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MIC2590B2YTQ Datasheet(PDF) 19 Page  Micrel Semiconductor 

MIC2590B2YTQ Datasheet(HTML) 19 Page  Micrel Semiconductor 
19 / 23 page Micrel, Inc. MIC2590B September 2008 19 M9999091808 The second breakdown voltage criteria which must be met is a bit subtler than simple drainsource breakdown voltage, but is not hard to meet. Lowvoltage MOSFETs generally have low breakdown voltage ratings from gate to source as well. In MIC2590B applications, the gates of the external MOSFETs are driven from the +12V input to the IC. That supply may well be at 12V + (5% x 12V) = 12.6V. At the same time, if the output of the MOSFET (its source) is suddenly shorted to ground, the gatesource voltage will go to (12.6V – 0V) = 12.6V. This means that the external MOSFETs must be chosen to have a gate source breakdown voltage in excess of 13V; after 12V absolute maximum the next commonly available voltage class has a permissible gatesource voltage of 20V maximum. This is a very suitable class of device. At the present time, most power MOSFETs with a 20V gate source voltage rating have a 30V drainsource breakdown rating or higher. As a general tip, look to surface mount devices with a drainsource rating of 30V as a starting point. MOSFET Maximum OnState Resistance The MOSFETs in the +3.3V and +5V MAIN power paths will have a finite voltage drop, which must be taken into account during component selection. A suitable MOSFET’s datasheet will almost always give a value of on resistance for the MOSFET at a gatesource voltage of 4.5V, and another value at a gatesource voltage of 10V. As a first approximation, add the two values together and divide by two to get the on resistance of the device with 7 Volts of enhancement (keep this in mind; we’ll use it in the following Thermal Issues sections). The resulting value is conservative, but close enough. Call this value RON. Since a heavily enhanced MOSFET acts as an ohmic (resistive) device, almost all that is required to calculate the voltage drop across the MOSFET is to multiply the maximum current times the MOSFET’s RON. The one addendum to this is that MOSFETs have a slight increase in RON with increasing die temperature. A good approximation for this value is 0.5% increase in RON per °C rise in junction temperature above the point at which RON was initially specified by the manufacturer. For instance, the Vishay (Siliconix) Si4430DY, which is a commonly used part in this type of application, has a specified RDS(ON) of 8.0mΩ max. at VGS = 4.5V, and RDS(ON) of 4.7mΩ max. at VGS =10V. Then RON is calculated as: () 6.35mΩ 2 8.0mΩ 4.7mΩ RON = + = at 25°C TJ. If the actual junction temperature is estimated to be 110°C, a reasonable approximation of RON for the Si4430DY at temperature is: () () 9.05mΩ C 0.5% 85 1 6.35mΩ C 0.5% 25 100 1 6.35mΩ ≅ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ° ° + = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ° ° − ° + Note that this is not a closedform equation; if more precision were required, several iterations of the calculation might be necessary. This is demonstrated in the section “MOSFET Transient Thermal Issues.” For the given case, if Si4430DY is operated at an IDRAIN of 7.6A, the voltage drop across the part will be approximately (7.6A)(9.05mΩ) = 69mV. MOSFET SteadyState Thermal Issues The selection of a MOSFET to meet the maximum continuous current is a fairly straightforward exercise. First, arm yourself with the following data: • The value of ILOAD(CONT, MAX) for the output in question (see Sense Resistor Selection). • The manufacturer’s data sheet for the candidate MOSFET. • The maximum ambient temperature in which the device will be required to operate. • Any knowledge you can get about the heat sinking available to the device (e.g., Can heat be dissipated into the ground plane or power plane, if using a surface mount part? Is any airflow available?). Now it gets easy: steadystate power dissipation is found by calculating I 2R. As noted in “MOSFET Maximum On State Resistance,” above, the one further concern is the MOSFET’s increase in RON with increasing die temperature. Again, use the Si4430DY MOSFET as an example, and assume that the actual junction temperature ends up at 110°C. Then RON at temperature is again approximately 9.05mΩ. Again, allow a maximum IDRAIN of 7.6A: () 0.523W 9.05mΩ 7.6A R I n Dissipatio Power 2 ON 2 DRAIN ≅ × = × ≅ The next step is to make sure that the heat sinking available to the MOSFET is capable of dissipating at least as much power (rated in °C/W) as that with which the MOSFET’s performance was specified by the manufacturer. Formally put, the steadystate electrical model of power dissipated at the MOSFET junction is analogous to a current source, and anything in the path of that power being dissipated as heat into the environment is analogous to a resistor. It’s therefore necessary to verify that the thermal resistance from the junction to the ambient is equal to or lower than that value of thermal resistance (often referred to as Rθ(JA)) for which the operation of the part is guaranteed. As an applications issue, surface mount MOSFETs are often less than ideally specified in this regard—it’s become common practice simply to state that the thermal data for the part is specified under the conditions “Surface mounted on FR4 board, t≤10seconds,” or something equally mystifying. So here are a few practical tips: 1. The heat from a surface mount device such as an SO8 MOSFET flows almost entirely out of the drain leads. If the drain leads can be soldered down to one square inch or more of copper the copper will act as the heat sink for the part. This copper must be on the same layer of the board as 
