DEMO MANUAL DC242 DC/DC CONVERTER LT1777 High Voltage, Low Noise Step-Down DC/DC Converter U DESCRIPTIO Demonstration Circuit DC242 is a high input voltage, low noise step-down (buck) regulator using the LT(R)1777 switching regulator IC. Typical applications are automotive cellular and GPS receivers, low noise telecom and industrial-instrument power supplies. This controller includes an onboard 700mA peak-current switch and is optimized for use with a high supply voltage. The input voltage can range from 7.4V to 48V. In order to achieve low noise, the LT1777 is equipped with dI/dt limiting circuitry, which is programmed via a small external inductor in the power path. It also contains internal circuitry to limit the W U WW PERFOR A CE SU ARY dV/dt turn-on and turn-off ramp rates. By slowing down the sharp edges during turn-on and turn-off of the power switch, conducted and radiated EMI are significantly reduced. The output voltage is jumper selectable to either 3.3V or 5V. The LT1777 uses a 100kHz switching frequency and current mode control, which offers excellent dynamic input supply rejection and short-circuit protection. Additionally, the supply current can be shut down to less than 30A in standby mode. Gerber files for this circuit are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. TA = 25C PARAMETER CONDITIONS Input Voltage Range IOUT = 0mA to 500mA VALUE 7.4V to 48V Output Voltage (Jumper Selectable) 3.3V, 5V Maximum Output Load Current VIN > 12V, VOUT = 5V Typical Output Ripple Voltage IOUT = 500mA, VIN = 24V Nominal Operating Frequency VIN = 7.4V to 48V IO = 500mA* 17mVP-P 100kHz IQ, Supply Current in Shutdown 30A *IO = 450mA for VIN 12V and VOUT = 5V W U U TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO VSW Switching Waveforms Component Side VSW VOLTAGE 10V/DIV VSW CURRENT 200mA/DIV 500ns/DIV DC242 TA01 1 DEMO MANUAL DC242 DC/DC CONVERTER W W U SCHE ATIC A D PACKAGE DIAGRA S TP2 VIN 7.4V TO 48V TP1 GND + C4 100F 63V C5 0.1F 100V TP6 SYNC 10 12 3 TP5 SHDN 14 R4 68k C3 100pF JP2 OPEN TO EXT SYNC C6 100pF C7 680pF 2 11 15 7 VIN 4 VCC SYNC VSW SHDN VD VC U1 LT1777 FB NC GND NC GND NC GND SGND GND 6 L1 200H CTX200-4 L2 0.47H SML32-470k 5 4 3 1 2 D1 10BQ100 13 1 + C2 100F 10V C1 1F 10V TP3 VOUT 3.3V OR 5V 500mA R3 35.7k 1% TP4 GND 8 9 16 JP1 VOSEL OPEN = 3.3V CLOSE = 5V R2 26.1k 1% R1 21.5k 1% DC242 F01 Figure 1. 100kHz Low Noise Step-Down Switching Regulator TOP VIEW 16 GND GND 1 NC 2 15 NC SHDN 3 14 VC VCC 4 13 FB 12 SYNC VD 5 VSW 6 11 NC SGND 7 10 VIN 9 GND 8 GND S PACKAGE 16-LEAD PLASTIC SO LT1777CS PARTS LIST REFERENCE DESIGNATOR PART NUMBER DESCRIPTION VENDOR C1 QUANTITY 1 0805ZC105MAT1A 1F 10V 20% X7R Capacitor AVX TELEPHONE C2 1 TPSD107M010R0065 100F 10V 20% Tantalum Capacitor AVX (207) 282-5111 C3, C6 2 06035A101JAT1A 100pF 50V 5% NPO Capacitor AVX (843) 946-0362 (619) 661-6835 (843) 946-0362 C4 1 63CV100BS 100F 63V 20% Electrolytic Capacitor Sanyo C5 1 12061C104MAT1A 0.1F 100V 20% X7R Capacitor AVX (843) 946-0362 C7 1 06033A681JAT1A 680pF 25V 5% NPO Capacitor AVX (843) 946-0362 D1 1 10BQ100TR 100V 1A Schottky Diode IR (310) 322-3331 JP1, JP2 2 2802S-02G2 2mm 2-Pin Jumper Comm Con (626) 301-4200 JP1, JP2 2 CCIJ2mm-138-G 2mm 2-Pin Shunt Comm Con (626) 301-4200 L1 1 CTX200-4 200H 0.6A SMT Inductor Coiltronics (561) 241-7876 L2 1 SML32-470K 470nH 450mA SMT Inductor Gowanda (716) 532-2234 2 DEMO MANUAL DC242 DC/DC CONVERTER PARTS LIST REFERENCE DESIGNATOR PART NUMBER DESCRIPTION VENDOR TELEPHONE L3 QUANTITY 1 DT1608C-152 1.5H Inductor (Alternate for L2) Coilcraft (847) 639-6400 R1 1 CR16-2152FM 21.5k 1/16W 1% Chip Resistor Tad (714) 255-9123 R2 1 CR16-2612FM 26.1k 1/16W 1% Chip Resistor Tad (714) 255-9123 R3 1 CR16-3572FM 35.7k 1/16W 1% Chip Resistor Tad (714) 255-9123 R4 1 CR16-683JM 68k 1/16W 5% Tad (714) 255-9123 TP1 to TP6 6 2501-2 0.090 Turret Testpoint Mill-Max (516) 922-6000 U1 1 LT1777CS 16-Lead SO DC/DC Controller IC LTC (408) 432-1900 QUICK START GUIDE Refer to Figure 2 for proper measurement equipment set-up and follow the procedure outlined below: 6. Leave jumper JP2 connected for switching the LT1777 at its nominal frequency of 100kHz. 1. Connect the input power supply to the VIN and GND terminals. Input voltage is limited to between 7.4V and 48V. 7. Set the output voltage with jumper JP1, as shown in Table 1. 