Datasheet LTC3824 (Analog Devices) - 10

LTC3824
applicaTions inForMaTion Inductor Selection
The power dissipated by the MOSFET when the LTC3824 The maximum inductor current is determined by : is in continuous mode is given by : I VOUT V + D I RIPPLE P (I L(MAX) = IOUT(MAX) + MOSFET = OUT )2(1+ δ)RDS(ON) 2 VIN + VD (V where I IN – VOUT ) • D + K(VIN)2(IOUT)(CRSS)(f) RIPPLE = f •L V The first term in the equation represents the I2R losses in and Duty Cycle D= OUT + VD the device and the second term is the switching losses. K VIN + VD (estimated as 1.7) is an empirical factor inversely related to the gate drive current and has the unit of 1/Amps. The δ VD is the catch diode D1 forward voltage and f is the term accounts for the temperature coefficient of the R switching frequency. DS(ON) of the MOSFET, which is typically 0.4%/°C. CRSS is the A small inductance will result in larger ripple current, MOSFET reverse transfer capacitance. Figure 1 illustrates output ripple voltage and also larger inductor core loss. the variation of normalized RDS(ON) over temperature for An empirical starting point for the inductor ripple current a typical power MOSFET. is about 40% of maximum DC current. 2.0 (V L = IN– VOUT ) • D f • 0.4 •I ANCE 1.5 OUT(MAX) The saturation current level of the inductor should be 1.0 sufficiently larger than IL(MAX).
Power MOSFET Selection
0.5 δ NORMALIZED ON-RESIST Important parameters for the power MOSFET include the drain-to-source breakdown voltage (BVDSS), the threshold 0–50 0 50 100 150 voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate- JUNCTION TEMPERATURE (°C) to-source voltage, the gate-to-source and gate-to-drain 3824 F01 charges (QGS and QGD, respectively), the maximum drain
Figure 1. Normalized RDS(ON) vs Temperature
current (ID(MAX)) and the MOSFET’s thermal resistance (R From a known power dissipated in the power MOSFET, its TH(JC)) and RTH(JA). junction temperature can be obtained using the following The gate drive voltage is set by the 8V internal regulator. formula: Consequently, at least 10V VGS rated MOSFETs are required in high voltage applications. TJ = TA + PMOSFET • RTH(JA) In order to calculate the junction temperature of the power The RTH(JA) to be used in this equation normally includes MOSFET, the power dissipated by the device must be known. the RTH(JC) for the device plus the thermal resistance from This power dissipation is a function of the duty cycle, the the case to the ambient temperature (RTH(CA)). This value load current and the junction temperature itself (due to the of TJ can then be compared to the original assumed value positive temperature coefficient of R used in the calculation. DS(ON)). The power dissipation calculation should be based on the worst-cast specifications for V
Output Diode Selection
SENSE(MAX), the required load current at maximum duty cycle, the voltage and temperature ranges, The catch diode carries load current during the switch and the RDS(ON) of the MOSFET listed in the data sheet. off-time. The average diode current is therefore dependent 3824fh 10 For more information www.linear.com/LTC3824 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Applications Information Typical Application Related Parts