DEMO CIRCUIT 1300A-A LTC3725 / QUICK START GUIDE LTC3725 / LTC3726 100W Isolated Forward Converter with Synchronous Rectification DESCRIPTION Demonstration circuit 1300A-A is a 100W Isolated DC1300A-B/C (12V 8.4A / 15V 6.7A) 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 200kHz and produces a regulated 5.0V, 20A 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 5.0 V OUT I Output Current Range 200LFM 0 20 A OUT F Switching (Clock) Frequency 200 kHz SW V Output Ripple V = 18V, I = 20A (20MHz BW) 60 mV OUT P-P IN OUT PP I Output Regulation Line and Load (9-36V, 0-20A) 0.2 % REG P /P Efficiency (see Figure 3) V =18V, I = 18A 90 % OUT IN IN OUT Isolation Basic 1500 Vdc Approximate Size Component Area x Top Component Height 2.3 x 1.45 x 0.40 Inches *Typical minimum startup is 9.3V OPERATING PRINCIPLES The LTC3725 Single-Switch Forward Controller is and protection functions. Once start-up is accom- used on the primary and provides start-up, gate drive, plished, the LTC3726 Secondary-Side Synchronous 1LTC3725 / Forward Controller takes over, and provides the output capacitance needed to meet transient response LTC3725 with timing information and bias power requirements. through a small pulse transformer. An optional LC filter stage on the input lowers rms input current. The filter must have output impedance that is When input voltage is applied, the LTC3725 com- less than the converter input impedance to assure stabili- mences soft-start of the output voltage. When the ty. This may require a damping impedance. (See Linear secondary bias source reaches the undervoltage thre- Technology Application Note AN19 for a discussion of shold, the LTC3726 comes alive and takes control by input filter stability.) A source with a 50mOhm or higher sending encoded PWM gate pulses to the LTC3725 ESR at the filter resonant frequency is one way of provid- through T2. These pulses also provide primary bias ing damping for the filter elements provided on the power efficiently over a wide input voltage range. DC1300A. For bench testing, adding an electrolytic capa- citor such as a Sanyo 50ME470AX to the input terminals The transition from primary to secondary control oc- will provide suitable damping and ripple current capabili- curs at a fraction of the nominal output voltage. From ty. The values selected have a filter resonant frequency then on, operation and design is simplified to that of a that is below the converter switching frequency, thus simple buck converter. Secondary control eliminates avoiding high circulating currents in the filter. delays, tames large-signal overshoot, and reduces QUICK START PROCEDURE Demonstration circuit 1300 is easy to set up to eva- a. Input voltages lower than 9V can keep the con- luate the performance of the LTC3725 / LTC3726. Re- verter from turning on due to the undervoltage fer to Figure 1 for proper measurement equipment lockout feature of the LTC3725 / LTC3726. setup and follow the procedure below: b. If efficiency measurements are desired, an am- NOTE. When measuring the output voltage ripple, care must be taken to meter capable of measuring 7Adc or a resistor avoid a long ground lead on the oscilloscope probe. Measure the output shunt can be put in series with the input supply voltage ripple by touching the probe tip and ground ring directly across in order to measure the DC1300As input cur- the last output capacitor as shown in Figure 12. rent. 1. Set an input power supply that is capable of 9V to c. A voltmeter with a capability of measuring at 36V to 18V. Then turn off the supply. least 36V can be placed across the input ter- 2. Direct an airflow of 200lfm across the unit for sus- minals in order to get an accurate input voltage tained operation at full load. measurement. 3. With power off, connect the supply to the input 4. Turn on the power at the input. terminals +Vin and Vin. 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