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LP3205-25J5F Datasheet(PDF) 8 Page - Lowpower Semiconductor inc |
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LP3205-25J5F Datasheet(HTML) 8 Page - Lowpower Semiconductor inc |
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8 / 10 page  Preliminary Datasheet LP3205 LP3205 – Ver. 1.0 Datasheet Feb.-2007 Page 8 of 10 Efficiency= 100%- (L1+L2+L3…) Where L1、L2, etc. are the individual losses as a percentage of Input power .although all dissipative elements in the for most of losses: VIN quiescent current and 1 2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence. 1.The VIN quiescent current is due to two components: the DC Bias current as given in the electrical characteristics and the Internal main switch and synchronous switch gate charge currents. the gate charge current results from switching the gate capacitance of the internal power MOSFET switches .Each time the gate charge current. results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switches from high to low to high again, a packet of charge △Q moves from VIN to ground. The resulting △Q/△t is the current out of VIN that is typically larger than the DC bias current. In continuous mode. LGATCHG=f(QT+QB) Where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. 1 2Rlosses tae 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 LX pin is a function of both top and bottom MOSFER RDS(ON) and the duty cycle (DC) as follows: The RDS(ON) for both the top and bottom MOSFETS can be obtained from the typical performance characteristics curves. thus, to obtain 1 2R losses, simply add R SW to RL and multiply 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% of the total loss. |
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