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I am very new to EE and have a question. In this circuit, below a MOSFET is used. Based on the schematic, how would I know how to choose the right equivalent MOSFET?

enter image description here

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    \$\begingroup\$ google.com/search?q=tn0604 \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 6 '13 at 0:09
  • \$\begingroup\$ Thanks but could you explain how to figure out an equivalent? like I said I am new. I clicked your link, but I would like to know also based on the circuit, how to determine what type of MOSFET to use. I will edit the question to reflect this comment. \$\endgroup\$ – PhillyNJ Oct 6 '13 at 0:16
  • \$\begingroup\$ The MOSFET used will be determined by the current draw of the device it is controlling and the voltage range that device also expects. You will as a minimum need to provide details of the device part number or the current draw etc. \$\endgroup\$ – placeholder Oct 6 '13 at 0:21
  • \$\begingroup\$ @rawbrawb - thx - I believe that would be 5V \$\endgroup\$ – PhillyNJ Oct 6 '13 at 0:25
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Two main issues: Provided you give your MOSFET enough voltage at the gate to turn it fully on, you reach the lowest possible resistance of your type of MOSFET and allow it to work with as little losses as possible.

  • First, your MOSFET must be able to work with the "on" and "off" logic levels provided by IC1. "Off" isn't critical, you want to take care for "on". Look for the minimum HI output voltage provided by IC1 in its data sheet. The value is likely called VOH,min. This value must be higher than the MOSFETs maximum threshold voltage VTH,max. Allow about 0.5 V of safety margin between the two parameters. This way, when your IC outputs a high voltage, your MOSFET is guaranteed to fully turn on.

  • Second, The MOSFET must be able to handle the current required by the load. Find out what current ILoadyour module (LCD1) requires on the pin connected to the MOSFET. Check the MOSFETs on resistance RDS,on. Calculate the loss dissipated by the MOSFET using PMOSFET = ILoad2 * RDS,on and check if the MOSFET package can handle it (cf. thermal data in the MOSFET's data sheet).

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