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LTC1872 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1872 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1872 For most applications, an 80k resistor is suggested for R1. To prevent stray pickup, locate resistors R1 and R2 close to LTC1872. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% – ( η1 + η2 + η3 + ...) where η1, η2, etc. are the individual losses as a percent- age of input power. Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1872 circuits: 1) LTC1872 DC bias current, 2) MOSFET gate charge current, 3) I2R losses and 4) voltage drop of the output diode. 1. The VIN current is the DC supply current, given in the electrical characteristics, that excludes MOSFET driver and control currents. VIN current results in a small loss which increases with VIN. APPLICATIONS INFORMATION 2. MOSFET gate charge current results from switching the gate capacitance of the power MOSFET. Each time a MOSFET gate is switched from low to high to low again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is a current out of VIN which is typically much larger than the contoller’s DC supply current. In continuous mode, IGATECHG = f(Qp). 3. I2R losses are predicted from the DC resistances of the MOSFET, inductor and current sense resistor. The MOSFET RDS(ON) multiplied by duty cycle times the average output current squared can be summed with I2R losses in the inductor ESR in series with the current sense resistor. 4. The output diode is a major source of power loss at high currents. The diode loss is calculated by multiply- ing the forward voltage by the load current. 5. Transition losses apply to the external MOSFET and increase at higher operating frequencies and input voltages. Transition losses can be estimated from: Transition Loss = 2(VIN)2IIN(MAX)CRSS(f) Other losses, including CIN and COUT ESR dissipative losses, and inductor core losses, generally account for less than 2% total additional loss. |
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Similar Description - LTC1872 |
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