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LTC3854EMSE 数据表(PDF) 16 Page - Linear Technology |
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LTC3854EMSE 数据表(HTML) 16 Page - Linear Technology |
16 / 28 page LTC3854 3854fa applicaTions inForMaTion sense voltage is progressively lowered from its maximum programmedvalueto25%ofthemaximumvalue.Foldback current limiting is disabled during soft-start. Minimum and Maximum On-Time Considerations Minimum on-time tON(MIN) is the smallest time duration that the LTC3854 is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that VOUT VIN • fSW > tON(MIN) If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. The minimum on-time for the LTC3854 is approximately 75ns. However, as the peak sense voltage decreases the minimum on-time gradually increases. This is of particu- lar concern in forced continuous applications with low ripple current at light loads. If the duty cycle drops below the minimum on-time limit in this situation, a significant amount of cycle skipping can occur with correspondingly larger current and voltage ripple. Care should also be taken for applications where the duty cycle can approach the maximum given in the data sheet (98%). In all low dropout applications, such as VOUT = 5V and VIN(MIN) = 4.5V, careful selection of the bottom syn- chronous MOSFET is required. For applications where the input voltage can drop below the targeted output voltage, and subsequently ramp up, a low threshold synchronous MOSFET with a small total gate charge should be chosen. This selection for the bottom synchronous MOSFET will insure that the bottom gate minimum on-time is sufficient in dropout to allow for the initial boost capacitor refresh that is needed to adequately turn on the top side driver and begin the switching cycle. Another method to guarantee performance in this type of application is to increase the minimum output load to 50mA. This minimum load will allow the user to choose larger MOSFETs for delivery of large currents when VIN is in the normal operating range yetstillprovideanadequatesafetymarginandgoodoverall performance in dropout with a slow ramping VIN. 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. Percent efficiency can be expressed as: %Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, 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 LTC3854 circuits: 1) IC VIN current, 2) INTVCC regulator current, 3) I2R losses, 4) Topside MOSFET transition losses. 1. The VIN current is the DC supply current given in the ElectricalCharacteristicstable,whichexcludesMOSFET driver and control currents. VIN current typically results in a small (<0.1%) loss. 2. INTVCC current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from INTVCC to ground. The resulting dQ/dt is a cur- rent out of INTVCC that is typically much larger than the control circuit current. In continuous mode, IGATECHG = f(QT + QB), where QT and QB are the gate charges of the topside and bottom side MOSFETs. 3. I2R losses are predicted from the DC resistances of the fuse (if used), MOSFET, inductor, current sense resistor. In continuous mode, the average output current flows through L and RSENSE, but is “chopped” between the topside MOSFET and the synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L and RSENSE to obtain |
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