2. Connect an ammeter in series with the input supply to measure input current. 3. Connect either power resistors or an electronic load to the VOUT and GND terminals. 4. Connect an ammeter in series with the output load to measure output current. 8. After all connections are made, turn on the input power and verify that the output voltage is correct. Table 1 POSITION OUTPUT VOLTAGE Jumper JP1 Closed 5V Jumper JP1 Open 3.3V 5. The SHDN pin should be left floating for normal operation or tied to GND for shutdown. A VOUT LOAD GND GND VIN A LT1777CS DC242 DC242 F02 Figure 2. Proper Measurement Set-Up 3 DEMO MANUAL DC242 DC/DC CONVERTER U OPERATIO The schematic in Figure 1 highlights the capabilities of the LT1777. The application circuit is set up for an output voltage of either 3.3V or 5V, set by jumper JP1.The demo board comes equipped with input, output, GND, SYNC and SHDN terminals to make bench testing convenient. Since the LT1777 allows such a wide input range, from 7.4V to 48V, the internal control circuitry draws power from the VCC pin, which is normally connected to the output supply. During start-up, the LT1777 draws power from VIN. However, after the switching supply output voltage reaches 2.9V, the LT1777 uses the output to supply its internal control circuitry, thereby reducing quiescent power by hundreds of milliwatts when operating at high input voltage. This helps maximize efficiency at high line voltages. Theory of Operation During normal operation, the internal power transistor is turned on during each cycle when the oscillator sets a latch and turned off when the main current comparator resets the latch. While the internal switch is off, Schottky diode (D1) carries the inductor current until it tries to reverse or until the beginning of the next cycle. The voltage on the VC pin, which is the output of the error amplifier, controls the peak inductor current. The FB pin provides the error amplifier an output feedback voltage, VFB, from an external resistor divider. When the load current increases, it causes a slight decrease in VFB relative to the 1.24V reference, which, in turn, causes the voltage on the VC pin to increase until the average inductor current matches the new load current. Sense Inductor The LT1777 uses an external sense inductor to program the dI/dt limiting circuitry. The minimum usefull inductance value for the sense inductor is 470nH, which produces a dI/dt of approximately 2.2A/s. Larger sense inductors yield lower current slew rates, resulting in reduced high frequency RFI emissions at the expense of slightly reduced efficiency. Refer to the "Selecting Sense 4 Inductor" section in the LT1777 data sheet for information on programming a different dI/dt rate through the main power inductor. SYNC Pin This pin is used to synchronize the internal oscillator to an external clock with a frequency between 130kHz and 250kHz. If a switching frequency higher than nominal is desired, remove jumper JP2 and tie an external oscillator of the desired frequency between the SYNC terminal and the input GND terminal. The external oscillator can be a TTL compatible level or 3.3V, with a duty cycle between 10% and 90%. Refer to the "Selecting Main Inductor" section in the LT1777 data sheet for information on optimizing the inductor value for running at a higher frequency. How to Measure Voltage Regulation and Efficiency When trying to measure load regulation or efficiency, voltage measurements should be made