DEMO MANUAL DC187/DC188 NO DESIGN SWITCHER DEMO MANUAL DC187/DC188 DESCRIPTIO LT1374 Monolithic 4A Switcher 5.5V to 25V Input 3.3V or 5V Output U Demonstration circuits DC187/DC188 are complete DC/DC step-down regulators using the LT ®1374, constant frequency, high efficiency converter in 7-pin DD (DC187) and SO-8 (DC188) packages. These circuits are primarily used in personal computers, disk drives, portable hand- held devices and in larger systems, as local onboard regulators. High frequency switching allows the use of small inductors, making these all surface mount solutions ideal for space-conscious systems. , LTC and LT are registered trademarks of Linear Technology Corporation. WW U W PERFORmANCE SU ARY TA = 25°C, VIN = 10V, ILOAD = 2A, VOUT = 3.3V (Jumper J1 inserted), SHDN pin open unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Output Voltage Jumper J1 Removed (Note 1) Jumper J1 Inserted 4.91 3.23 5.02 3.30 5.20 3.42 V V Maximum ILOAD (Note 2) 4 Input Voltage Range (Note 3) 5.5 Switching Frequency 460 Output Ripple Voltage A 25 500 V 540 kHz 60 mVP-P 8 mV Line Regulation 5.5V to 25V Load Regulation ILOAD = 10mA to 4A SHDN Lockout Threshold Shutdown Option Boards 2.3 2.38 2.46 V SHDN Shutdown Threshold Shutdown Option Boards 0.15 0.37 0.6 V Synchronization Range SYNC Option Boards 580 Supply Current SHDN = 0V Note 1: Output voltage variations include the ±1% tolerance of the feedback-divider network. For tighter voltage range, use higher tolerance resistors or a fixed 5V output device, the LT1374-5. 5 mV 1000 20 kHz µA Note 2: For DC188 additional thermal restrictions apply. Note 3: For operating voltages down to 4V consult LTC Marketing for details on the LT1506. BOARD PHOTOS DC187 DC188 1 DEMO MANUAL DC187/DC188 U W TYPICAL PERFOR A CE CHARACTERISTICS Temperature Rise vs Load Current 90 90 DIE TEMPERATURE RISE (°C) EFFICIENCY (%) 86 84 82 80 78 100 VIN = 10V VOUT = 5V 80 88 Temperature Rise vs Time, DC188 DC188 DIE TEMPERATURE RISE (°C) 10VIN, 5VOUT Efficiency 70 60 DC187 50 40 30 20 VIN = 10V VOUT = 5V IOUT = 4A 80 60 IOUT = 3A 40 IOUT = 2A 20 IOUT = 1A 10 76 0 0 1 2 4 3 0 2 1 3 0 4 20 10 TIME (SECONDS) 0 LOAD CURRENT (A) LOAD CURRENT (A) 187/88 TA02 187/88 TA01 187/88 TA03 W W U PACKAGE A D SCHE ATIC DIAGRA SM DC187 D2 MMBD914LT1 E1 VIN 5.5V TO 25V + C3 10µF 50V 2* BOOST SW VIN U1 LT1374CR SHDN GND E3 GND 4 L1 6.8µH + 6 5 E2 S/D C4 0.33µF 16V D3 OPTIONAL FB 3 D1 MBRD835L + C7 OPTIONAL R2 5.36k 1% R4 2.74k 1% + C6 0.47µF 25V DC187 SD R3 4.99k 1% VC 1 C1 1500pF 50V FRONT VIEW TAB IS GND 7 6 5 4 3 2 1 R PACKAGE 7-LEAD PLASTIC DD PAK LT1374CR (LT1374CR-SYNC) 2 E5 VOUT 3.3V/4A OR 5V/4A E4 GND J1 3.3V/5V LINK 7 R1 OPTIONAL C2 OPTIONAL *SYNC FUNCTION REPLACES SHDN FOR LT1374CR-SYNC PARTS C5 100µF 10V 30 FB BOOST VIN GND VSW SHDN (SYNC) VC DEMO MANUAL DC187/DC188 W W U PACKAGE A D SCHE ATIC DIAGRA SM DC188 D2 MMBD914LT1 C4 0.33µF 16V D3 OPTIONAL E1 VIN 5.5V TO 25V + C3 10µF 50V 7* E2 S/D + 2 1 BOOST U1 SW LT1374CS8 BIAS VIN SHDN GND FB 8 D1 MBRD835L C5 100µF 10V + C7 OPTIONAL R2 5.36k 1% R4 2.74k 1% + 5 3 R3 4.99k 1% 6 R1 OPTIONAL C2 OPTIONAL E5 VOUT 3.3V/4A OR 5V/4A C6 0.47µF 25V E4 GND J1 3.3V/5V LINK VC 4 E3 GND L1 6.8µH DC188 SD TOP VIEW VIN 1 8 VSW BOOST 2 C1 1500pF 50V *SYNC FUNCTION REPLACES SHDN FOR LT1374CS8-SYNC PARTS 7 SHDN (SYNC) FB 3 6 VC FGND 4 5 BIAS S8 PACKAGE 8-LEAD PLASTIC SO LT1374CS8 (LT1374CS8-SYNC) PARTS LIST REFERENCE DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR 08055C152MAT2S 1500pF 50V X7R Chip Capacitor AVX (803) 946-0362 10µF 50V Y5U Chip Capacitor Marcon (847) 696-2000 0.33µF 16V X7R Chip Capacitor AVX (803) 946-0362 100µF 10V TPS Tantalum Capacitor AVX (207) 282-5111 0.47µF 25V Y5V Chip Capacitor AVX (803) 946-0362 MBRD835L SMT Diode Motorola (602) 244-3576 MMBD914LT1 Diode Motorola (602) 244-3576 C1 1 C2, C7 2 C3 1 THCR60E1H106ZT C4 1 0805YC334MAT2S C5 1 TPSD107M010R0100 C6 1 12063G474MAT3S D1 1 D2 1 TELEPHONE Optional Capacitor D3 1 E1 to E5 5 2501-2 Turret Optional Diode Mill-Max (516) 922-6000 J1 1 2802S-02-G2 0.079" Center 2-Pin Header Comm Con (818) 301-4200 P1 1 CCIJ2MM-138-G 0.079" Center 2-Pin Shunt Comm Con (818) 301-4200 L1 1 DO3316P-682 6.8µH 20% Inductor Coilcraft (847) 639-1469 R1 1 R2 1 CR10-5361F-T 5.36k 1/8W 1% Chip Resistor TAD (714) 255-9123 R3 1 CR10-4991F-T 4.99k 1/8W 1% Chip Resistor TAD (714) 255-9123 R4 1 CR10-2741F-T 2.74k 1/8W 1% Chip Resistor TAD (714) 255-9123 U1 (DC187) 1 LT1374CR (-SYNC) IC LTC (408) 432-1900 U1 (DC188) 1 LT1374CS8 (-SYNC) IC LTC (408) 432-1900 Optional Resistor 3 DEMO MANUAL DC187/DC188 U OPERATIO DC187 vs DC188 (Temperature vs Package Size) The DC187 and DC188 demonstration boards are intended for evaluation of the LT1374 switching regulator in the 7-pin DD and SO-8 packages, respectively. The boards are electrically identical; they differ only in package layout (the BIAS pin is not available in the DD package). The primary reason for choosing the SO-8 over the DD package is board space. The DC187 (DD package) occupies an active board area of approximately 0.75 square inches. Optimizing the DC188 board, by using a Sumida coil and removing the layout options and voltage-selector jumper, a total active area of 0.4 square inches is possible. The DD package is more suitable for higher power or higher ambient-temperature applications. Although both boards will supply 4A of output current, the DC188 must be thermally derated to 3A continuous at 40°C ambient to prevent excessive die temperatures. The DC187 can run at 70°C ambient at 4A output current. The SO-8 package can, however, be used for dynamic loads up to the full rated switch current. The Temperature Rise vs Time, DC188 graph shows the dynamic thermal response of the DC188 board for loads up to 4A. Shutdown Option (Device Markings LT1374CR or 1374) For normal operation, the S/D pin can be left floating. S/D has two output-disable modes, lockout and shutdown. When the pin is taken below the lockout threshold, switching is disabled. This is typically used for input undervoltage lockout. Grounding the S/D pin places the LT1374 in shutdown mode. This reduces total board supply current to 20µA. Synchronization Option (Device Markings LT1374CR-SYNC or 1374SN) For normal operation the S/D pin can be left floating. To synchronize switching to an external clock, apply a logiclevel signal to the S/D pin. Amplitude must be from a logical low to greater than 2.2V with a duty cycle from 10% to 90%. Synchronization frequency must be greater than that of the free-running oscillator and less than 1MHz. Additional circuitry may be required to prevent subharmonic oscillation—refer to the data sheet for more details. COMPONENTS LT1374 Operation Inductor L1 The LT1374 data sheet gives a complete description of the part, operation and applications information. The data sheet must be read in conjunction with this demo manual. The inductor is a Coilcraft DO3316P-682, a 6.8µH unshielded ferrite unit. It is selected for its low cost, small size and 4.6A ISAT rating. The equivalent Coiltronics UP26R8 unit can be substituted. If board space is at a premium and higher ripple current is acceptable, DC188 has the solder pads available for the Sumida CD43-1R8 inductor. This 1.8µH