MIC2582/MIC2583/MIC2583R Single Channel Hot Swap Controller - Micrel - #19

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Micrel, Inc. MIC2582/MIC2583 April 2009 19 M9999-043009-C MOSFET Selection Selecting the proper external MOSFET for use with the MIC2582/83 involves three straightforward tasks: • Choice of a MOSFET which meets minimum voltage requirements. • Selection of a device to handle the maximum continuous current (steady-state thermal issues). • Verify the selected part’s ability to withstand any peak currents (transient thermal issues). MOSFET Voltage Requirements The first voltage requirement for the MOSFET is easily stated: the drain-source breakdown voltage of the MOSFET must be greater than VIN(MAX). For instance, a 12V input may reasonably be expected to see highfrequency transients as high as 18V. Therefore, the drain-source breakdown voltage of the MOSFET must be at least 19V. For ample safety margin and standard availability, the closest value will be 20V. The second breakdown voltage criterion that must be met is a bit subtler than simple drain-source breakdown voltage, but is not hard to meet. In MIC2582/83 applications, the gate of the external MOSFET is driven up to approximately 19.5V by the internal output MOSFET (again, assuming 12V operation). At the same time, if the output of the external MOSFET (its source) is suddenly subjected to a short, the gatesource voltage will go to (19.5V – 0V) = 19.5V. This means that the external MOSFET must be chosen to have a gate-source breakdown voltage of 20V or more, which is an available standard maximum value. However, if operation is at or above 13V, the 20V gatesource maximum will likely be exceeded. As a result, an external Zener diode clamp should be used to prevent breakdown of the external MOSFET when operating at voltages above 8V. A Zener diode with 10V rating is recommended as shown in Figure 9. At the present time, most power MOSFETs with a 20V gate-source voltage rating have a 30V drain-source breakdown rating or higher. As a general tip, choose surface-mount devices with a drain-source rating of 30V as a starting point. Finally, the external gate drive of the MIC2582/83 requires a low-voltage logic level MOSFET when operating at voltages lower than 3V. There are 2.5V logic level MOSFETs available. Please see Table 4 “MOSFET and Sense Resistor Vendors” for suggested manufacturers. Figure 9. Zener Clamped MOSFET Gate