DEMO CIRCUIT 1174A-C LTC3725 / LTC3726 QUICK START GUIDE LTC3725 / LTC3726 50W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1174A-C is a 50W Isolated Forward Converter with Synchronous Rectification featuring the LTC3725 / LTC3726. tion helps to attain efficiency exceeding 90%. Isolation is 1500VDC basic. 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 250kHz and produces a regulated 15V, 3.3A output from an input voltage range of 9 to 36V: suitable for automotive, industrial, and other applications. It has an eighth-brick footprint area. Synchronous rectifica- 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 Size CONDITIONS MIN 9* VIN = 9 –36V VIN = 18V, IOUT = 4.2A (20MHz BW) Line and Load (9-36V, 0-3.3A) VIN =18V, IOUT = 3A Basic Component Area x Top Component Height TYP MAX 36 UNITS V 15.0 V 0 3.3 A 250 kHz 100 mVP–P ±0.04 % 89 % 1500 Vdc 2.3 x 0.9 x 0.35 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 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. 1 LTC3725 / LTC3726 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 DC1174A. 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 1174 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. 1. Set an input power supply that is capable of 9V to 36V to 18V. Then turn off the supply. 2. Direct an airflow of 200lfm across the unit for sustained operation at full load. 3. With power off, connect the supply to the input terminals +Vin and –Vin. 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 DC1174A’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. NOTE. Make sure that the input voltage never exceeds 36V. 5. Check for the proper output voltage of 15V. Turn off the power at the input. 6. Once the proper output voltages are established, connect a variable load capable of sinking 3.3A at 15V to the output terminals +Vout and –Vout. Set the current for 0A. a. If efficiency measurements are desired, an am- meter or a resistor shunt that is capable of handling 3.3Adc can be put in series with the output load in order to measure the DC1174A’s output current. b. A voltmeter with a capability of measuring at least 15V 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. 4. Turn on the power at the input. 2 LTC3725 / LTC3726 Figure 1. Proper Measurement Equipment Setup Figure 2. Proper Noise Measurement Setup 3 Efficiency (%) LTC3725 / LTC3726 92 90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 9V IN 18V IN 36V IN 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Curre nt in Amps Figure 3. Efficiency Figure 4. Output Ripple at 18Vin and 4.2Aout (50mV, 5us / div, 25MHz) 4 LTC3725 / LTC3726 Figure 5. Transient Response Waveform at 18Vin and 2.1 - 4.2Aout (1A, 100mV, 200us / div) 5 LTC3725 / LTC3726 Figure 6. Thermal Map, Frontside at 18Vin and 3.3Aout (Ta = 25 degrees C) Figure 7. Thermal Map, Backside at 18Vin and 3.3Aout (Ta = 25 degrees C) 6 D4 4.7V MMSZ5230BS E2 D3 22V MMSZ5251B 28.7K R22 C55 470pF -VOUT D2 1N4148WS 1 C3 3.3uF 50V 1210 2 Q28 FMMT38C R18 147K C2 3.3uF 50V 1210 3 R89 511 C82 75pF 2 C27 2.2nF R3 100 R49 0 1 R93 147 1% VIN 5 4 VCC FS/IN- FB/IN+ R13 0 1206 VSB Q27C R63 90.9K R61 100 C73 470pF C72 0.1uF R58 5.1K R48 .010 1W C66 2.2nF 100V 7 8 9 10 11 D6 4.3V MMSZ4687T1 C9 0.22uF C7 (Opt.) C6 (Opt.) Q11 (Opt.) Q8 Si7852DP U1 LTC3725EMSE Q37 MMBT2907A R94 147 1% 1 7 R87 0 R86 261K SSFLT ULVO VCC R88 47pF 2 1 8 10K R6 10K VIN D29 PT1N4148WS SG C29 33nF 1 Ra Rb 162K 1 CUT C81 10pF C5 3.3uF 50V 1210 Q6 MMBFJ201 3 C4 3.3uF 50V 1210 Q34 2N7002 VSW C24 4.7uF 2 1 2 -Vin 1 2 10 IS 9V-36V 1 2 5 6 4 3 C8 470pF R91 10.0K R90 7.50K -VOUT C30 2.2nF 250V -VOUT R7 47K 8 1 R2 196 1/4W R1 196 1/4W R51 196 1/4W 1/4W R52 196 T2 PA0297 2 3 4 1 5 6 R92 47K 5 14 15 PT- PT+ R84 0 SS VA VSW -VOUT 2 C70 1nF C78 4.7nF SS 3 1 Q12 Si7450DP 3 D1 (Opt.) Q32 FMMT718 3 2 R66 100K R55 100 1% R85 (Opt.) R54 100 1% R50 0.012 1W 1 D25 (Opt.) C75 47pF R69 113K R83 (Opt.) R56 100 FB/PH 4 Q27 FCX491 1 R76 680 1206 C76 (Opt.) C77 10uF E3 R46 619 E4 47uF 20V + C80 +VOUT R41 14.7K VCC -VOUT C31 (Opt.) 1210 +VOUT R9 (Opt.) C10 (Opt.) -Vout 15V/3.3A +Vout LINEAR TECHNOLOGY CORPORATION C79 470pF R68 27.4K SG VSB Q27C C33 22uF 16V D28 MMSZ5236BS 7.5V R4 10 1/4W 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 1174A-C A This Circuit Is Proprietary To Linear Technology And Wednesday, August 12, 2009 1 1 Date: Sheet Supplied For Use With Linear Technology Parts. of Customer Notice C67 4.7uF 25V VCC Q26 FMMT619 2 3 L2 12uH VA LTC CONFIDENTIAL - FOR CUSTOMER USE ONLY Q36 MBT3946DW1T1 -VOUT U2 LTC3726EGN PT- C71 1uF C69 100pF 200V C1 390pF 200V Q14 Si7456DP R79 3.3K 10 E1 3 2 2 FG 12 SW 3 NDRV VSLMT 9 GND 8 GATE PGND 6 GND 11 PGND 13 ISRUN/SS 6 11 IS+ L1 0.68uH 4 3 SLP 7 1 FS/SYNC 9 1 SG T1 PA0810 1 +Vin 2 3 1 2 6 1 ITH 5 2 3 16 VCC 3 MODE 2 1 L3 (Opt.) LTC3725 / LTC3726 7