DEMO CIRCUIT 1300A-C LTC3725 / LTC3726 QUICK START GUIDE LTC3725 / LTC3726 100W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1300A-C is a 100W Isolated DC1300A-A/B (5V 20A / 12V 8.4A) or DC1174A- Forward Converter with Synchronous Rectification A/B/C (5V 10A /12V 4.2A / 15V 3.3A). For telecom featuring the LTC3725 / LTC3726. input requirements, see DC1031A-A/B/C (2.5V/3.3V/5V 20A), or DC1032A-A (12V 12A), or This circuit was designed to demonstrate the high le- DC888A-A/B/C (3.3V 50A / 5V 35A /12V 20A). vels 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, 6.7A output Design files for this circuit board are available. Call from an input voltage range of 9 to 36V: suitable for the LTC factory. automotive, industrial, and other applications. It has a , 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 quarter-brick footprint area. Synchronous rectification Drive, FilterCAD, Hot Swap, LinearView, Module, Micropower SwitcherCAD, Multimode Dimming, No Latency , No Latency Delta-Sigma, No R , Operational Filter, PanelProtect, helps to attain efficiency exceeding 90%. Secondary- SENSE PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, side control eliminates complex optocoupler feedback, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. providing fast transient response with minimum out- put capacitance. For other output requirements, see PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS V Input Supply Range 9* 36 V IN V Output Voltage 15.0 V OUT I Output Current Range 200LFM 0 6.7 A OUT F Switching (Clock) Frequency 250 kHz SW V Output Ripple V = 18V, I =6.7A (20MHz BW) 50 mV OUT P-P IN OUT PP I Output Regulation Line and Load (9-36V, 0-6.7A) 0.04* % REG P /P Efficiency (see Figure 3) V =18V, I = 6.7A 89* % OUT IN IN OUT Isolation Basic 1500 Vdc Approximate Size Component Area x Top Component Height 2.3 x 1.45 x 0.47 Inches *Typical minimum startup is 9.3V 1 L TC3725 / LTC3726 OPERATING PRINCIPLES The LTC3725 Single-Switch Forward Controller is simple buck converter. Secondary control eliminates used on the primary and provides start-up, gate drive, delays, tames large-signal overshoot, and reduces and protection functions. Once start-up is accom- output capacitance needed to meet transient response plished, the LTC3726 Secondary-Side Synchronous requirements. Forward Controller takes over, and provides the An optional LC filter stage on the input lowers rms input LTC3725 with timing information and bias power current. The filter must have output impedance that is through a small pulse transformer. less than the converter input impedance to assure stabili- ty. This may require a damping impedance. (See Linear When input voltage is applied, the LTC3725 com- Technology Application Note AN19 for a discussion of mences soft-start of the output voltage. When the input filter stability.) A source with a 50mOhm or higher secondary bias source reaches the undervoltage thre- ESR at the filter resonant frequency is one way of provid- shold, the LTC3726 comes alive and takes control by ing damping for the filter elements provided on the sending encoded PWM gate pulses to the LTC3725 DC1300A. For bench testing, adding an electrolytic capa- through T2. These pulses also provide primary bias citor such as a Sanyo 50ME470AX to the input terminals power efficiently over a wide input voltage range. will provide suitable damping and ripple current capabili- ty. The values selected have a filter resonant frequency The transition from primary to secondary control oc- that is below the converter switching frequency, thus curs at a fraction of the nominal output voltage. From avoiding high circulating currents in the filter. then on, operation and design is simplified to that of a QUICK START PROCEDURE Demonstration circuit 1300 is easy to set up to eva- shunt can be put in series with the input supply luate the performance of the LTC3725 / LTC3726. Re- in order to measure the DC1300As input cur- rent. fer to Figure 1 for proper measurement equipment setup and follow the procedure below: c. A voltmeter with a capability of measuring at NOTE. When measuring the output voltage ripple, care must be taken to least 36V can be placed across the input ter- avoid a long ground lead on the oscilloscope probe. Measure the output minals in order to get an accurate input voltage voltage ripple by touching the probe tip and ground ring directly across measurement. the last output capacitor as shown in Figure 12. 4. Turn on the power at the input. 1. Set an input power supply that is capable of 9V to 36V to 18V. Then turn off the supply. NOTE. Make sure that the input voltage never exceeds 36V. 2. Direct an airflow of 200lfm across the unit for sus- 5. Check for the proper output voltage of 15V. Turn tained operation at full load. off the power at the input. 3. With power off, connect the supply to the input 6. Once the proper output voltages are established, terminals +Vin and Vin. connect a variable load capable of sinking 6.7A at 15V to the output terminals +Vout and Vout. Set a. Input voltages lower than 9V can keep the con- the current for 0A. verter from turning on due to the undervoltage lockout feature of the LTC3725 / LTC3726. a. If efficiency measurements are desired, an am- meter or a resistor shunt that is capable of han- b. If efficiency measurements are desired, an am- dling 6.7Adc can be put in series with the out- meter capable of measuring 7Adc or a resistor 2