Capacitor Discharging with CMOS Gate

The old 4000A stuff was incredibly fragile this way. The 4000B series was much better hardened against scr latchup, as is HC and such. Lots of analog chips and adc/dac types are terrible. National's LM35 is positively eager to latch.

Maybe just use a 2N7002 to discharge the cap and end the debate?

John

In the Motorola 4000B family guide they still explicitly warn about that latch-up condition. Yes, the B series is more robust but I would not let a design like this get into production.

Since the OP is tight on board space maybe he could use a BAW56 instead and run the other cathode to the supply rail. Then the cap would dump into the rail directly in case of a sag instead of via the output. Or use a SC75 variety if it has to be really tiny.

Hey, surprise, this thread hasn't turned towards politics :-)

In a TO-220 :P Arrrff! I've been freaked enough to use a decharge resistor. On the silkscreen I'm labeling the decharge resistor 'PitaR1' :P

D from BC British Columbia Canada.

SOT-23. About as big as a resistor.

If you're concerned about discharge current damaging the gate, that's not going to happen. If you're worried about latchup, the resistor's not guaranteed to help.

So there.

John

Or just use a double diode. Since they come in the same package as single diodes if you chose SOT23 or SC75 nobody would notice since there is no extra package :-)

But a BAW56 would. So there :-)

I built a sonar unit using a UV-erasable MC68HC11, and had ongoing problems during development with unstable A/D readings... until I covered the chip :-) Seems pretty obvious now, but it was a learning experience.

OK, I admit I've had too much beer, Maredsous and then Stella, both on draft, but I can't see how a diode helps either problem.

John

I still recall your 1st response. That latch up stuff is wayyy beyond my experience. So I'm getting thrown off..

No chuckle for decharging a little cap with a TO-220? :) I do have bunch of SOT23 2n7002s if I'm really chicken.

D from BC British Columbia Canada.

Model: 14V signal _| SW1 |-----| \\ D1 / R | | |O--|

:) I've seen a machine driven by a 8749 (EPROM microcontroller) crash due to a camera flash - but it didn't destroy itself!

A Shottky to Vdd would be better.. that one would probably still allow significant current through the ESD network.

Best regards, Spehro Pefhany

Hunh? That didoe won't help latch-up. You could put one betwen the 14V and 'Vdd', maybe with 1K in parallel

Best regards, Spehro Pefhany

He could insert an SD from C (+) to Vdd, depending on the application, the small signal SD reverse leakage may even ameliorate the C self-discharge a bit, and the low VF will not even turn on his output diode, which could be a 1N4148.

Right, old news. That parasitic SCR to the substrate is activated by a distributed ohmic drop turning on a PN junction and will require 100s of milliamps if not more.

More like 10's of milliamps.

It's a variable number highly dependent on manufacturer and care of the designer in applying "well stops".

I've harangued about this before, followed by "put-downs" by those of you who think you know; but plain and simple it's BAD engineering practice.

...Jim Thompson

Most HC-type parts are spec'd for 50 mA ESD current. Some will latch at higher currents, some never. Some other parts, especially mixed-signal stuff, can be triggered by sub-mA currents. I have a product with a DAC7734 surrounded with diodes to keep it from latching.

If you pull microamps out of an LM35 signal pin (to measure negative temps, as the datasheet suggests) before you apply Vcc, it will usually latch. The older CD4000's would latch from tiny dc currents or spikes.

The newer Xilinx chips seem happy with all the current an ESD diode could reasonably stand.

John

No, thaddaway:

Hope it worked, had to copy your ASCII art and edit with keyboard.

Is not bad practice if the mfr guarantees the latchup current, which is typically 50 mA when they do. I haven't seen a standard cmos logic product (HC/HCT, even the HC405x analog multiplexers) latch, ever.

Most of the HC muxes have another shoot-through issue if you overdrive the inputs, but that doesn't latch.

John