 4 for proper measurement equipment setup. 1. Connect the input power supply to the VIN and GND terminals. 3. To shut down the circuit, connect the ITH/RUN pin to ground. U OPERATIO The circuit shown in Figure 1 operates from an input voltage between 2.5V and 9.8V. The output voltage of 2.5V is fixed. For other output voltages, resistor RF1 must be replaced (see Output Voltage Setup). This demonstration circuit has been optimized for efficiency and physical footprint. For other requirements, please contact the factory. This demonstration circuit is intended for the evaluation of the LTC1772 switching regulator IC and was not designed for any other purpose. OPERATION The LTC1772 uses the constant-frequency, pulse-widthmodulated, current mode architecture shown in Figure 2. Current mode operation provides the well known advantages of clean start-up and excellent line and load regulation. The LTC1772 is designed to operate down to approximately 2.2V input voltage, making it suitable for applications that are powered either by a low input supply or a single lithium-ion battery. The external MOSFET can limit the minimum input voltage; therefore, be careful when specifying the MOSFET. To prevent damage to a lithium-ion battery by deep discharge, an undervoltage lockout circuit is incorporated into the LTC1772. When the input supply drops to approximately 2.2V, all circuitry except the undervoltage detector block is turned off. The LTC1772 operates as follows: the external P-channel power MOSFET is turned on at the beginning of each cycle when the oscillator sets the latch (RS1) and is turned off when the current comparator (ICOMP) resets the latch. The peak inductor current at which ICOMP resets the RS latch is controlled by the voltage on the ITH/RUN pin, which is the output of the error amplifier, EAMP. An external resistive divider connected between VOUT and ground allows the EAMP to receive an output feedback voltage, VFB. When the load current increases, it causes a slight decrease in VFB relative to the 0.8V reference, which, in turn, causes the ITH/RUN voltage to increase until the average inductor current matches the new load current. The main control loop is shut down by pulling the ITH/RUN pin low. Releasing ITH/RUN allows an internal 0.5A current source to charge the external compensation network. When the ITH/RUN pin reaches 0.4V, the main control loop is enabled with the ITH/RUN voltage, and then pulled up to its zero-current level of approximately 0.7V. As the external compensation network continues to charge, the corresponding output current trip level follows, allowing normal operation. Comparator OVP guards against transient overshoots >7.5% of the target output voltage by turning off the P-channel power MOSFET and keeping it off until the fault is removed. 3 DEMO MANUAL DC270 NO-DESIGN SWITCHER U OPERATIO VIN SENSE - 5 4 + ICMP - VIN RS1 SLOPE COMP OSC PGATE SWITCHING LOGIC AND BLANKING CIRCUIT R Q S 6 - FREQ FOLDBACK SCD BURST CMP + 0.3V SLEEP - + 0.15V OVP + - VREF + 60mV + VREF 0.8V VIN EAMP 0.5A VFB + - 1 ITH/RUN 3 VIN VIN 0.3V - 0.4V VOLTAGE REFERENCE + SHDN CMP VREF 0.8V - GND SHDN UV 2 UNDERVOLTAGE LOCKOUT 1.2V DC270 F02 Figure 2. LTC1772 Block Diagram Burst Mode Operation Undervoltage Lockout The LTC1772 enters Burst Mode operation at low load currents. In this mode, the peak current of the inductor is set as if VITH/RUN = 1V (at low duty cycles), even though the voltage at the ITH/RUN pin is at a lower value. If the inductor's average current is greater than the load requirement, the voltage at the ITH/RUN pin will drop. When the ITH/RUN voltage goes below 0.85V, the sleep signal goes high, turning off the external MOSFET. The sleep signal goes low when the ITH/RUN voltage goes above 0.925V and the LTC1772 resumes normal operation. The next oscillator cycle will turn the external MOSFET on and the switching cycle repeats. To prevent deep discharge of a lithium-ion battery when it is near its end of charge, an undervoltage lockout circuit is incorporated into the LTC1772. When the input supply voltage drops below approximately 2.2V, the UV lockout feature turns off the P-channel MOSFET and all circuitry except the undervoltage block, which draws only several microamperes. 4 DEMO MANUAL DC270 NO-DESIGN SWITCHER U OPERATIO Short-Circuit Protection HOW TO MEASURE VOLTAGE REGULATION When the output is shorted to ground, the frequency of the oscillator is reduced to about 90kHz. This low frequency allows the inductor current to safely discharge, thereby preventing current runaway. The oscillator's frequency will gradually increase to its designed rate when the feedback voltage again approaches 0.8V. When measuring voltage regulation, all measurements must be taken at the point of regulation. This point is where the LTC1772 control loop looks for the information to keep the output voltage constant. This information appears between Pin 3 and Pin 2 of the LTC1772. For output voltages above 0.8V, the voltage at Pin 3 can be adjusted by the resistor divider network. These points correspond to the output terminals of the demonstration board. Test leads should be attached to these terminals and the load should be attached as close to these terminals as possible. This applies to line regulation (input-to-output voltage regulation) as well as load regulation tests. In performing line regulation tests, always look at the input voltage across the input terminals. Refer to Figure 4 for proper monitoring equipment configuration. Output Voltage Setup In this demonstration circuit, the output voltage is set for 2.5V. Output voltages other than 2.5V can be obtained by removing component RF1 = 174k and replacing it with a resistor of the value: V RF 1 = 80.6k OUT - 1 0.8 Note that output votlages below 0.8V are not possible with this topology. Higher output voltages may require a