QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER LT3431 DESCRIPTION Demonstration circuit 549 is a monolithic step-down DC/DC switching converter using the LT3431. The board is optimized for 5V output at up to 2A load current for a steady state input voltage range of 7.5V to 20V. The LT3431 is capable of withstanding input transients to 60V and runs at 500kHz with steady state input voltages that require duty cycles between 25% and 75%. For duty cycles below 25% the part may pulse skip to effectively average the lower duty cycle and still provide minimum on-time switch cycles. With its wide input voltage range, 3A internal power switch, high 500kHz switching frequency and thermally enhanced package, the LT3431 is a very versatile and powerful IC for DC/DC converters that require compact size, high efficiency and tolerance to high input voltage transients. The LT3431 500kHz switching frequency allows all of the components to be small, surface mount devices. Synchronization with an external clock of up to 700kHz is possible. The current-mode control topology creates fast transient response and good loop stability with a minimum number of external compensation components. In particular, the current-mode architecture allows the use of ceramic input and output capacitors for increased reliability, extremely low output ripple voltage and small component size. The low resistance internal power switch (0.1Ω) maintains high efficiencies of up to 87%. The SHDN pin can be used to program undervoltage lockout or place the part in micropower shutdown, reducing supply current to 30µA by driving the pin low. The LT3431 datasheet gives a complete description of the part, operation and applications information. The datasheet must be read in conjunction with this Quick Start Guide for demonstration circuit 549. In particular, the datasheet section on “Thermal Calculations” is important for estimating whether a given application’s combination of input voltage, load current and frequency will cause the LT3431 to exceed it’s absolute maximum rated junction temperature. The part is assembled in a small 16pin thermally enhanced package with exposed pad where proper board layout is essential for maximum thermal performance. See the datasheet section “Layout Considerations”. NOTE: Do not hot-plug the input voltage terminal VIN. The absolute maximum voltage on VIN is 60V and hotplugging a power supply through wire leads to the demonstration card can cause the voltage on the extremely low-ESR ceramic input capacitor to ring to twice its DC value. This is due to high currents instantaneously generated in the inductive supply leads from an input voltage step on the low-ESR ceramic input capacitor. An over-voltage can result if care is not taken to ensure that this does not happen. A bulky higher-ESR capacitor, and an additional inductive 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER filter between the bulk capacitor and the input capacitor can be added to the circuit to dampen hot-plug transient ringing. See Application Note 88 for more details. Design files for this circuit board are available. Call the LTC factory. Table 1. Typical Performance Summary PARAMETER CONDITION Steady State Input Voltage Range VOUT = 5V, IOUT ≤ 2A, TA = 25°C 7.5–20V Maximum Transient Input Voltage VOUT VALUE 60V IOUT ≤ 2A 5V ±3% Maximum Output Current 2A Typical Switching Frequency 500kHz Typical Efficiency VIN = 12V, VOUT = 5V, IOUT = 2A 83% 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER QUICK START PROCEDURE Demonstration circuit 549 is easy to set up to evaluate the performance of the LT3431. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE: Make sure that the input voltage does not exceed 60V. NOTE: The synchronization and shutdown can be left floating (disconnected) if their functions are not being used. 1. Connect the power supply, load, and meters shown in Figure 1. 2. After all connections are made, turn on input power and verify that the output voltage is 5.0V. functions are optional and their pins + + – – BENCH SUPPLY + – + LOAD – – + + – – + OPTIONAL Figure 1. Proper Measurement Equipment Setup CUSTOMIZING THE DEMONSTRATION CIRCUIT CUSTOMIZING THE OUTPUT VOLTAGE The components assembled on the board are optimized for a wide input voltage range and a 5V output. The feedback resistors (R2, R3) can be changed to adjust the output voltage according to the following equation: VOUT = 1.22 (1 + R2/R3) The Thevenin resistance seen at the FB pin should be less than 3.8k to maximize frequency foldback start-up and short-circuit. during For output voltages below 3V, the boost diode should be moved from D2 to D3 to provide the minimum boost voltage required for the internal power switch. Make sure that the boost capacitor (C4) has a voltage rating greater than the output voltage for applications