Tell me more ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.
up vote 1 down vote favorite

I am using a P-channel MOSFET as a switch in my design & I have connected the drain to the load. The load needs 0.8mA of current. How can I decide whether the MOSFET is conducting that much current if I connect the source of the MOSFET to 3V ? The MOSFET used is BSS84 from Fairchild & the link for BSS84 datasheet is given below.

BSS84 P-channel MOSFET

share|improve this question
More info needed. LOad is what> resistor ? what value ? If you connect load from 3V to ground what curr ent does it draw? have you got atest meter ? – Russell McMahon Mar 16 '12 at 7:52
Madhu, are you sure that you have to drive 0.8 mA? It's a pretty small current, can I ask you what are you driving? – clabacchio♦ Mar 16 '12 at 10:08

1 Answer

up vote 1 down vote

I think this is the graph of your datasheet that gives all the information you need:

I_D vs. V_GS

So assuming this circuit

enter image description here

V_GS is -3V which allows I_D to be up to ca. 0.5 A.

EDIT:
clabacchio is right pointing out that the graph is actually for V_DS = 5V.
Of course a graph for V_DS = 3V would be better, but there isn't one in the datasheet. So the question is: Will the resistance R_DSON be different for very low drain currents?
Looking at Figure 2 ("On-Resistance Variation with Drain Current and Gate Voltage") gives the answer:
it does change vs. I_DS, but only for currents above ca. 0.1 A and: resistance even gets lower (=better) for lower I_DS.

share|improve this answer
The current spec is 0.8 mA (may be wrong?) and the VDS for that graph is 5V, too high for this application – clabacchio♦ Mar 16 '12 at 10:07
I think Curd is write with that 2 figures. From the graph, Id for 3V Vgs is approximately 0.5A to 0.6A. But I need only 0.8mA for programming an EEPROM (i.e, Programming Current). Now, I want to know how much current that EEPROM will take? – Madhu Mar 16 '12 at 10:46
@clabacchio: concerning I_D: The OP wants to use the MOSFET as switch (not as current source). Since 0.5 A >> 0.8mA this is ok. Concerning V_DS: see my EDIT – Curd Mar 16 '12 at 11:16
@Madhu ok, my assumption about current sources was wrong, I read a different thing :) – clabacchio♦ Mar 16 '12 at 12:30
Anyway I think it would be better posting Fig 1 and saying that at the specified current level the VDS is very small (close to 0). – clabacchio♦ Mar 16 '12 at 15:57

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.