galapogos Posted February 1, 2007 Report Share Posted February 1, 2007 Hi,I have an application where I have to switch on/off 2 3.3V low powerdevices(~50mA max combined) with a 3.3V MCU signal. I am currentlyusing a TP0610K that I have lying around with the followingconnections.Source - 3.3VGate - 50K pullup to 3.3V, and MCU control signalDrain - 1M pulldown and device VccThis seems to work, but according to the TP0610K datasheet (http://www.vishay.com/docs/71411/71411.pdf), 3.3V doesn't even begin to bringthe fet into saturation mode. Here are the relevant specs of theTP0610K (at least those that I think is relevant):Vgs(th) = -1.0/-3.0V min/maxId(on) = -50mA (Vgs = -4.5V, Vds = -10V)Rds(on) = 10ohm (Vgs = -4.5V, Id = -25mA)*According to the Rds(on)-Vgs graph, Rds(on) rises sharply at aboutVgs = 5VNow, while the switch seems to work now, there are a couple of thingsthat worry me.1) The Id(on) of 50mA is probably lower when Vgs, Vds = -3.3V, andthis may be a problem since my devices consume about 50mA max2) Rds(on) rises almost to an asymptote when Vgs < 5VWith that said, I looked at a few other mosfets that I could drop intomy existing design, and came up with the following:Fairchild FDV304P - Vth -0.86V, Id -460mA, Rds(on) [email protected] VgsFairchild FDV302P - Vth 1V, Id -120mAm Rds(on) [email protected] VgsON NTR1P02LT1 - Vth -1V, Id -1.3A, Rds(on) curve bends at -4V Vgs,0.19ohm-2.5V VgsON NTR2101P Vth -1V, Id -3.7A, Rds(on) [email protected] VgsON NTR4101P - Vth -0.72V, Id -3.2A, Rds(on) [email protected] VgsVishay Si2323DS - Vth -1V, Id -4.1A, Rds(on) [email protected] VgsVishay Si2301BDS - Vth -0.95V, Id -2A?, Rds(on) [email protected] VgsThere are a few more, but I just want to make sure if I have the rightidea. Basically what I'm thinking is that I need a mosfet that has thefollowing characteristics:1) low threshold voltage(all the above have Vgs(th) >= -1V2) Sufficient Id(-120mA and less should be sufficient?)3) Low Rds(on)(single digit, or even <1ohm shouldn't cause anynoticable voltage dip at device Vcc)Some of the above mosfets are for logic rather than power switching,but I'm guessing that as long as the current capacity(Id) issufficient, it shouldn't matter?Is there anything else I should worry about?Thanks! Quote Link to comment Share on other sites More sharing options...
indulis Posted February 1, 2007 Report Share Posted February 1, 2007 Take a look herehttp://www.vishay.com/mosfets/lteq-20-v/micro-foot-package/Note the Rds on with a Vgs of only 1.5V!! Even at 2.5V they are all below .05 ohmsIn general, you'll find that N channel MOSFET's have lower Rds on numbers than P channel. Quote Link to comment Share on other sites More sharing options...
galapogos Posted February 1, 2007 Author Report Share Posted February 1, 2007 Thanks, but due to the existing design I have to use a P channel transistor. Is a low RDS and Vth all that I should be looking at? I guess I just need to know some guidelines on what parameters are important when choosing a transistor. I might even go for a PNP BJT since they seem to be more recommended. Quote Link to comment Share on other sites More sharing options...
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