where the boost diode is placed in D2 and greater 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER than the input voltage when the boost diode is placed in position D3. For output voltages greater than 5V, the optional ‘blocking’ zener diode D4 can be used to reduce the boost voltage across C4 from the output voltage to some lower voltage between 3V and 5V. This diode transfers power dissipation from inside the IC to the discrete component outside the IC, allowing higher ambient temperature operation for the part. It is recommended that an SMAZ7V5 zener diode is used in D4 when VOUT = 12V. To properly install D4, the small trace shorting the anode to the cathode of D4 on the board must be opened (an Exacto knife works well) before D4 is soldered to the board. In the ‘Thermal Calculations’ section of Applications Information in the datasheet, the boost power consumption in the IC must account for the zener blocking diode VZ, so the boost voltage changes from VOUT to (VOUT – VZ). COMPENSATION Demonstration Circuit 549 has a frequency compensation network that is optimized for the ceramic output capacitor C5, the wide input voltage range 7.5V to 20V, and 5V output. Improved loop bandwidth can be achieved for various output voltages, output capacitors, and input voltage ranges by adjusting R1, C1, and C2. Additional optional component locations for feedforward capacitor (C8) and resistor (R4 for short circuit feedback pin protection when feedforward capacitor is used) are located in parallel with R2. For more information, see the ‘Frequency Compensation’ section in the Applications Information in the datasheet, Application Notes 19 or Application Note 76. SOFT START When the ratio of the input voltage divided by the output voltage (plus the forward voltage of the catch diode) is greater than four VIN/(VOUT + VF) > 4 the soft start circuit made up of components C9, R5, R6, and Q1 should be used to control output voltage rise time and overshoot during startup. The soft start circuit is covered in detail in ‘Buck Converter with Adjustable Soft Start’ in the Applications Information in the datasheet. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 549 2A, 500KHZ HIGH VOLTAGE BUCK CONVERTER 100% 90% 80% EFFICIENCY (%) 70% 60% 50% 8.0 V in 40% 12.0 Vin 30% 20.0 Vin 20% 10% 0% 0 500 1000 1500 2000 LO AD CURRENT (mA) Figure 2. Efficiency 2800 Tj = 125C Maximum Load Current (mA) 2600 2A Load, Tj < 125C 2400 2200 2000 1800 1600 1400 1200 7 9 11 13 15 17 19 21 23 25 VI N ( V ) Figure 3. Maximum Load Current Capabilities (TA = 25°C) 5 5 D3 MMSD914T1 (Opt.) 2 1 E1 + C10 (Opt.) E2 3 4 6 Vin Vin BOOST 15 SHDN SW 2 14 SYNC SW 5 E4 C 7 NC 13 NC 1 Gnd 8 Gnd BIAS Gnd Vc 9 16 11 R4 (Opt.) Gnd C D1 30BQ060 SMC R6 (Opt.) 3 C1 220pF 1 2 C2 .015uF Q1 MMBT2222A (Opt.) NOTE: UNLESS OTHERWISE SPECIFIED R5 (Opt.) B LINEAR TECHNOLOGY CORPORATION For All Optional Components On Boards: Q1,R5,R6,C9 Show Ref. Des. And Outline D3,D4 Show Ref. Des. With No Outline C6,C7,C8,C10,R4 Show No Ref. Des. And No Outline 1630 McCARTHY BLVD. MILPITAS, CA 95035 PHONE (408) 432-1900 FAX (408) 434-0507 * High Vin Possible ( Up to 60V) With Thermal Constraints. See Quick Start Guide Title Size 4 3 A 3A, 500kHz High Voltage Buck Converter Document Number DC549A Date: Tuesday, September 03, 2002 5 E6 Vout 5V@2A C5 47uF 6.3V Cer C8 (Opt.) 12 R1 1.5K A C6 (Opt.) 0805 10 R3 4.99k 1% B E5 2 C9 (Opt.) R2 15.4k 1% Gnd D L1 10uH UP2B-100 1 U1 LT3431EFE FB 1 D4 SMAZ7V5 (Opt.) 2 1 C4 0.22uF 16V E3 SYNC D2 MMSD914T1 SOD123 1 2 C3 2.2uF 100V Gnd SHDN 2 1 Vin 7.5V-20V* 3 2 D 4 2 LT3431EFE Sheet 1 of 1 Rev 1 Bill Of Material Demo Bd. # 549A Linear Technology Corporation LT3431EFE 6/17/2005 1:38 PM Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Qty Reference 1 1 1 1 1 0 0 0 1 1 0 0 6 1 0 0 1 1 1 0 1 4 4 C1 C2 C3 C4 C5 C6 C8,C9 C10 D1 D2 D3 D4 E1-E6 L1 L1 Q1 R1 R2 R3 R4,R5,R6 U1 4 CORNERS Part Description CAP, NPO 220pF 25V CAP, X7R 0.015uF 50V CAP, Y5U 2.2uF 100V CAP, X7R 0.22uF 16V CAP, CER 47uF 6.3V CAP, 0805 ( Opt. ) CAP, 0603 ( Opt. ) CAP, C-CASE ( Opt. ) DIODE SCHOTTKY,3A, 60V,SMA DIODE, 200mA, 100V DIODE, ( Opt. ) DIODE, ( Opt. ) TP, TURRET, .094" INDUCTOR, SMT, 10uH, L-COIL-UP2B INDUCTOR, ( Opt. ) TRANS, ( Opt. ) RES, 1.5K 1%, 0603 RES, 15.4k 1%, 0603 RES, 4.99k 1%, 0603 RES, ( Opt. ) IC, LT3431EFE STANDOFF, NYLON HEX #4-40 x 1/4" SCREW, #4-40 x 1/4 Page 1 - of - 1 Manufacture / Part # AVX, 06033A221KAT1A 0603 AVX, 06035C153KAT1A 0603 UNITED CHEMI-CON, THCS50E2A225ZT TAIYO YUDEN, EMK212BJ224MG 0805 TDK, C4532X5R0J476M 1812 IR, 30BQ060 SMC DIODES INC. 1N4148W-7 SOD-123 ON SEMI., MMSD914T1-7 SOD-123 DIODES INC., SMAZ7V5 SMA MILL-MAX, 2501-2 COOPER ELECT. UP2B-100 SUMIDA, CDRH8D28 DIODES INC., MMBT2222A-7 SOT-23 AAC, CR16-1501FM AAC, CR16-1542FM AAC, CR16-4991FM LINEAR TECH., LT3431EFE TSSOPEF16 MICRO PLASTICS, #14HTSP101 ANY