DEMO MANUAL DC2490A LT8303 Micropower No-Opto Isolated Flyback Converter Description Demonstration circuit 2490A is a micropower no-opto directly from the primary-side flyback waveform, the part isolated flyback converter featuring the LT 8303. This requires no third winding or opto-isolator for regulation. demo circuit outputs 5V, and maintains tight regulation The output voltage is programmed with a single external with a load current from 2.5mA to 0.84A over an input resistor. Boundary mode operation provides a small mag- voltage from 36V to 72V. The output current capabil- netic solution with excellent load regulation. Low ripple ity increases with the input voltage, as shown in the Burst Mode operation maintains high efficiency at light Performance Summary table. load while minimizing the output voltage ripple. A 0.45A, 150V DMOS power switch is integrated along with all The DC2490A needs less than 0.5% of its full output the high voltage circuitry and control logic into a 5-lead power as a minimum load to maintain good output voltage ThinSOT package. regulation. On the DC2490A, in order to avoid preload- + ing, a 5.6V Zener diode is placed between its V and The LT8303 data sheet gives a complete description of OUT V to serve as a minimum load. the part, operation and application information. The data OUT sheet must be read in conjunction with this quick start The Performance Summary table summarizes the perfor- guide for demo circuit 2490A. mance of the demo board at room temperature. The demo circuit can be easily modified for different applications with Design files for this circuit board are available at some predesigned transformers. DEMO MANUAL DC2490A Quick s tart p D Demonstration circuit 2490A is easy to set up to evalu- 3. Check for the proper output voltages. The output should ate the performance of the LT8303. Refer to Figure 1 for be regulated at 5V (5%). proper measurement equipment setup and follow the NOTE: The LT8303 requires very small minimum load procedure below: to maintain good output voltage regulation. A Zener 1. With power off, connect the input power supply to the diode is placed on the output to clamp the voltage to board through V (E1) and GND (E2) terminals. Con- 5.6V. This Zener can be replaced with a 2000 resistor IN + nect the load to the terminals V (E3) and V (E4) at the trade-off of lower efficiency. OUT OUT on the board. 4. Once the proper output voltage is established, adjust 2. Turn on the power at the input. Increase V slowly to 36V. the input voltage and load current within the operating IN range and observe the output voltage regulation, ripple NOTE: Make sure that the input voltage is always within voltage, efficiency and other parameters. spec. To operate the board with higher input/output voltage, input capacitor, output capacitor and output NOTE: When measuring the input or output voltage diode with higher voltage ratings are needed. ripples, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly + across the V (E1) and GND (E2), or V (E3) and IN OUT V (E4) terminals. See Figure 2 for proper scope OUT probe technique. dc2490af 2 ure roce