DEMO CIRCUIT 1300A-B LTC3725 / LTC3726 QUICK START GUIDE LTC3725 / LTC3726 100W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1300A-B is a 100W Isolated Forward Converter with Synchronous Rectification featuring the LTC3725 / LTC3726. This circuit was designed to demonstrate the high levels of performance, efficiency, and small solution size attainable using this part in a Resonant-Reset Forward Converter power supply. It operates at 200kHz and produces a regulated 12V, 8.4A output from an input voltage range of 9 to 36V: suitable for automotive, industrial, and other applications. It has a quarter-brick footprint area. Synchronous rectification helps to attain efficiency exceeding 90%. Secondaryside control eliminates complex optocoupler feedback, providing fast transient response with minimum output capacitance. For other output requirements, see DC1300A-A/C (5V@20A / [email protected]) or DC1174AA/B/C (5V@10A /[email protected] / [email protected]). For telecom input requirements, see DC1031A-A/B/C (2.5V/3.3V/5V@20A), or DC1032A-A (12V@12A), or DC888A-A/B/C (3.3V@50A / 5V@35A /12V@20A). Design files for this circuit board are available. Call the LTC factory. , LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, μModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. PERFORMANCE SUMMARY Specifications are at TA = 25°C SYMBOL VIN VOUT IOUT FSW VOUT P-P IREG POUT/PIN PARAMETER Input Supply Range Output Voltage Output Current Range Switching (Clock) Frequency Output Ripple Output Regulation Efficiency (see Figure 3) Isolation Approximate Size CONDITIONS 200LFM VIN = 18V, IOUT = 8.4A (20MHz BW) Line and Load (9-36V, 0-8.4A) VIN =18V, IOUT = 8.4A Basic Component Area x Top Component Height MIN 9* TYP MAX 36 UNITS V 12.0 V 0 8.4 A 200 kHz 60 mVP–P ±0.06 % 90 % 1500 Vdc 2.3 x 1.45 x 0.47 Inches *Typical minimum startup is 9.3V 1 LTC3725 / LTC3726 OPERATING PRINCIPLES The LTC3725 Single-Switch Forward Controller is used on the primary and provides start-up, gate drive, and protection functions. Once start-up is accomplished, the LTC3726 Secondary-Side Synchronous Forward Controller takes over, and provides the LTC3725 with timing information and bias power through a small pulse transformer. When input voltage is applied, the LTC3725 commences soft-start of the output voltage. When the secondary bias source reaches the undervoltage threshold, the LTC3726 comes alive and takes control by sending encoded PWM gate pulses to the LTC3725 through T2. These pulses also provide primary bias power efficiently over a wide input voltage range. The transition from primary to secondary control occurs at a fraction of the nominal output voltage. From then on, operation and design is simplified to that of a simple buck converter. Secondary control eliminates delays, tames large-signal overshoot, and reduces output capacitance needed to meet transient response requirements. An optional LC filter stage on the input lowers rms input current. The filter must have output impedance that is less than the converter input impedance to assure stability. This may require a damping impedance. (See Linear Technology Application Note AN19 for a discussion of input filter stability.) A source with a 50mOhm or higher ESR at the filter resonant frequency is one way of providing damping for the filter elements provided on the DC1300A. For bench testing, adding an electrolytic capacitor such as a Sanyo 50ME470AX to the input terminals will provide suitable damping and ripple current capability. The values selected have a filter resonant frequency that is below the converter switching frequency, thus avoiding high circulating currents in the filter. QUICK START PROCEDURE Demonstration circuit 1300 is easy to set up to evaluate the performance of the LTC3725 / LTC3726. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: NOTE. When measuring the output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the output voltage ripple by touching the probe tip and ground ring directly across the last output capacitor as shown in Figure 12. shunt can be put in series with the input supply in order to measure the DC1300A’s input current. c. A voltmeter with a capability of measuring at least 36V can be placed across the input terminals in order to get an accurate input voltage measurement. 1. Set an input power supply that is capable of 9V to 36V to 18V. Then turn off the supply. 4. Turn on the power at the input. 2. Direct an airflow of 200lfm across the unit for sustained operation at full load. 5. Check for the proper output voltage of 12V. Turn off the power at the input. 