DEMO CIRCUIT 1300A-A QUICK START GUIDE LTC3725 / LTC3725 / LTC3726 100W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1300A-A 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 5.0V, 20A 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-B/C ([email protected] / [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 MIN 9* 200LFM VIN = 18V, IOUT = 20A (20MHz BW) Line and Load (9-36V, 0-20A) VIN =18V, IOUT = 18A Basic Component Area x Top Component Height TYP MAX 36 UNITS V 5.0 V 0 20 A 200 kHz 60 mVP–P ±0.2 % 90 % 1500 Vdc 2.3 x 1.45 x 0.40 Inches *Typical minimum startup is 9.3V 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 1 LTC3725 / 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. 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 shunt can be put in series with the input supply in order to measure the DC1300A’s input current. 1. Set an input power supply that is capable of 9V to 36V to 18V. Then turn off the supply. c. A voltmeter with a capability of measuring at 2. Direct an airflow of 200lfm across the unit for sustained operation at full load. least 36V can be placed across the input terminals in order to get an accurate input voltage measurement. 3. With power off, connect the supply to the input terminals +Vin and –Vin. 4. Turn on the power at the input. NOTE. Make sure that the input voltage never exceeds 36V. 5. Check for the proper output voltage of 5V. Turn off the power at the input. 6. Once the proper output voltages are established, connect a variable load capable of sinking 20A at 5V to the output terminals +Vout and –Vout. Set the current for 0A. 2 LTC3725 / a. If efficiency measurements are desired, an am- meter or a resistor shunt that is capable of handling 20Adc can be put in series with the output load in order to measure the DC1300A’s output current. b. A voltmeter with a capability of measuring at least 5V can be placed across the output terminals in order to get an accurate output voltage measurement. 7. Turn on the power at the input. NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 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. Figure 1. Proper Measurement Equipment Setup 3 LTC3725 / LTC3726 Figure 2. Proper Noise Measurement Setup Efficiency vs. Load Current 92% 90% 88% Efficiency (%) 86% 84% 82% 9VIN 80% 18VIN 78% 36VIN 76% 74% 72% 70% 2 4 6 8 10 12 14 16 18 20 Load Curre nt (A) Figure 3. Efficiency 4 LTC3725 / LTC3726 Figure 4. Output Ripple at 18Vin and 20Aout (25MHz) (20mV, 5us / div, 25MHz) Figure 5. Transient Response Waveform at 18Vin and 10 - 20Aout (10A, 1000mV, 100us / div) 5 LTC3725 / LTC3726 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 8:20:51 AM 2/19/2008 e : 0.95 Bg : 28.8°C Figure 6. Thermal Map, Frontside at 18Vin and 20Aout (Ta = 25 degrees C, 200 LFM) 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 9:00:28 AM 2/19/2008 e : 0.95 Bg : 29.6°C Figure 7. Thermal Map, Backside at 18Vin and 20Aout (Ta = 25 degrees C, 200 LFM) 6 D3 12V MMSZ5242BS -VOUTS CUT 1 C81 10pF C82 75pF SG 2 C88 47pF 2 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 3.3nF 100V 1206 47K 5 14 15 PT- PT+ R84 0 Q14 HAT2169H Q15 (Opt.) VSW SS VA R79 1.0K -VOUTS 2 Q36 MBT3946DW1T1 R50 0.002 1W 1 D25 BAS21 R55 100 1% C70 2.2nF R85 (Opt.) C78 4.7nF SS R54 100 1% -VOUTP1 -VOUTS -VOUTP U2 LTC3726EGN PT- C71 1uF C20 1.5nF 200V -VOUTP C30 2.2nF 250V 5 6 4 3 R24 R23 16.2 1/4W T2 PA0297 2 3 4 1 5 -VOUTS R7 47K Q27C R63 90.9K R61 100 C73 470pF 1 C66 5.6nF 100V C72 0.1uF R58 7 8 9 10 11 T1 PA0901.005 6 D6 4.3V MMSZ4687T1 C7 100pF C6 100pF R95 U1 LTC3725EMSE FB/IN+ R13 (Opt.) 1206 Q8 Q11Q39 Si7852DP C89 0.22uF FMMT718 Q2 VCp C27 2.2nF R98 22 1 (Opt.) R49 Q37 MMBT2907A 1% R94 147 R89 604 1 1 T3 PA1005.100 Q1 FMMT619 1 SSFLT ULVO R87 0 2 2.2K D7 BAS21 D29 PT1N4148WS C29 33nF 1 8 3 R97 VCC 10K 10K R6 Ra VCp Rb 162K 1 R3 100 C3 C4 C5 6.8uF 50V 1812 1 Q6 MMBFJ201 3 R11 R12 1.0 Q34 2N7002 VSW D2 1N4148WS 1 C24 4.7uF 2 Q28 FMMT38C VIN R18 147K E2 C2 6.8uF 50V 1812 1 2 3 7 46 5 8 7 VIN 10 3 2 2 3 E1 3 2 C55 1nF 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 2.2nF 200V 1206 1 R56 100 C79 2.2nF R68 6.81K SG VSB Q27C VCC 2.4uH PA1494.242 L2 25V C67 4.7uF R46 619 E4 -Vout +Vout 5V/20A LTC CONFIDENTIAL - FOR CUSTOMER USE ONLY R9 100 C10 1.0nF 220uF 6.3V -VOUTS C77 22uF 1210 -VOUTP C76 (Opt.) R41 4.53K C80 E3 + C83 +VOUT -VOUTP VCC C31 C33 100uF 6.3V 1210 +VOUT LINEAR TECHNOLOGY CORPORATION FB/PH 4 Q27 FCX491A 1 R76 470 1206 D28 MMSZ5236BS 7.5V R4 (Opt.) 1630 McCARTHY BLVD. MILPITAS, CA. 95035 408-432-1900 Linear Technology Has Made A Best Ef f ort To Design A 408-434-0507 FAX Circuit That Meets Customer-Supplied Specif ications; 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 Document Number Rev Perf ormance Or Reliability . Contact Linear Technology Size Applications Engineering For Assistance. Demo Circuit 1300A-A This Circuit Is Proprietary To Linear Technology And Friday, June 05, 2009 1 3 Date: Sheet Supplied For Use With Linear Technology Parts. of Customer Notice -VOUTS C75 68pF R69 0 R83 (Opt.) VA Q26 FMMT619 2 3 Q3 Q12 Q38 Si7852DP D1 BAS21 R1 R2 R51 R52 31.6 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 470uH LTC3725 / LTC3726 7