8 July 2007

We have selected a moderately low-rated mosfet for our circuit application which has a max Vds as 60V and a max Id as 0.4A. But it was mentioned charging the capacitor in the MOSFET in order to switch it on requires a large amount of current at the very beginning, which causes the following two concerns:
1. The maximum current drawn from the PIC is not large enough to charge the MOSFET fast enough
2. The large drawn of current would damage the PIC

Based on these two concerns, we selected http://www.farnell.com/datasheets/67079.pdf from farnell which can be used as a CMOS current source driver. It will be able to provide a large current to the MOSFET, and also be a separator between MOSFET and PIC. The previous concern was the output impedance and the limited current carrying capability of a CMOS chip are not suitable when high switching speeds are required. (Low frequency range: 50 Hz < f < 500 Hz, High frequency range : f > 20 kHz) However, as our application is not high frequency switching, we do not have to consider that. Hence, the CMOS logic buffer we are using is quite reasonable for our circuit.

We have decided on ordering the temp04 from farnell as well the CMOS logic buffer from Farnell and will be ordering them tomorrow.

The next step I would do before the component arrives, is to test out the MOSFET we ordered, and figure out the PIC programming part for the temperature sensor.

sha
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