directly across the VOUT and GND terminals rather than at the end of test leads at the load. Similarly, input voltage should be measured directly at the VIN and GND terminals of the DC242. Input and output current should be measured by placing an ammeter in series with the input supply and load. Figure 3 shows the typical efficiency curve for L2 (sense inductor) = 470nH, VIN = 12V, VOUT = 5V. 85 VIN = 12V VOUT = 5V L2 = 470nH 80 EFFICIENCY (%) Introduction 75 70 65 60 0 100 300 400 200 LOAD CURRENT (mA) 500 DC242 F03 Figure 3. LT1777 Output Efficiency DEMO MANUAL DC242 DC/DC CONVERTER U OPERATIO How to Measure Output Voltage Ripple Heat Dissipation Issues In order to measure output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. A sturdy wire should be soldered on the output GND terminal. The other end of the wire is looped around the ground connection of the probe and should be kept as short as possible. The tip of the probe is touched directly to VOUT (see Figure 4). Bandwidth is generally limited to 20MHz for ripple measurements. Also, if multiple pieces of line-powered test equipment are used, be sure to use isolation transformers on their power lines to prevent ground loops, which can cause erroneous results. Figure 5 shows the output voltage ripple with a steady state load of 500mA. Since the LT1777 includes a 500mA onboard power switch, care must be taken not to exceed the maximum junction temperature for the part. A simple technique is to use the PC board as a heat sink. The four corner pins on the LT1777 IC are connected to the ground plane on both sides of the PC board. The two sides are connected through vias to better handle the power dissipation. If the LT1777 is placed on a multilayer board, it is desirable to have metal on the inner layers directly underneath the LT1777. This helps in spreading heat and improves the power dissipation capability of the PCB. Layout Guidelines Since the LT1777 is a switching regulator, attention to proper layout is essential for good load regulation and to minimize radiated/conducted noise. Be sure to follow these layout guidelines: PROBE DC242 LT1777 VOUT GND DC242 F04 Figure 4. Measuring Output Voltage Ripple 1. The LT1777's VSW pin, sense inductor L2 and Schottky diode D1 should be placed as close as possible to each other. 2. The anode of Schottky diode D1 should be tied directly to the ground plane. 3. Keep the trace from the FB pin to the junction of R3 and R2 short and use a long trace from the top of resistor R3 to the output terminal, rather than vice versa. 4. Output capacitor C2's grounds should be tied directly to the ground plane. 5. The ground of the feedback resistors and the loop compensation resistor/capacitor (connected to the VC pin) should be referenced to the chip SGND pin, which, in turn, is directly tied to the ground plane. DC242 F05 6. C5 should be as close as possible to Pin 10. Figure 5. Output Voltage Ripple for the LT1777, IL = 500mA 5 DEMO MANUAL DC242 DC/DC CONVERTER W U PCB LAYOUT A D FIL Component Side Silkscreen Component Side Solder Mask Component Side Paste Mask 6 DEMO MANUAL DC242 DC/DC CONVERTER W U PCB LAYOUT A D FIL Copper Layer 1 (Top) Copper Layer 2 Solder Side Solder Mask 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 DC242 DC/DC CONVERTER U PC FAB DRAWI G 2000 300 225 C NOTES: UNLESS OTHERWISE SPECIFIED 1. WORKMANSHIP SHALL BE IN ACCORDANCE WITH IPC-A-600E 2. FINISHED MATERIAL IS FR4, 0.062 THICK 2 OZ COPPER, 2 LAYERS PLATED HOLE WALL THICKNESS 0.001 MIN 3. PROCESS AND PLATING: SMOBC, TIN-LEAD 4. SOLDER MASK BOTH SIDES USING GLOSSY GREEN LPI 5. SILKSCREEN WHITE NONCONDUCTIVE EPOXY INK COMPONENT SIDE D A A 2000 D A D A A B B C SYMBOL DIAMETER NUMBER OF HOLES A 15 9 B 30 4 C 70 2 D 94 6 TOTAL HOLES 21 ALL DIMENSIONS ARE IN MILS 8 Linear Technology Corporation dc242f LT/TP 0599 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 1999