substitute output capacitor, since the installed output capacitor is rated for 6V. For the purposes of these tests, the demonstration circuit should be powered by a regulated DC bench supply so additional variation on the DC input does not add an error to the regulation measurements. LTC1772 A 0.8V VOUT + EAMP VFB - RF1 174k RF2 80.6k LTC1772 + +VIN +VOUT + V + A + GND LOAD V SD GND DC270 F03 DC232 F04 Figure 3. Output Voltage Setting Figure 4. Correct Measurement Setup 5 DEMO MANUAL DC270 NO-DESIGN SWITCHER U OPERATIO CHECKING TRANSIENT RESPONSE Switching regulators take several cycles to respond to a step in DC 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 would indicate a stability problem. The external components shown in Figure 1 will prove adequate for most applications. A second, more severe transient is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25)(CLOAD). Thus, a 10F capacitor would require a 250s rise time, limiting the charging current to about 200mA. COMPONENTS Component selection can be very critical in switching power supply applications. This section discusses some of the guidelines for selecting the different components. The LTC1772 data sheet details more specific selection criteria for most of the external components surrounding the IC. Refer to the data sheet if changes to this demo circuit are anticipated. Capacitors The most common component uncertainty with switching power supplies involves capacitors. In this circuit (refer to Figure 1) CIN and CO1 are low ESR, high ripple-current 6 capacitors. ESR (or equivalent series resistance) is the parasitic series resistance in the capacitor. Often this resistance is the limiting element in reducing ripple at the output or input of the supply. The capacitors used in this circuit are specifically designed for switching power supplies. One other choice of capacitors is organic semiconductor types (OS-CON) that are specifically made for power supply applications. They have very low ESR and are ~1/2 the size of equivalent wet electrolytics. Power MOSFET Since the LTC1772 is designed for operation down to approximately 2.2V, a sublogic threshold MOSFET (RDS(ON) guaranteed atVGS = 2.5V) is required for applications that work close to this voltage. When these MOSFETs are used, make sure that the input supply to the LTC1772 is less than the absolute maximum VGS ratings, typically 8V. Inductor Although the inductor used in the demo board is from Coilcraft, a wide variety of inductors are available from other manufacturers. Many inductors will work in this circuit; the only fixed requirement is that the inductor be able to support the output DC current and still maintain its inductance value. Each inductor design will have a different physical size, different loss characteristics and different stray field patterns. Therefore, the circuit must be recharacterized for efficiency if any of the alternate inductors are used in place of the existing one. Because of the aforementioned variations in design and cost of the inductor, we suggest you contact some of the inductor manufacturers in Table 1 to discuss your needs. Often, a standard, low cost solution that will meet your needs is available. DEMO MANUAL DC270 NO-DESIGN SWITCHER U OPERATIO Table 1. Inductor Manufacturers Sense Resistor The current sense resistor specified in the component list is manufactured by International Resistive Company. Alternate resistors can be obtained from Dale. Schottky Diode The catch diode D1 carries load current during the offtime. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages, the diode conducts most of the time. As VIN approaches VOUT, the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short circuited. Under this condition, the diode must safely handle IPEAK at close to 100% duty cycle. A high speed switching diode optimizes efficiency. Schottky diodes are a good choice for low forward drop and fast switching times. Component Manufacturers Besides those components that are used on the demonstration board, other components may also be used. Below is a partial list of the manufacturers whose components can be used for the switching regulator. Using components other than the ones on the demonstration board requires recharacterizing the circuit for performance. MANUFACTURER PART NUMBERS Coilcraft 1102 Silver Lake Road, Cary, IL 60013 (847) 639-6400, FAX: (847) 639-1469 D01608 Series Coiltronics International 6000 Park of Commerce Blvd., Boca Raton, FL 33487 (561) 241-7876, FAX: (561) 241-9339 Econo-Pac Octa-Pac API Delevan 270 Quaker Road, East Aurora, NY 14052 (716) 652-3600, FAX: (716) 652-4814 4501 Series Sumida Electric Co. Ltd. 5999 New Wilke Rd., Suite 110, Rolling Meadows, IL 60008 (847) 956-0667, FAX: (847) 956-0702 CD 43 Series CDH 53 Series CDRH62B Murata Electronics 1900 W. College Ave., State College, PA 16801-2799 (814) 237-1431, FAX: (814) 238-0490 LQN6C Series Table 2. Capacitor Manufacturers MANUFACTURER PART NUMBERS AVX Corporation P.O. Box 867, Myrtle Beach, SC 29578 (843) 946-0362, FAX: (843) 448-1943 TPS Series Sanyo Video Components 2001 Sanyo Avenue, San Diego, CA 92173 (619) 661-6835, FAX: (619) 661-1055 OS-CON Series POSCAP Series Sprague 678 Main Street, Sanford, ME 04073 (207) 324-4140, FAX: (207) 324-7223 593D Series Murata Electronics 1900 W. College Ave., State College, PA 16801-2799 (814) 237-1431, FAX: (814) 238-0490 GRM 200 Series 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. 7 DEMO MANUAL DC270 NO-DESIGN SWITCHER U W PCB LAYOUT A D FIL Component Side Silkscreen Component Side Solder Mask Top Layer Component Side Paste Solder Side Solder Mask Solder Side U PC FAB DRAWI G 2.0" A B B C B B 1.7" D B SYMBOL DIAMETER NUMBER OF HOLES A 0.070 2 B 0.065 5 C 0.025 6 D 0.015 1 TOTAL HOLES 14 A 8 Linear Technology Corporation dc270 LT/TP 0300 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