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LTC3704 数据表(PDF) 26 Page - Linear Technology |
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LTC3704 数据表(HTML) 26 Page - Linear Technology |
26 / 36 page LTC1871 26 1871fe APPLICATIONS INFORMATION During the MOSFET’s on-time, the control circuit limits the maximum voltage drop across the power MOSFET to about 150mV (at low duty cycle). The peak inductor current is therefore limited to 150mV/RDS(ON). The relationship between the maximum load current, duty cycle and the RDS(ON) of the power MOSFET is: RDS(ON) VSENSE(MAX) IO(MAX) • 1 1 + 2 • T • 1 VO + VD VIN(MIN) + 1 The VSENSE(MAX) term is typically 150mV at low duty cycle and is reduced to about 100mV at a duty cycle of 92% due to slope compensation, as shown in Figure 8. The constant ‘χ’ in the denominator represents the ripple current in the inductors relative to their maximum current. For example, if 30% ripple current is chosen, then χ = 0.30. The ρT term accounts for the temperature coefficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C. Figure 9 illustrates the variation of normalized RDS(ON) over temperature for a typical power MOSFET. Another method of choosing which power MOSFET to use is to check what the maximum output current is for a given RDS(ON) since MOSFET on-resistances are available in discrete values. IO(MAX) VSENSE(MAX) RDS(ON) • 1 1 + 2 • T • 1 VO + VD VIN(MIN) + 1 Calculating Power MOSFET Switching and Conduction Losses and Junction Temperatures In order to calculate the junction temperature of the power MOSFET, the power dissipated by the device must be known. This power dissipation is a function of the duty cycle, the load current and the junction temperature itself. As a result, some iterative calculation is normally required to determine a reasonably accurate value. Since the controller is using the MOSFET as both a switching and a sensing element, care should be taken to ensure that the converter is capable of delivering the required load current over all operating conditions (load, line and temperature) and for the worst-case specifications for VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the manufacturer’s data sheet. The power dissipated by the MOSFET in a SEPIC converter is: PFET = IO(MAX) • DMAX 1– DMAX 2 •RDS(ON) •DMAX • T + k• VIN(MIN) + VO ()1.85•I O(MAX) • DMAX 1– DMAX •CRSS •f The first term in the equation above represents the I2R losses in the device and the second term, the switching losses. The constant k = 1.7 is an empirical factor inversely related to the gate drive current and has the dimension of 1/current. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PFET •RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. This value of TJ can then be used to check the original assumption for the junction temperature in the iterative calculation process. SEPIC Converter: Output Diode Selection To maximize efficiency, a fast-switching diode with low forward drop and low reverse leakage is desired. The output diode in a SEPIC converter conducts current during the switch off-time. The peak reverse voltage that the diode must withstand is equal to VIN(MAX) + VO. The average forward current in normal operation is equal to the output current, and the peak current is equal to: ID(PEAK) = 1+ 2 •IO(MAX) • VO + VD VIN(MIN) + 1 The power dissipated by the diode is: PD = IO(MAX) • VD and the diode junction temperature is: TJ = TA + PD • RTH(JA) The RTH(JA) to be used in this equation normally includes the RTH(JC) for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. |
类似零件编号 - LTC3704 |
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类似说明 - LTC3704 |
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