unit has a 2.9A ISAT rating. With this coil, ripple at 10VIN, 5VOUT is ±1.5A. This gives a maximum output current of (4.5A – 1.5A) = 3A. At input voltages above 15V, C7 should be inserted in parallel with C5 to increase output capacitor ripple rating. Hook-Up Solid turret terminals are provided for easy connection to supplies and test equipment. The jumper should be inserted if a 3.3V output is required. It should be removed for 5V operation. Connect a 0V to 25V, 4.5A power supply across the VIN and GND terminals and the load across the VOUT and GND terminals. When measuring load/line regulation, remember to Kelvin connect to the turrets. Also, when measuring output ripple voltage with an oscilloscope probe, the wire from the probe to the ground clip will act as an antenna, picking up noise. For improved results, the ground clip should be removed from the probe. The tip should be touched against the output turret, with the bare ground shield pressed against the ground turret. This reduces the noise seen on the waveform. 4 Input/Output Capacitors C3, C5, C6 and C7 The input capacitor, C3, is a Tokin ceramic capacitor. It was selected for its small size, high voltage rating and low ESR (effective series resistance). The input ripple current for a buck converter is high, typically IOUT/2. Tantalum capacitors become resistive at higher frequencies, requiring careful ripple-rating selection to prevent excessive DEMO MANUAL DC187/DC188 U OPERATIO heating. Ceramic capacitors’ ESL (effective series inductance) tends to dominate their ESR, making them less susceptible to ripple-induced heating. The output capacitor, C5, is an AVX tantalum capacitor. A ceramic is not recommended as the main output capacitor since loop stability relies on a resistive characteristic at higher frequencies to form a zero. The AVX TPS series, specifically designed for use in switch mode power supplies, has very low ESR. At switching frequencies, ripple voltage is more a function of ESR than of absolute capacitance value. If lower output ripple voltage is required, use the optional capacitor, C7, to reduce ESR, rather than increasing the capacitance of C5. For very low ripple, an additional LC filter on the output may be a cheaper solution. The output contains very narrow voltage spikes because of the parasitic inductance of C5. A small ceramic capacitor, C6, removes these spikes on the demo board. In application circuits, trace inductance and local bypass capacitors may perform this function, negating the need for C6. Catch Diode D1 Use diodes designed for switching applications, such as Schottky or ultrafast diodes, with adequate current rating and fast turn-on times. In selecting a diode, the basic parameters of interest are forward voltage, maximum reverse voltage, average operating current and peak current. Lower forward voltage yields higher circuit efficiency and lowers power dissipation in the diode. The MBRD835L has a maximum forward drop of 0.4V at 3A. The reverse voltage rating must be greater than the input voltage. Average diode current is always less than output current, but under a shorted output condition, diode current can equal the switch current limit. If the application must withstand this condition, the diode must be rated for maximum switch current. Compensation: C1, C2 and R1 A detailed discussion of frequency compensation can be found in the LT1374 data sheet. C1, a 1500pF capacitor from VC to ground, gives a stable loop response over a wide range of input and output conditions. Options R1 and C2 are included to optimize the dynamic response for specific