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LT3585EDDB-2 Datasheet(PDF) 11 Page - Linear Technology |
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LT3585EDDB-2 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LT3585-0/LT3585-1 LT3585-2/LT3585-3 11 3585f APPLICATIO S I FOR ATIO Choosing the Right Device (LT3585-0/LT3585-1/LT3585-2/LT3585-3) The only difference between the four versions of the LT3585 series is the peak current level. For the fastest possible charge time, use the LT3585-3. The LT3585-1 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 LT3585-1 can use a physically smaller transformer. The LT3585-0 and LT3585-2 have a current limit in between that of the LT3585-1 and the LT3585-3. Transformer Design The flyback transformer is a key element for any LT3585-0/ LT3585-1/LT3585-2/LT3585-3 design. It must be designed carefully and checked that it does not cause excessive cur- rent 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 LT3585-0/LT3585-1/LT3585-2/LT3585-3 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 halts power delivery. Thus, the choice of N sets the target output voltage and changes the amplitude gain of the reflected voltage from the output to the SW pin. Choose N according 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 forward 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 (LT3585-0), 0.7 (LT3585-1), 1 (LT3585-2) and 2 (LT3585-3). LPRI needs to be equal or larger than this value to ensure that the LT3585 series has adequate 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 important parameter is the leakage inductance, LLEAK. When the power switch of the LT3585 series turns off, the leakage inductance on the primary of the transformer causes a voltage spike to occur on the SW pin. The height of this spike must not exceed 50V, even though the absolute maximum rating of the SW pin is 60V. The 60V absolute maximum rating is a DC blocking voltage specification, which assumes that the current in the power NPN is zero. Figure 4 shows the SW voltage waveform for the circuit of Figure 8 (LT3585-0). Note that the absolute maximum rating of the SW pin is not exceeded. Make sure to check the SW voltage waveform with VOUT near the target output voltage, as this is the worst-case condition for SW volt- age. Figure 5 shows the various limits on the SW voltage during switch turn off. Table 1. Recommended Transformer Parameters PARAMETER NAME TYPICAL RANGE LT3585-0 TYPICAL RANGE LT3585-1 TYPICAL RANGE LT3585-2 TYPICAL RANGE LT3585-3 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 |
Similar Part No. - LT3585EDDB-2 |
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Similar Description - LT3585EDDB-2 |
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