Hy all,
The other day I was doing a design which included an LM555 and LMC555 powered from a 12V supply.
I would have liked to have pulled the 'Control' pin down to 3.33V to alter the comparator references to 3.33V and 3.33V/2, but I could find nothing in the data sheet that defines the use of the Control pin or the voltage limits.
Does anyone have any information/experience in this area?
spec
Thanks for the simulation result.The datasheet specifies the maximum CONT voltage as Vcc but does not give a minimum value, implying that down to 0V is ok. Simulation, for what it's worth, shows that for Vcc=12V the 555 will oscillate with 2.1V<Vcont<=12V.
You could test an individual chip and get an empirical answer, but the trouble is, the next chip you fit to the circuit may not be the same. Even the schematic you see in the TI data sheet, for example is notional, although it may be true for the TI (National Semi) version.
spec
Hi,
A couple notes about using the control voltage pin.
First, once you use the control voltage the temperature characteristic of the circuit then depends entirely on the temperature dependence of the control voltage signal.
Second and for some applications a little more important, once you use the control voltage you loose the independence from power supply voltage variations. In normal operation the oscillation frequency is independent from the power supply voltage level, so in theory at 5v you'll get the same frequency output as at 10v. Once the control voltage pin is used to change the output frequency however, power supply variations begin to have an impact on the output frequency again due to the loss of the Vcc/3 discharge and 2*Vcc/3 charge symmetricalness of the charge and discharge threshold levels. As the power supply level changes so does the frequency, so some testing and/or simulation is a good idea. If your supply voltage does not vary that much it may not matter as much, but keep in mind that not all voltage regulator chips are perfect.
I did a plot on the control voltage vs output timing a long time ago and i think i posted to this site. Not sure where it is now but we can do that again if needed. One thing i remember is that it is not linear with control voltage input.
For a free running oscillator there are other ways to do it too which are not very hard to do, such as using an LM339 or similar and set up the three resistors yourself.
BTW, the CMOS version has the 100k resistors but i dont know if they are subject to change in any version, bipolar or CMOS.
LATER:
Here is a graph of the normalized output frequency vs normalized control voltage. Note that setting the control voltage to zero will mean there is no charge/discharge difference so it wont work well. This is for the 50 percent duty cycle version of the Astable oscillator connection (that's where the output drives one resistor to the capacitor).
Hy,
Some interesting observations from you all.
Guess what, the answer to my question has been staring me in the face all along. It is on the datasheet- it is just that I have been reading it incorrectly.
The 'Control Voltage Level' shows the permitted control voltages, both for 5V and 15V supply lines. I had been reading this as the level that the control voltage would assume on this pin. It is not; it is the maximum range that the control voltage can be set too.
So, to answer the initial question, the lowest voltage that control can be made with a 15V supply is 9V and the lowest voltage that control can be made with a 5V supply is 2.6V. I bet, in practice, you could go lower than 9V with a 15V supply but that would be outside the specification stated low limit. Of course, the low voltage limit may be constrained by something as basic as the current flowing through the upper resistor in the divider chain.
Thanks once again for all your inputs.
spec
Hello again,
Say whaaaaaa????
Where are you reading this spec from spec <chuckle> ?
The listing shows the possible OUTPUT voltage of the control pin doesnt it? It should be roughly Vcc*2/3 for any Vcc, so for Vcc=6v it should be 'around' 2v min and 4v max, but you can apply as INPUT any voltage from -0.3v to Vcc+0.3v without damage.
Hy,
I too have been looking around for data on the 555 families: there is a lot of it. I had forgotten about the round tin can version that we used for the first few years.
As a result of the information in this ETO thread, and on the net, my personal conservative rules for using the control pin will now tend to be:
(1) Nominal value of resistor chain: 5K Ohms
(2) Maximum input voltage (operate): Vcc-1V
(3) Minimum input voltage (operate): 1.7V
(4) Linear voltage to frequency range Vcntrl: To be defined, but probably a delta of +-1.5V
But, I needn't have bothered about the Control input levels. While searching the net, I came across this circuit which does the job of turning a 555 into a buffer with a low level drive, without any critical aspects, and for any supply line voltage: http://electronicdesign.com/power/lm555-makes-inexpensive-power-driver
Here are some links that may be of interest:
(1) http://en.wikipedia.org/wiki/555_timer_IC
(2) http://en.wikipedia.org/wiki/Hans_R._Camenzind
(3) http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page3.htm
(4) http://www.taydaelectronics.com/datasheets/A-249.pdf (more definitive about parameters, especially control pin)
(5) **broken link removed**
Additional
(1) /1979_Signetics_Analog_Applications.pdf (an old friend and a treasured document from way back)
(2) http://archive.org/details/Signetics555556Timers (cute little booklet)
This data flowed functional block diagram, modified from the net, sums up the 555 nicely:
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?