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LTC3872EDDB Datasheet(PDF) 10 Page - Linear Technology |
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LTC3872EDDB Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page LTC3872 10 3872fa 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 (line voltage 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 boost converter is: P I D RD k FET OMAX MAX DS ON MAX T = ⎛ ⎝⎜ ⎞ ⎠⎟ + () () – •• • 1 2 ρ ••• – •• . () V I D Cf O OMAX MAX RSS 185 1 () 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 case to the ambient temperature (RTH(CA)). This value of TJ can then be compared to the original, assumed value used in the iterative calculation process. 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 boost converter conducts current during the switch off-time. The peak reverse voltage that the diode must withstand is equal to the regulator output voltage. The average forward current in normal operation is equal to the output current, and the peak current is equal to the peak inductor current. II I D D PEAK L PEAK OMAX MAX () () () • – == + ⎛ ⎝⎜ ⎞ ⎠⎟ 1 21 χ 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. Remember to keep the diode lead lengths short and to observe proper switch-node layout (see Board Layout Checklist) to avoid excessive ringing and increased dis- sipation. Output Capacitor Selection Contributions of ESR (equivalent series resistance), ESL (equivalent series inductance) and the bulk capacitance must be considered when choosing the correct component for a given output ripple voltage. The effects of these three parameters (ESR, ESL and bulk C) on the output voltage ripple waveform are illustrated in Figure 5e for a typical boost converter. The choice of component(s) begins with the maximum acceptable ripple voltage (expressed as a percentage of the output voltage), and how this ripple should be divided between the ESR step and the charging/discharging ΔV. For the purpose of simplicity we will choose 2% for the maximum output ripple, to be divided equally between the ESR step and the charging/discharging ΔV. This percent- age ripple will change, depending on the requirements of the application, and the equations provided below can easily be modified. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the fol- lowing equation: ESR V I COUT O IN PEAK ≤ 001 .• () APPLICATIO S I FOR ATIO |
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