Influence of P-MOSFET hi-z drain to connected logic clock signal

Thread Starter

josip

Joined Mar 6, 2014
67
Hi,
I have P-MOSFET that is used to generate VPP (6V) for microcontroler blow fuse (secure device). Line for VPP is shared with 1.8-3.3V programming clock signal. What is the influence of turned off P-MOSFET (Vg=Vs=4.5-6V) drain connected to logic clock signal (1-8Mhz square, 1.8-3.3V generated by LVC logic family)? I am worried about extra capacitance, but not sure if the right parameter (that I am looking for) in datasheet is Coss. I need P-MOSFET with low Rdson (less than 100mOhm) and with minimum impact on logic clock signal when P-MOSFET is off. Switching characteristic of P-MOSFET is irelevan in my case, only important is MOSFET on state (minimum Rdson, clock signal is off) and MOSFET off state (minimum impact on clock signal).

Regards,
Josip
 
Last edited:

Thread Starter

josip

Joined Mar 6, 2014
67
Still not totally clear.
What, exactly, is the circuit supposed to do?
Device flashing target microcontroller and can blow fuse on target microcontroller, to lock it for flash rewrite / readout. There is data line beside clock, but clock line is used also (with special entry data / clock sequence) for fuse blow with VPP.

During normal use, LVC logic generate clock signal and MOSFET is disabled (hi-z).
During fuse blow, clock is disabled (hi-z), and MOSFET is turned on.
Just looking for MOSFET (disabled, hi-z) that will not have any (or minimal) impact (capacitance) on clock signal.
 

crutschow

Joined Mar 14, 2008
29,779
Okay, the MOSFET capacitance could have a loading effect on the 1.3MJHz clock signal.
You should use a small MOSFET with a low capacitance.

Alternately, you might consider using a CMOS switch, (e.g. CD4066) instead, which would minimize the capacitance.
 

Thread Starter

josip

Joined Mar 6, 2014
67
Problem with CMOS switches is resistance of few ohms (big voltage drop in my case) and that they can not supply 100mA current needed for fuse blow. However, I see that CD4066 Cos output capacitance is very low, 8pF.

So I need to select MOSFET with minimal Coss in datasheet?
 
Top