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LTC1265-3.3 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1265-3.3 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC1265/LTC1265-3.3/LTC1265-5 APPLICATIONS INFORMATION output voltage can potentially float above the maximum allowable tolerance. To prevent this from occuring, a resistor must be connected between VOUT and ground with a value low enough to sink the maximum possible leakage current. THERMAL CONSIDERATIONS In a majority of applications, the LTC1265 does not dissipate much heat due to its high efficiency. However, in applications where the switching regulator is running at high duty cycles or the part is in dropout with the switch turned on continuously (DC), the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated by the regulator exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = P(θJA) where P is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature is simply given by: TJ = TR + TA As an example, consider the LTC1265 is in dropout at an input voltage of 4V with a load current of 0.5A. From the Typical Performance Characteristics graph of Switch Re- sistance, the ON resistance of the P-channel is 0.55 Ω. Therefore power dissipated by the part is: P = I2(RDSON) = 0.1375W For the SO package, the θJA is 110°C/W. Therefore the junction temperature of the regulator when it is operating in ambient temperature of 25 °C is: TJ = 0.1375(110) + 25 = 40.1°C Remembering that the above junction temperature is obtained from a RDSON at 25°C, we need to recalculate the junction temperature based on a higher RDSON since it increases with temperature. However, we can safely as- sume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125 °C. Now consider the case of a 1A regulator with VIN = 4V and TA = 65°C. Starting with the same 0.55Ω assumption for RDSON, the TJ calculation will yield 125°C. But from the graph, this will increase the RDSON to 0.76Ω, which when used in the above calculation yields an actual TJ > 148°C. Therefore the LTC1265 would be unsuitable for a 4V input, 1A output regulator operating at TA = 65°C. Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1265. These items are also illustrated graphically in the layout diagram of Figure 6. Check the following in your layout: 1. Are the signal and power grounds segregated? The LTC1265 signal ground (Pin 11) must return to the (–) plate of COUT. The power ground (Pin 12) returns to the anode of the Schottky diode, and the (–) plate of CIN, whose leads should be as short as possible. 2. Does the (+) plate of the CIN connect to the power VIN (Pins 1,13) as close as possible? This capacitor pro- vides the AC current to the internal P-channel MOSFET and its driver. 3. Is the input decoupling capacitor (0.1 µF) connected closely between power VIN (Pins 1,13) and power ground (Pin 12)? This capacitor carries the high fre- quency peak currents. 4. Is the Schottky diode closely connected between the power ground (Pin 12) and switch (Pin 14)? 5. Does the LTC1265 SENSE– (Pin 7) connect to a point close to RSENSE and the (+) plate of COUT? In adjustable applications, the resistive divider, R1 and R2, must be connected between the (+) plate of COUT and signal ground. 6. Are the SENSE– and SENSE+ leads routed together with minimum PC trace spacing? The 1000pF capacitor between Pins 7 and 8 should be as close as possible to the LTC1265. 7. Is SHDN (Pin 10) actively pulled to ground during normal operation? The SHDN pin is high impedance and must not be allowed to float. |
Similar Part No. - LTC1265-3.3_15 |
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Similar Description - LTC1265-3.3_15 |
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