Datasheet LTC1265, LTC1265-3.3, LTC1265-5 (Analog Devices) - 10

LTC1265/LTC1265-3.3/LTC1265-5
U U W U APPLICATIONS INFORMATION
output voltage can potentially float above the maximum Now consider the case of a 1A regulator with VIN = 4V and allowable tolerance. To prevent this from occuring, a TA = 65°C. Starting with the same 0.55Ω assumption for resistor must be connected between VOUT and ground RDSON, the TJ calculation will yield 125°C. But from the with a value low enough to sink the maximum possible graph, this will increase the RDSON to 0.76Ω, which when leakage current. used in the above calculation yields an actual TJ > 148°C. Therefore the LTC1265 would be unsuitable for a 4V input,
THERMAL CONSIDERATIONS
1A output regulator operating at TA = 65°C. In a majority of applications, the LTC1265 does not
Board Layout Checklist
dissipate much heat due to its high efficiency. However, in When laying out the printed circuit board, the following applications where the switching regulator is running at checklist should be used to ensure proper operation of the high duty cycles or the part is in dropout with the switch LTC1265. These items are also illustrated graphically in turned on continuously (DC), the user will need to do some the layout diagram of Figure 6. Check the following in your thermal analysis. The goal of the thermal analysis is to layout: determine whether the power dissipated by the regulator exceeds the maximum junction temperature of the part. 1. Are the signal and power grounds segregated? The The temperature rise is given by: LTC1265 signal ground (Pin 11) must return to the (–) plate of C T OUT. The power ground (Pin 12) returns to the R = P(θJA) anode of the Schottky diode, and the (–) plate of CIN, where P is the power dissipated by the regulator and θJA whose leads should be as short as possible. is the thermal resistance from the junction of the die to the 2. Does the (+) plate of the C ambient temperature. IN connect to the power VIN (Pins 1,13) as close as possible? This capacitor pro- The junction temperature is simply given by: vides the AC current to the internal P-channel MOSFET T and its driver. J = TR + TA As an example, consider the LTC1265 is in dropout at an 3. Is the input decoupling capacitor (0.1µF) connected input voltage of 4V with a load current of 0.5A. From the closely between power VIN (Pins 1,13) and power Typical Performance Characteristics graph of Switch Re- ground (Pin 12)? This capacitor carries the high fre- sistance, the ON resistance of the P-channel is 0.55Ω. quency peak currents. Therefore power dissipated by the part is: 4. Is the Schottky diode closely connected between the P = I2(R power ground (Pin 12) and switch (Pin 14)? DSON) = 0.1375W For the SO package, the θ 5. Does the LTC1265 SENSE– (Pin 7) connect to a point JA is 110°C/W. close to RSENSE and the (+) plate of COUT? In adjustable Therefore the junction temperature of the regulator when applications, the resistive divider, R1 and R2, must be it is operating in ambient temperature of 25°C is: connected between the (+) plate of COUT and signal T ground. J = 0.1375(110) + 25 = 40.1°C Remembering that the above junction temperature is 6. Are the SENSE– and SENSE+ leads routed together with obtained from a R minimum PC trace spacing? The 1000pF capacitor DSON at 25°C, we need to recalculate the junction temperature based on a higher R between Pins 7 and 8 should be as close as possible to DSON since it increases with temperature. However, we can safely as- the LTC1265. sume that the actual junction temperature will not exceed 7. Is SHDN (Pin 10) actively pulled to ground during the absolute maximum junction temperature of 125°C. normal operation? The SHDN pin is high impedance and must not be allowed to float. 10