applications. Boost: D2, D3 and C4 A boost voltage of at least 2.8V is required throughout the on time of the switch to guarantee that it remains saturated. For output voltages of 3.3V or more, diode D2 provides sufficient boost voltage to C4. Below 3.3V, D2 can be moved to position D3, powering boost from VIN. PCB LAYOUT In many cases, the layout of the demonstration board may be dropped directly into the application with minimal changes. If this is not practical, there are several precautions that must be taken when laying out high frequency converter circuits. The high frequency switching path runs from ground, through C3 to the VIN pin of the LT1374, out of the SW pin, through D1 and back to ground. This loop acts as an antenna and will radiate noise if not kept as short as possible. Also, at higher switching currents the associated trace inductance can cause excessive voltage spikes across the switch. The use of a ground plane will reduce many noise problems. The ground pin of the LT1374 contains some high frequency signal currents, but more importantly, it is the 0V reference for the output voltage. Connect the ground pin directly to the ground plane. The FB and VC components should be kept away from the power components as much as possible. The ground for these components should be separated from power grounds. Run a Kelvin sense to VOUT as required, but keep the divider network close to the LT1374 to prevent noise pick-up on the FB node. Noise pickup on the VC pin appears as various problems, including poor load regulation, subharmonic oscillation and instability. Thermal management must also be considered. The SO-8 package has a fused ground pin. Soldering this pin to a large copper area will significantly reduce its thermal resistance. Solder filled feedthroughs close to the ground pin provide a good thermal path to the ground plane. For the DD package, the grounded tab should be treated in the same way. For more information or advice, contact the LTC Applications department. 5 DEMO MANUAL DC187/DC188 U W PCB LAYOUT A D FIL DC187 6 Component Side Silkscreen Component Side Component Side Solder Mask Solder Side Solder Side Solder Mask Pastemask DEMO MANUAL DC187/DC188 U W PCB LAYOUT A D FIL DC188 Component Side Silkscreen Component Side Component Side Solder Mask Solder Side Solder Side Solder Mask Pastemask 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 DC187/DC188 U PC FAB DRAWI GS DC187 2.000 C D A D B A A 2.000 NOTES: UNLESS OTHERWISE SPECIFIED 1. MATERIAL: FR4 OR EQUIVALENT EPOXY, 2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006 TOTAL OF 2 LAYERS 2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC) 3. SOLDER MASK: BOTH SIDES USING GREEN SR1020 OR EQUIVALENT 4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK 5. ALL DIMENSIONS IN INCHES A SYMBOL DIAMETER NUMBER OF HOLES A 0.020 29 B 0.037 2 C 0.072 2 D 0.095 5 TOTAL HOLES 38 C 187 FD DC188 2.000 D E E A A C 2.000 NOTES: UNLESS OTHERWISE SPECIFIED 1. MATERIAL: FR4 OR EQUIVALENT EPOXY, 2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006 TOTAL OF 2 LAYERS 2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC) 3. SOLDER MASK: BOTH SIDES USING GREEN SR1020 OR EQUIVALENT 4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK 5. ALL DIMENSIONS IN INCHES SYMBOL DIAMETER NUMBER OF HOLES A 0.020 38 B 0.037 2 C 0.025 5 D 0.072 2 E 0.095 5 TOTAL HOLES 38 B C C D 8 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417● (408) 432-1900 FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com 188 FD dc1878 LT/TP 0498 500 • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 1998