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LT3468-2 Datasheet(PDF) 8 Page - Linear Technology |
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LT3468-2 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT3468/LT3468-1/LT3468-2 8 346812fa 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 transformer turns ratio. IPK is 1.4 (LT3468), 0.7 (LT3468-1), and 1.0 (LT3468-2). LPRI needs to be equal or larger than this value to ensure that the LT3468/LT3468-1/LT3468-2 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 impor- tant parameter is the leakage inductance, LLEAK. When the power switch of the LT3468/LT3468-1/LT3468-2 turns off, the leakage inductance on the primary of the trans- former 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 speci- fication, which assumes that the current in the power NPN is zero. Figure 3 shows the SW voltage waveform for the circuit of Figure 6(LT3468). Note that the absolute maxi- mum 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 voltage. Figure 4 shows the various limits on the SW voltage during switch turn off. It is important not to minimize the leakage inductance to a very low level. Although this would result in a very low leakage spike on the SW pin, the parasitic capacitance of the transformer would become large. This will adversely effect the charge time of the photoflash circuit. Linear Technology has worked with several leading mag- netic component manufacturers to produce pre-designed flyback transformers for use with the LT3468/LT3468-1/ LT3468-2. Table 2 shows the details of several of these transformers. APPLICATIO S I FOR ATIO Figure 4. New Transformer Design Check (Not to Scale). Figure 3. LT3468 SW Voltage Waveform VIN = 5V VOUT = 320V 3468 G18 100ns/DIV VSW 10V/DIV Table 1. Recommended Transformer Parameters TYPICAL RANGE TYPICAL RANGE TYPICAL RANGE PARAMETER NAME LT3468 LT3468-1 LT3468-2 UNITS LPRI Primary Inductance >5 >10 >7 µH LLEAK Primary Leakage Inductance 100 to 300 200 to 500 200 to 500 nH N Secondary: Primary Turns Ratio 8 to 12 8 to 12 8 to 12 VISO Secondary to Primary Isolation Voltage >500 >500 >500 V ISAT Primary Saturation Current >1.6 >0.8 >1.0 A RPRI Primary Winding Resistance <300 <500 <400 mΩ RSEC Secondary Winding Resistance <40 <80 <60 Ω VSW 3420 F07 0V “A” “B” MUST BE LESS THAN 40V MUST BE LESS THAN 50V |
Similar Part No. - LT3468-2_15 |
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Similar Description - LT3468-2_15 |
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