Datasheet LTC3406A (Analog Devices) - 10

LTC3406A
APPLICATIONS INFORMATION
Although all dissipative elements in the circuit produce 2. I2R losses are calculated from the resistances of the losses, two main sources usually account for most of internal switches, RSW, and external inductor RL. In the losses in LTC3406A circuits: VIN quiescent current continuous mode, the average output current fl owing and I2R losses. The VIN quiescent current loss dominates through inductor L is “chopped” between the main the effi ciency loss at very low load currents whereas the switch and the synchronous switch. Thus, the series I2R loss dominates the effi ciency loss at medium to high resistance looking into the SW pin is a function of both load currents. In a typical effi ciency plot, the effi ciency top and bottom MOSFET RDS(ON) and the duty cycle curve at very low load currents can be misleading since (DC) as follows: the actual power lost is of no consequence as illustrated R in Figure 2. SW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be 1 VIN = 3.6V obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to 0.1 RL and multiply the result by the square of the average output current. 0.01 Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than POWER LOSS (W) 0.001 2% total additional loss. VOUT = 1.2V VOUT = 1.8V VOUT = 2.5V
Thermal Considerations
0.0001 0.1 1 10 100 1000 In most applications the LTC3406A does not dissipate OUTPUT CURRENT (mA) 3406A F02 much heat due to its high effi ciency. But, in applications where the LTC3406A is running at high ambient tem-
Figure 2. Power Lost vs Load Current
perature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the 1. The VIN quiescent current is due to two components: maximum junction temperature of the part. If the junction the DC bias current as given in the electrical charac- temperature reaches approximately 150°C, both power teristics and the internal main switch and synchronous switches will be turned off and the SW node will become switch gate charge currents. The gate charge current high impedance. results from switching the gate capacitance of the To avoid the LTC3406A from exceeding the maximum junc- internal power MOSFET switches. Each time the gate tion temperature, the user will need to do some thermal is switched from high to low to high again, a packet of analysis. The goal of the thermal analysis is to determine charge, dQ, moves from VIN to ground. The resulting whether the power dissipated exceeds the maximum dQ/dt is the current out of VIN that is typically larger junction temperature of the part. The temperature rise is than the DC bias current. In continuous mode, IGATECHG given by: = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias TR = (PD)(θJA) and gate charge losses are proportional to VIN and thus where PD is the power dissipated by the regulator and θJA their effects will be more pronounced at higher supply is the thermal resistance from the junction of the die to voltages. the ambient temperature. 3406afa 10