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LTC3824EMSE-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC3824EMSE-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 12 page  LTC3824 8 3824fc APPLICATIONS INFORMATION Inductor Selection The maximum inductor current is determined by : IL(MAX) =IOUT(MAX) + IRIPPLE 2 where IRIPPLE = (VIN –VOUT)•D f• L and Duty Cycle D = VOUT + VD VIN + VD VD is the catch diode D1 forward voltage and f is the switching frequency. A small inductance will result in larger ripple current, output ripple voltage and also larger inductor core loss. An empirical starting point for the inductor ripple current is about 40% of maximum DC current. L = (VIN–VOUT)•D f • 0.4 •IOUT(MAX) The saturation current level of the inductor should be sufficiently larger than IL(MAX). Power MOSFET Selection Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate- to-source voltage, the gate-to-source and gate-to-drain charges (QGS and QGD, respectively), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistance (RTH(JC)) and RTH(JA). The gate drive voltage is set by the 8V internal regulator. Consequently, at least 10V VGSratedMOSFETsarerequired in high voltage applications. 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 (due to the positive temperature coefficient of RDS(ON)). The power dissipation calculation should be based on the worst-cast specifications for VSENSE(MAX), the required load current at maximum duty cycle, the voltage and temperature ranges, and the RDS(ON) of the MOSFET listed in the data sheet. The power dissipated by the MOSFET when the LTC3824 is in continuous mode is given by : P MOSFET = V OUT + VD V IN + VD (I OUT ) 2(1+δ)R DS(ON) + K(V IN ) 2(I OUT )(CRSS )(f) The first term in the equation represents the I2R losses in the device and the second term is the switching losses. K (estimated as 1.7) is an empirical factor inversely related to the gate drive current and has the unit of 1/Amps. The δ term accounts for the temperature coefficient of the RDS(ON) of the MOSFET, which is typically 0.4%/°C. CRSS is the MOSFET reverse transfer capacitance. Figure 1 illustrates the variation of normalized RDS(ON) over temperature for a typical power MOSFET. Figure 1. Normalized RDS(ON) vs Temperature From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + PMOSFET • 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 case to the ambient temperature (RTH(CA)). This value of TJ can then be compared to the original assumed value used in the calculation. Output Diode Selection The catch diode carries load current during the switch off-time. The average diode current is therefore dependent JUNCTION TEMPERATURE (°C) –50 1.0 1.5 150 3824 F01 0.5 0 0 50 100 2.0 |
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