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LT3485EDD-1 Datasheet(PDF) 10 Page - Linear Technology |
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LT3485EDD-1 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3485-0/LT3485-1/ LT3485-2/LT3485-3 10 34850123fb APPLICATIO S I FOR ATIO Choosing the Right Device (LT3485-0/LT3485-1/LT3485-2/LT3485-3) The only difference between the four versions of the LT3485 is the peak current level. For the fastest possible charge time, use the LT3485-3. The LT3485-1 has the lowest peak current capability, and is designed for applications that need a more limited drain on the batteries. Due to the lower peak current, the LT3485-1 can use a physically smaller transformer. The LT3485-0 and LT3485-2 have a current limit in between that of the LT3485-3 and the LT3485-1. Transformer Design The flyback transformer is a key element for any LT3485-0/LT3485-1/LT3485-2/LT3485-3 design. It must be designed carefully and checked that it does not cause excessive current or voltage on any pin of the part. The main parameters that need to be designed are shown in Table 1. The first transformer parameter that needs to be set is the turns ratio N. The LT3485-0/LT3485-1/LT3485-2/LT3485- 3 accomplish output voltage detection by monitoring the flyback waveform on the SW pin. When the SW voltage reaches 31.5V higher than the VBAT voltage, the part will halt power delivery. Thus, the choice of N sets the target output voltage as it changes the amplitude of the reflected voltage from the output to the SW pin. Choose N accord- ing to the following equation: N VOUT = + 2 31 5 . where VOUT is the desired output voltage. The number 2 in the numerator is used to include the effect of the voltage drop across the output diode(s). Thus for a 320V output, N should be 322/31.5 or 10.2. For a 300V output, choose N equal to 302/31.5 or 9.6. The next parameter that needs to be set is the primary inductance, LPRI. Choose LPRI according to the following formula: L V NI PRI OUT PK ≥ − •• • 200 10 9 where VOUT is the desired output voltage. N is the trans- former turns ratio. IPK is 1.4 (LT3485-0), 0.7 (LT3485-1), 1 (LT3485-2) and 2 (LT3485-3). LPRI needs to be equal or larger than this value to ensure that the LT3485-0/LT3485-1/LT3485-2/LT3485-3 has ad- equate time to respond to the flyback waveform. All other parameters need to meet or exceed the recom- mended limits as shown in Table 1. A particularly impor- tant parameter is the leakage inductance, LLEAK. When the power switch of the LT3485-0/LT3485-1/LT3485-2/ LT3485-3 turns off, the leakage inductance on the pri- mary of the transformer causes a voltage spike to occur on the SW pin. The height of this spike must not exceed 40V, even though the absolute maximum rating of the SW Pin is 50V. The 50V absolute maximum rating is a DC blocking voltage specification, which assumes that the current in the power NPN is zero. Figure 3 shows the SW voltage waveform for the circuit of Figure 8 (LT3485-0). Table 1. Recommended Transformer Parameters TYPICAL RANGE TYPICAL RANGE TYPICAL RANGE TYPICAL RANGE PARAMETER NAME LT3485-0 LT3485-1 LT3485-2 LT3485-3 UNITS LPRI Primary Inductance >5 >10 >7 >3.5 µH LLEAK Primary Leakage Inductance 100 to 300 200 to 500 200 to 500 100 to 300 nH N Secondary: Primary Turns Ratio 8 to 12 8 to 12 8 to 12 8 to 12 VISO Secondary to Primary Isolation Voltage >500 >500 >500 >500 V ISAT Primary Saturation Current >1.6 >0.8 >1.0 >2 A RPRI Primary Winding Resistance <300 <500 <400 <200 m Ω RSEC Secondary Winding Resistance <40 <80 <60 <30 Ω |
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