It doesn't work well except with CMOS 555s, and then only when the output is lightly loaded. The logic levels of the bipolar part don't track the power supply voltage, so the supply rejection goes south.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
-- The problem isn't that the logic levels of the bipolar part don't track the supply voltage, because they do. They _have_ to because the references for the threshold and trigger comparators are generated by a three-resistor voltage divider connected between Vcc and ground with one of the comparators' inputs connected to the two junctions. That way the references are ratiometric and are _always_ 1/3 Vcc and 2/3 Vcc. The problem is that if the bipolar part is operated at a low voltage, Vce(sat) of the upper totem-pole emitter follower can fall below 2/3Vcc, making it impossible to switch the front end.
_Logic_ levels, not comparator thresholds--the output high and low levels are not too close to the rails on the bipolar part, which makes the ratiometric property fail to work.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
-- But the output _does_ track the supply in that for any given current out, the output voltage will vary as the supply does. It's exactly the same with CMOS but, since the CMOS output is closer to the rails, it extends the working range of the ratiometric property.
You're trumping up a silly semantic issue by foisting your definition of 'track' where it doesn't fit. That winds up being nonsense, as you say, but it's your nonsense and not mine.
If the output high level is, say, two VBEs below the positive supply, the charging current and the upper comparator threshold won't track each other as the supply voltage changes. That makes the frequency and duty cycle supply voltage dependent.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
John Fields stated in post snipped-for-privacy@4ax.com on
5/24/11 1:25 PM:
Thanks!
-- [INSERT .SIG HERE]
-- Really? If the supply voltage changes and the output voltage of the logic follows the change, how is that not "tracking"?
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