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LT1425 数据表(PDF) 12 Page - Linear Technology |
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LT1425 数据表(HTML) 12 Page - Linear Technology |
12 / 20 page 12 LT1425 APPLICATIONS INFORMATION amount when output switch current is zero. Please refer to Figure 1 which shows nominal reference current shift at zero load for a range of ROCOMP values. Example: for a load compensation resistor of 12k, the graph indi- cates a 1.0% shift in reference current. The RFB resistor value should be adjusted down by about 1.0% to restore the original target output voltage. integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which yield more freedom in setting total turns and mutual inductance. Turns ratio can then be chosen on the basis of desired duty cycle. However, remember that the input supply voltage plus the secondary-to-primary referred version of the flyback pulse (including leakage spike) must not exceed the allowed output switch breakdown rating. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a spike after output switch turn-off. This is increasingly prominent at higher load currents where more stored energy must be dissipated. In many cases a “snubber” circuit will be required to avoid over- voltage breakdown at the output switch node. LTC’s Application Note 19 is a good reference on snubber design. In situations where the flyback pulse extends beyond the enable delay time, the output voltage regulation will be affected to some degree. It is important to realize that the feedback system has a deliberately limited input range, roughly ±50mV referred to the RREF node, and this works to the user’s advantage in rejecting large, i.e., higher voltage leakage spikes. In other words, once a leakage spike is several volts in amplitude, a further increase in amplitude has little effect on the feedback system. So the user is generally advised to arrange the snubber circuit to clamp at as high a voltage as comfortably possible, observing switch breakdown, such that leakage spike duration is as short as possible. As a rough guide, total leakage inductances of several percent (of mutual inductance) or less may require a snubber, but exhibit little to no regulation error due to leakage spike behavior. Inductances from several percent up to perhaps ten percent cause increasing regulation error. Severe leakage inductances in the double digit percentage range should be avoided if at all possible as there is a potential for abrupt loss of control at high load current. This curious condition potentially occurs when the leak- age spike becomes such a large portion of the flyback waveform that the processing circuitry is fooled into thinking that the leakage spike itself is the real flyback In less critical applications, or when output current remains relatively constant, the load compensation func- tion may be deemed unnecessary. In such cases, a reduced component solution may be obtained as follows: Leave the ROCOMP node open (ROCOMP = ∞), and replace the filter capacitor normally on the RCCOMP node with a short to ground. TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LT1425 successfully. In addi- tion to the usual list of caveats dealing with high frequency isolated power supply transformer design, the following information should prove useful. Turns Ratio Note that due to the use of an RFB/RREF resistor ratio to set output voltage, the user has relative freedom in selecting transformer turns ratio to suit a given application. In other words, “screwball” turns ratios like “1.736:1.0” can scru- pulously be avoided! In contrast, simpler ratios of small ROCOMP (kΩ) 1 2 10 100 1000 1425 F01 1 0 Figure 1 |
类似零件编号 - LT1425 |
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类似说明 - LT1425 |
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