This section aims to guide engineers on how to get the input parameters in calculation tool to select a proper MOSFET.

First input the application specifications, entering parameters such as V_IN(min/max), V_OUT, I_OUT, switching frequency and the LC parameters. Then choose a candidate MOSFET and manually enter the key parameters as shown in Figure 3-1.

Figure 3-1 Input MOSFET Parameters in Calculation Tool

The first parameter in the design calculator is the on-state resistance (RDS(on)). The calculator asks for R_DS(on) at 5.1V driving voltage. R_DS(on) is typically given in the data sheet either in the electrical characteristics section or in a graph that plots the V_gs versus R_DS(on).

The total gate charge (Q_G) refers to the charge (in coulombs) necessary to charge the gate capacitance to turn on the MOSFET so that the actual gate voltage of the MOSFET matches the driving voltage. This is different from the switching charge. Q_G is typically given in the MOSFET data sheet.

The internal gate resistance R_G can be modeled by a resistor in series with the MOSFET gate, while RG_PCB is the external gate resistance introduced by PCB copper trace or external resistor.

The C_iss, C_rss, C_oss is the input, reverse and output junction capacitance. Noted that the dynamic characteristic of a MOSFET mainly varies with the drain-source voltage V_DS, Figure 3-2 shows the dependency of C_iss, C_rss, C_oss on V_DS. The junction capacitance is typically given in a graph that plots the V_DS versus capacitance.

Figure 3-2 MOSFET C_iss, C_rss, C_oss versus V_DS

The gate-source threshold voltage V_GSTH, is the threshold voltage that the MOSFET is turned on, while the gate plateau voltage or miller voltage is the voltage that the C_rss is charged. These two voltages are typically given in the MOSFET data sheet.