Re: Initializing a Flip-Flop on Power-up
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 17 Feb 2006 21:26:39 GMT
Hello Frank,
My circuit du jour is a BSS123. Drain tugs on input that needs reset
low. Gate has cap to VCC and resistor to GND. Plus discharge path if
some BOR is required as well. If the 4-5c are too much I'll sometimes
use a BJT. There must be a discharge path from VCC to GND if power is
turned off. If there isn't that needs some extra parts.
If there is no discharge path of importance, the 4013 stays in position
for weeks. If there is a load on the power supply, the cap gets discharged
through the internal protection diode.
Yes, with CMOS this cane become a concern. I have had designs that could have properly idled for hours on a bunch of bypass caps.
Spending an extra 4-5 cents doesn't sound like you, what have you been
drinking this afternoon? ;)
Oh, just coffee. But not the Douwe Egberts brand :-(
It's a toss-up between cost and battery life. The BJT needs some base current. Only for a second or two but it can matter. Also, you need one additional resistor to avoid frying the base on power-up and this adds almost a cent back into it. So, if you can haggle the FET down to under 3c it'll break even. A lot of times I use the same FET for boost conversion and stuff like that and when there are no BJTs anywhere else it can be cheaper to use a FET for POR even if it's over 3c, by lower inventory and rigging costs.
So far the discrete solution was always more economical than one of the ritzy POR chips. Until one comes out that does it for around 5c.
Regards, Joerg
http://www.analogconsultants.com
.
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