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LT1534-1 Datasheet(PDF) 10 Page - Linear Technology |
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LT1534-1 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LT1534/LT1534-1 APPLICATIONS INFORMATION Thermal Considerations Computing power dissipation for this IC requires careful attention to detail. Reduced output slewing causes the part to dissipate more power than would occur with fast edges. However, much improvement in noise can be produced with modest decrease in supply efficiency. Power dissipation is a function of topology, input voltage, switch current and slew rates. It is impractical to come up with an all-encompassing formula. It is therefore recom- mended that package temperature be measured in each application. The part has an internal thermal shutdown to prevent device destruction, but this should not replace careful thermal design. 1. Dissipation due to input current: PV mA I VIN IN =+ 11 60 where I is the average switch current. 2. Dissipation due to the driver saturation: PVSAT = (VSAT)(I)(DCMAX) where VSAT is the output saturation voltage which is approximately 0.1 + (0.2)(I), DCMAX is the maximum duty cycle. 3. Dissipation due to output slew using approximations for slew rates: P VI I R IV V Rf SLEW IN CSL IN SAT VSL OSC = () + () ()+ () − () () () 2 2 9 2 2 9 4 33 10 4 220 10 ∆ Note if VSAT and ∆I are small with respect to VIN and I, then: P IR V R fV I SLEW CSL IN VSL OSC IN = ()( ) () + ()() () ()( )() 33 10 220 10 99 One word of caution. Sometimes a feedback zero is added to the control loop by placing a capacitor across R1 above. If the feedback zero capacitively pulls the FB pin above the internal regulator voltage (2.4V typ), output regulation may be disrupted. A series resistance with the feedback pin can eliminate this potential problem. Negative Output Voltage Setting Negative output voltage can be sensed using the NFB pin. In this case regulation will occur when the NFB pin is at – 2.5V. The input bias current for the NFB pin is –25 µA (INFB) and must be accounted for when selecting divider resistor values. Figure 3 NFB PIN INFB 1534 F02 –VOUT R2 R1 Referring to Figure 3, R1 is chosen such that: RR V RA OUT 12 25 25 2 25 = − +µ • . .• A suggested value for R2 is 2.5k. The NFB pin is normally left open if the FB pin is being used. Dual Polarity Output Voltage Sensing Certain applications may benefit from sensing both posi- tive and negative output voltages. When doing this each output voltage resistor divider is individually set as previ- ously described. When both FB and NFB pins are used, the LT1534 will act to prevent either output from going beyond its set output voltage. The highest output (lightest load) will dominate control of the regulator. This technique would prevent either output from going unregulated high at no load. However, this technique will also compromise output load regulation. Shutdown If the shutdown pin is pulled low, the regulator will turn off. The supply current will be reduced to less than 20 µA. |
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Similar Description - LT1534-1 |
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