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LTC3405AES6-1.5 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3405AES6-1.5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC3405A-1.5/LTC3405A-1.8 sn3405a1518 3405a1518fs APPLICATIO S I FOR ATIO 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is “chopped” between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Charateristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. Thermal Considerations In most applications, the LTC3405A series parts do not dissipate much heat due to their high efficiency. But, in applications where they run at high ambient temperature with low supply voltage, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150 °C,both power switches will be turned off and the SW node will become high impedance. To keep the LTC3405A series parts from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The tempera- ture rise is given by: TR = (PD)(θJA) where PD 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, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3405A-1.8 with an input voltage of 2.7V, a load current of 300mA and an ambient temperature of 70 °C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70 °C is approximately 0.94Ω and the RDS(ON) of the N-channel synchronous switch is approximately 0.75 Ω. The series resistance looking into the SW pin is: RSW = 0.95Ω (0.67) + 0.75Ω (0.33) = 0.88Ω Therefore, power dissipated by the part is: PD = ILOAD2 • RSW = 79.2mW For the SOT-23 package, the θJA is 250°C/W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.0792)(250) = 89.8°C which is well below the maximum junction temperature of 125 °C. Note that at higher supply voltages, the junction tempera- ture is lower due to reduced switch resistance (RDS(ON)). Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ( ∆ILOAD • ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady- state value. During this recovery time VOUT can be moni- tored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76. |
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