Datasheet LTC3405A-1.5, LTC3405A-1.8 (Analog Devices) - 10

LTC3405A-1.5/LTC3405A-1.8
U U W U APPLICATIO S I FOR ATIO
2. I2R losses are calculated from the resistances of the The junction temperature, TJ, is given by: internal switches, RSW, and external inductor RL. In TJ = TA + TR continuous mode, the average output current flowing through inductor L is “chopped” between the main where TA is the ambient temperature. switch and the synchronous switch. Thus, the series As an example, consider the LTC3405A-1.8 with an input resistance looking into the SW pin is a function of both voltage of 2.7V, a load current of 300mA and an ambient top and bottom MOSFET RDS(ON) and the duty cycle temperature of 70°C. From the typical performance graph (DC) as follows: of switch resistance, the RDS(ON) of the P-channel switch R at 70°C is approximately 0.94Ω and the R SW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) DS(ON) of the N-channel synchronous switch is approximately 0.75Ω. The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Charateristics The series resistance looking into the SW pin is: curves. Thus, to obtain I2R losses, simply add RSW to RSW = 0.95Ω (0.67) + 0.75Ω (0.33) = 0.88Ω RL and multiply the result by the square of the average Therefore, power dissipated by the part is: output current. P 2 • R Other losses including C D = ILOAD SW = 79.2mW IN and COUT ESR dissipative losses and inductor core losses generally account for less For the SOT-23 package, the θJA is 250°C/ W. Thus, the than 2% total additional loss. junction temperature of the regulator is: T
Thermal Considerations
J = 70°C + (0.0792)(250) = 89.8°C which is well below the maximum junction temperature of In most applications, the LTC3405A series parts do not 125 dissipate much heat due to their high efficiency. But, in °C. applications where they run at high ambient temperature Note that at higher supply voltages, the junction tempera- with low supply voltage, the heat dissipated may exceed ture is lower due to reduced switch resistance (RDS(ON)). the maximum junction temperature of the part. If the junction temperature reaches approximately 150°C, both
Checking Transient Response
power switches will be turned off and the SW node will The regulator loop response can be checked by looking at become high impedance. the load transient response. Switching regulators take To keep the LTC3405A series parts from exceeding the several cycles to respond to a step in load current. When maximum junction temperature, the user will need to do a load step occurs, VOUT immediately shifts by an amount some thermal analysis. The goal of the thermal analysis is equal to (∆ILOAD • ESR), where ESR is the effective series to determine whether the power dissipated exceeds the resistance of COUT. ∆ILOAD also begins to charge or maximum junction temperature of the part. The tempera- discharge COUT, which generates a feedback error signal. ture rise is given by: The regulator loop then acts to return VOUT to its steady- state value. During this recovery time V T OUT can be moni- R = (PD)(θJA) tored for overshoot or ringing that would indicate a stability where PD is the power dissipated by the regulator and θJA problem. For a detailed explanation of switching control is the thermal resistance from the junction of the die to the loop theory, see Application Note 76. ambient temperature. 3405a1518fa 10