QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER LTC3705 and LTC3706 DESCRIPTION Demonstration circuit 776 is a 36V-72Vin, forward supply without the need of an optocoupler, self- starting architecture, input undervoltage lockout, and converter featuring the LTC3705 and LTC3706. This output overvoltage protection. circuit was designed specifically to attain a high step- down ratio power supply in one stage in order to ef- Design files for this circuit board are available. ficiently power 1.5V loads from a typical telecom in- Call the LTC factory. put voltage range. Isolation voltage is 1500VDC. , LTC and LT are registered trademarks of Linear Technology Corporation. This circuit features secondary-side control of the Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Minimum Input Voltage 36V Maximum Input Voltage 72V Output Voltage V V = 36V to 72V, I = 0A to 45A, 400LFM 1.5V 1% OUT IN OUT 400LFM 45A Maximum Output Current 200LFM 40A Typical Output Ripple V V = 48V, I = 30A (20MHz BW) 30mV OUT IN OUT PP Line 0.01% Output Regulation Load 0.18% Peak Deviation 65mV Dynamic Response Load Step of 20A to 40A (1A/us min) Settling Time (to within 15mV of set point) 50us Nominal Switching Frequency 215kHz Efficiency V = 48V, I = 45A, 400LFM 84.75% Typical IN OUT Logic Low Voltage-Off 1V MAX On/Off Control Logic High Voltage-On 2.5V MIN Isolation Voltage Basic Insulation 1500VDC OPERATING PRINCIPLES The LTC3706 controller is used on the secondary and the output voltage. As this voltage begins to rise, the the LTC3705 driver with self-starting capability is LTC3706 secondary controller is quickly powered up used on the primary. When an input voltage is ap- via T1, D3, and Q13. The LTC3706 then assumes plied, the LTC3705 begins a controlled soft-start of control of the output voltage by sending encoded 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 776 36V-72VIN, FORWARD CONVERTER PWM gate pulses to the LTC3705 primary driver via reduces output capacitance. This, while utilizing off- the small signal transformer, T3. The LTC3705 then the-shelf magnetics and attaining high efficiency. operates as a simple driver receiving both input sig- For large values of input inductance, a 100V, 47uF elec- nals and bias power through T3. trolytic capacitor can be added across the input termi- The transition from primary to secondary control oc- nals to damp the input filter and provide adequate stabil- curs seamlessly at a fraction of the output voltage. ity. See Linear Technology Application Note AN19 for a From that point on, operation and design simplifies to discussion on input filter stability analysis. A recom- that of a simple buck converter. Secondary sensing mended part is the Sanyo 100MV39AX. eliminates delays, tames large-signal overshoot and QUICK START PROCEDURE Demonstration circuit 776 is easy to set up to evalu- NOTE: Make sure that the input voltage never ex- ate the performance of the LTC3705 and LTC3706. ceeds 72V. Refer to Figure 1 for proper measurement equipment 4. Check for the proper output voltage of 1.5V 1%. setup and follow the procedure below: Turn off the power at the input. NOTE: When measuring the input or output voltage 5. Once the proper output voltages are established, ripple, care must be taken to avoid a long ground lead connect a variable load capable of sinking 50A at on the oscilloscope probe. Measure the output (or 1.5V to the output terminals +Vout and Vout. Set input) voltage ripple by touching the probe tip and the current for 0A. probe ground directly across the +Vout and Vout (or a. If efficiency measurements are desired, an am- +Vin and Vin) terminals. See Figure 2 for proper meter or a resistor shunt that is capable of han- scope probe technique. dling 50Adc can be put in series with the output 1. Set an input power supply that is capable of 36V to load in order to measure the DC776As output 72V to a voltage of 36V. Then turn off the supply. current. 2. With power off, connect the supply to the input b. A voltmeter with a capability of measuring at terminals +Vin and Vin. least 2V can be placed across the output termi- a. Input voltages lower than 36V can keep the con- nals in order to get an accurate output voltage verter from turning on due to the undervoltage measurement. lockout feature of the LTC3705. 6. Turn on the power at the input. b. If efficiency measurements are desired, an am- NOTE: If there is no output, temporarily disconnect meter capable of measuring 3Adc can be put in the load to make sure that the load is not set too series with the input supply in order to measure high. the DC776As input current. 7. Once the proper output voltage is again estab- c. A voltmeter with a capability of measuring at lished, adjust the load within the operating range least 72V can be placed across the input termi- and observe the output voltage regulation, ripple nals in order to get an accurate input voltage voltage, efficiency and other desired parameters. measurement. 3. Turn on the power at the input. 2