3. With power off, connect the supply to the input terminals +Vin and –Vin. 6. Once the proper output voltages are established, connect a variable load capable of sinking 8.4A at 12V to the output terminals +Vout and –Vout. Set the current for 0A. a. Input voltages lower than 9V can keep the con- verter from turning on due to the undervoltage lockout feature of the LTC3725 / LTC3726. b. If efficiency measurements are desired, an am- meter capable of measuring 7Adc or a resistor NOTE. Make sure that the input voltage never exceeds 36V. a. If efficiency measurements are desired, an am- meter or a resistor shunt that is capable of handling 8.4Adc can be put in series with the out- 2 LTC3725 / LTC3726 put load in order to measure the DC1300A’s output current. NOTE. If there is no output, temporarily disconnect the load to make b. A voltmeter with a capability of measuring at 8. Once the proper output voltage is again established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other desired parameters. least 5V can be placed across the output terminals in order to get an accurate output voltage measurement. sure that the load is not set too high. 7. Turn on the power at the input. Figure 1. Proper Measurement Equipment Setup 3 LTC3725 / LTC3726 Figure 2. Proper Noise Measurement Setup 92.00 90.00 88.00 Efficiency (%) 86.00 9VIN 84.00 18VIN 82.00 36VIN 80.00 78.00 76.00 74.00 72.00 70.00 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000 Curre nt in Am ps Figure 3. Efficiency 4 LTC3725 / LTC3726 Figure 4. Output Ripple at 18Vin and 8.4Aout (25MHz) (50mV, 5us / div, 25MHz) Figure 5. Transient Response Waveform at 18Vin and 4.2 – 8.4Aout (5A, 100mV, 100us / div) 5 LTC3725 / LTC3726 Ce lsius 153.0 90.0 85.0 80.0 75.0 70.0 65.0 60.0 55.0 50.0 45.0 -22.0 40.0 Th ermo teknix T V S-70 0 1:52:03 PM 2/26/2008 e : 0.95 Bg : 32.7°C Figure 6. Thermal Map, Frontside at 18Vin and 8.4Aout (Ta = 25 degrees C) Celsius 153.0 90.0 85.0 80.0 75.0 70.0 65.0 60.0 55.0 50.0 45.0 -22.0 40.0 Thermoteknix TVS-700 2:38:29 PM 2/26/2008 e : 0.95 Bg : 33.0°C Figure 7. Thermal Map, Backside at 18Vin and 8.4Aout (Ta = 25 degrees C) 6 D3 22V MMSZ5251BS C55 4.7nF 1 2 Q28 FMMT38C VIN R18 147K E2 -VOUTS 1 C81 10pF 2 1 8 2 C88 47pF R49 FS/IN- 1 5 4 VSB C9 0.22uF VIN VCC R86 261K R93 147 1% 0 5.1K 8 R92 C8 470pF R91 10.0K R90 7.50K C1 1.5nF 100V 1206 47K 5 VSW 14 15 PT- PT+ R84 0 Q14 Q15 Si7852DP SS VA R79 3.3K -VOUTS 2 Q36 MBT3946DW1T1 R50 0.005 1W 1 D25 (Opt.) R55 100 1% C70 3.9nF R85 (Opt.) C78 4.7nF SS R54 100 1% -VOUTP1 -VOUTS -VOUTP U2 LTC3726EGN PT- C71 1uF C20 (Opt.) 200V -VOUTP C30 2.2nF 250V 5 6 4 3 R23 R24 (Opt.) 1/4W T2 PA0297 2 3 4 1 5 6 -VOUTS R7 47K Q27C R63 90.9K R61 100 C73 470pF 1 C66 3.3nF 100V T1 PA0910 C72 0.1uF R58 7 8 9 10 11 D6 4.3V MMSZ4687T1 C7 100pF C6 100pF R95 U1 LTC3725EMSE FB/IN+ R13 0 1206 Q8 Q11 Q39 Si7370DP C89 0.22uF FMMT718 Q2 VCp C27 2.2nF R98 22 1 (Opt.) Q37 MMBT2907A 1% R94 147 R89 715 1 2.2K T3 PA1005.100 Q1 FMMT619 1 SSFLT ULVO 1 2 D7 BAS21 VCC R87 0 10K 10K 3 7 46 5 8 R97 D29 PT1N4148WS C29 33nF C82 82pF SG Q34 2N7002 VSW C24 4.7uF Ra R6 VCp Rb 162K 1 R3 100 Q6 MMBFJ201 3 CUT 1.0 R11 R12 1 3 C2 6.8uF 50V 1812 3 2 1 2 C3 C4 C5 6.8uF 50V 1812 7 VIN 10 3 2 2 3 E1 D2 1N4148WS R22 28.7K D4 4.7V MMSZ5230BS 1 2 10 -Vin 1 2 2 FG 12 SW 3 11 3 Q32 FMMT718 3 2 C69 220pF 200V 1206 1 R56 100 C79 2.2nF R68 19.6K SG VSB Q27C VCC 10uH PULSE, PA2050.103NL L2 25V C67 4.7uF R46 619 R9 (Opt.) C10 (Opt.) -Vout +Vout 12V/8.4A LTC CONFIDENTIAL - FOR CUSTOMER USE ONLY -VOUTS C77 22uF 1210 -VOUTP C76 (Opt.) R41 11.8K E4 68uF 16V C80 E3 + C83 +VOUT -VOUTP VCC C31 C33 22uF 16V 1210 +VOUT LINEAR TECHNOLOGY CORPORATION FB/PH 4 Q27 FCX491A 1 R76 470 1206 D28 MMSZ5236BS 7.5V R4 10 1630 McCARTHY BLVD. MILPITAS, CA. 95035 Linear Technology Has Made A Best Ef f ort To Design A 408-432-1900 Circuit That Meets Customer-Supplied Specif ications; 408-434-0507 FAX Howev er, It Remains The Customer's Responsibility To Verif y Proper And Reliable Operation In The Actual Title Application. Component Substitution And Printed LTC3725EMSE, LTC3726EGN 9V-36Vin Forward Converter Circuit Board Lay out May Signif icantly Af f ect Circuit Perf ormance Or Reliability . Contact Linear Technology Size Document Number Rev Applications Engineering For Assistance. Demo Circuit 1300A-B This Circuit Is Proprietary To Linear Technology And Friday, June 05, 2009 2 3 Supplied For Use With Linear Technology Parts. of Date: Sheet Customer Notice -VOUTS C75 68pF R69 0 R83 (Opt.) VA Q26 FMMT619 2 3 Q3 Q12 Q38 Si7450DP D1 (Opt.) R1 R2 R51 R52 75 R66 100K IS+ 3 NDRV VSLMT 9 GND 8 GATE PGND 6 PGND 13 ISRUN/SS 6 IS 11 SLP 7 1 FS/SYNC 9 1 SG +Vin 9V - 36V 3 2 1 L1 0.47uH 4 3 GND 2 3 1 2 6 1 ITH 5 2 3 16 VCC 3 MODE 2 1 L3 (Opt.) LTC3725 / LTC3726 7