QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 659 SYNCHRONOUS FLYBACK REGULATOR 4 OUTPUT LTC3806 DESCRIPTION Demonstration circuit DC659 is a four-output flyback regulator based on the LTC3806 control IC. Input volt- Design files for this circuit board are available. Call ages between 25 and 60 volts are stepped down to 12V, the LTC factory. 5V, 3.3V and 2.5V. Total load power is 22W. Thanks to LTC is a trademark of Linear Technology Corporation the use of synchronous rectification, efficiency and cross regulation are significantly improved versus a conventional flyback regulator. This demo circuit is suit- able for powered Ethernet or 42V automotive applica- tions. Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Output Voltage V V = 25V to 60V, I = 40mA to 400mA 12V 10% OUT1 IN OUT Output Voltage V V = 25V to 60V, I = 0mA to 400mA 5V 5% OUT2 IN OUT Output Voltage V V = 25V to 60V, I = 0A to 3.0A 3.3V 5% OUT3 IN OUT Output Voltage V V = 25V to 60V, I = 0A to 2.0A 2.5V 5% OUT4 IN OUT Nominal Switching Frequency 250kHz Efficiency V = 44V, Full load 89% Typical IN Logic Low Voltage-Off 0.6V MAX On/Off Control Logic High Voltage-On 1V MIN OPERATING PRINCIPLES DC659 steps a high input voltage down to multiple low are able to share power. This improves the cross regula- voltage outputs with high efficiency. The demo circuit tion of the outputs so that output voltage regulation is features the LTC3806 synchronous flyback controller. maintained even when one output is lightly loaded and The synchronous flyback topology replaces the normal the other outputs are heavily loaded. Normally with di- output diode of the flyback converter with a MOSFET. ode rectification, the lightly loaded output rises, as its The MOSFET has a lower forward voltage drop than the output capacitor is peak charged with voltage spikes caused by transformer leakage inductance. This effect diode it replaces, which improves efficiency particularly at low and very low output voltages. The second benefit can still be seen on the diode derived (non- achieved by synchronous rectification is that unlike a synchronous) 12V output on the demo circuit. This out- put will rise in the event that its load is relatively small diode, the MOSFET synchronous switch will allow cur- rent to flow in both directions. Therefore, during the ON compared to the other output loads. This output has a time of the secondary side switches, all of the outputs correspondingly loose output accuracy specification. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 659 SYNCHRONOUS FLYBACK REGULATOR 4 OUTPUT QUICK START PROCEDURE Demonstration circuit 659 is easy to set up to evaluate the performance of the LTC3806. Refer to Figure 1 for 2. Turn on the power at the input. proper measurement equipment setup and follow the 3. Check for the proper output voltages. Vout1 = 12V , procedure below: Vout2 = 5V Vout3=3.3V Vout4=2.5V NOTE: When measuring the input or output voltage rip- NOTE: If there is no output, temporarily disconnect the ple, care must be taken to avoid a long ground lead on load to make sure that the load is not set too high. the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across 4. Once the proper output voltages are established, ad- the Vin or Vout and GND terminals. See Figure 2 for just the loads within the operating range and observe proper scope probe technique. the output voltage regulation, ripple voltage, efficiency and other parameters. 1. With the power off connect the input power supply and the output loads as shown in Fig.1 Figure 1. Proper Measurement Equipment Setup 2