cyberfreak

Hi All, Please see attached 2 SRAM devices ; I want to replace or subtitute the HM62W8512B (512kB) with the HM62V8100I (1MB). Both have an access time of 55nS, but as we know the 1MB one has 1 address line more. What I need to know is by comparing the drawings, must the address lines be shifted or can it be kept symetrical. What I mean by example is must I connect A5 of the previous 512kB device to A18 of the new 1MB device? Or can I connect A5 to A5, etc. How does it work? Cyberfreak

Hi AudioGuru How my design fits together is the following: +12VI is connected to my load and my load return(gnd) is connected to the same ground as my battery. Another external 12V PSU (in-case no solar cells avialable) is connected to +12VI and same ground. My logic sense if there is an external PSU present or not, to turn the MOSFET on or off. The solar cells is connected to VBAT (which have a higher voltage ,round 14V,with trickle, bulk and float charge capability) and is also sensed by my logic. All the parts of the design work. It is just the MOSFET to keep the battery off or on at that conditions that I can't get to work. MOSFETs is not my strong point.

Hi AudioGuru Answer to your first question: Don't know really why I chose to use 2 Mosfets in series. Maybe because I've read somewhere that the diode inside the Mosfet can help stopping any leakage current. Something like that. I realized before my last post that it is not really necessary. Please find attached a new drawing, illustrating a final dawing what we were discussing. You will see I've changed the Logic driver to an OR gate, CD4071, and P-Channel Logic level MOSFET or an Enhanced Mode Mosfet, NDT454P. The Battery supply voltage is +-12V, where it also powers the Logic circuit, to decide when to switch the battery on or off. Applying a 0 at the Gate will turn the MOSFET on, hopefully hard enough to supply my load with 1.8A of power. And a High (12V) to switch the Mosfet off. Do I still need to apply a higher voltage to the Gate than Source? There is not really a frequency speed.. It is switch on and off at certain condition from the OR gate logic. It will stay on or off continuously at certain conditions. My load is GPRS/GPS circuitry with my own uP for Asset Tracking, with a charging circuit for solar panels. That's why I need to switch my battery backup on or off at certain conditions. Will the current driven by the OR gate be enough to drive the Mosfet. Current according to the datasheet is 2.25mA. Mosfets are voltage devices, but current must charge/discharge the gate capacitance, as you mentioned. http://www.fairchildsemi.com/ds/CD/CD4071BC.pdf

Hi AudioGuru Please find attached my schematic. As you can see, I must connect the Battery to Drain, so that the Battery's positive is isolated when the MOSFET is off. As you explained, that means I must use a P-Channel MOSFET instead and change my gate driver to an OR gate. But, how will I supply the MOSFET gate with 24V? Charge pump? How wil this charge pump look like?

Hi AudioGuru Thank you for showing me out that line on the datasheet. My Application is to switch a 12V Battery on or off by the logic NOR gate. Another interesing question arrises: There will be some voltdrop across the Source and Drain, due to Rds, when I apply 12V to the gate. Question is: Will the Source Voltage be, say 11.95V when 12V is applied to the gate and the 12V Battery connected to Drain? Reason I'm asking is that during the search for answers on the net, I've read that the gate voltage must be higher than the source, and in this case it's nearly the same.

Will logic level MOSFETS turn off when 0V is applied to the Gate? Using this MOSFET as an example: http://www.fairchildsemi.com/ds/FD/FDS6961A.pdf

Hi AudioGuru You helped me shine some light on the matter. The MOSFET that I am looking at using, is the FDS6961A. http://www.fairchildsemi.com/ds/FD/FDS6961A.pdf Looking through the datasheet and comparing it with the 2N7000, you used, I found that Vgs=4.5V will draw Id=2.8A, which is more enough. I only need 1.8A, but I have 12V to my disposal. As you said the higher Vgs the lower Rds(on); less powerloss, etc. BUT, what if I want it to be switched OFF completely? Can I apply a 0V? I'm using a NOR gate CD4001 to drive the MOSFET.

Hi I need to confirm the theory about Logic Level MOSFETs. How does it work? 0V to switch it off and 5V to switch it on, type of thing? Can someone please help me?