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CMOS 7555 Calculator ?

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ljcox

Well-Known Member
There are 2 ways you can do this.

1. Connect a diode and resistor in series and connect them across the timing resistor. This will make the charge and discharge time constants different, so you should be able to achieve a 5/95 mark/space ratio. This will change the frequency, so you will need to change the timing capacitor in order to correct it.

2. A more elegant and accurate method is to use a modulo 20 counter since 95/5 = 19. See attachment


Len
 

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Roff

Well-Known Member
DaleHall said:
I have data sheet and formula's
but was unable to get 7556 to output >50% duty, need 95%
when using them..
Generate a 5% duty cycle and run the output through an inverter. Voila! 95% duty cycle.
 

DaleHall

New Member
Thanks all,

I should have been more specific re: needs.

Design affects ~2000 of them, parts count is critical.

2 Applications (IR Emitter)
both 40 KHz 50-50 gated by 100Hz (90-10 & 80-20 for direct gate
one for 1mS burst, 2nd 2mS burst of 40KHz)

object: minimium component, minimum power i.e. batt powered,
use 7556 dual CMOS

Initially had a 2222 inv stage +2 R's,
was seeking to eliminate those 3 parts
by directly driving 2nd 7555's
need one 80 % and one 90% duty version to a 2nd 7555
rather than a 20 & 10 duty w/inverters to 7555
 

Roff

Well-Known Member
Duty cycle is normally specified as Tpw(pos)/Tper unless otherwise noted. When you say 90-10, does that mean 90% low and 10% high? I still don't understand what you're trying to do. Write it all out in sentences instead of outline form.
 

Styx

Active Member
do you need variable PWM duty or do you need fixed 95% duty?

if it is fixed 95% use a 74HC14 (or equive Hex inverting SCHMITT) use cap for the timing to one input then from the output to the input use two resistors in parallel. One of these resistors has a diode in series with it. Get the two resistors right and the cap right and you have 95% duty at yr required frequency
 

DaleHall

New Member
Design: dual CMOS 7556 (Maxim & Intersil PN ICM7556)

Design 1
Side 1, 100Hz, 90% high+5v (9000uS), 10%low_0v(1000uS) to /reset of
Side 2, 40kHz 50-50 direct input from 100Hz low Output for 1mS
Side 2 output: 40 ea 40kHz 50-50 duty +5V pulses followed by 9mS low

Design 2
Side 1, 100Hz, 80% high+5v (8000uS), 20%low_0v (2000uS) to /reset of
Side 2 40kHz 50-50 direct input from 100Hz low Output for 2mS
Side 2 output: 80 ea 40kHz 50-50 duty +5V pulses followed by 8mS low

Each side 2 output drives a logic level MOSFET driving IR LEDs.
 

DaleHall

New Member
Design: dual CMOS 7556 (Maxim & Intersil PN ICM7556)

Design 1
Side 1, 100Hz, 90% high+5v (9000uS), 10%low_0v(1000uS) to /reset of
Side 2, 40kHz 50-50 direct input from 100Hz low Output for 1mS
Side 2 output: 40 ea 40kHz 50-50 duty +5V pulses followed by 9mS low

Design 2
Side 1, 100Hz, 80% high+5v (8000uS), 20%low_0v (2000uS) to /reset of
Side 2 40kHz 50-50 direct input from 100Hz low Output for 2mS
Side 2 output: 80 ea 40kHz 50-50 duty +5V pulses followed by 8mS low

Each side 2 output drives a logic level MOSFET driving IR LEDs.
 

ljcox

Well-Known Member
Your requirements are rather hard to follow. Some additional text and some diagrams would assist us to understand.

Len
 

Roff

Well-Known Member
I don't know if this will really work. It simulates OK, but the 7555 model I have is a behavioral model.
Design the 100Hz, 80%/90% duty cycle oscillator using the two resistor/discharge pin circuit. Design the 40kHz oscillator using the one resistor circuit with feedback from pin 3 to pins 2 and 6 and the cap. Connect the anode of a 1N4148 diode to pin 3 of the 100Hz oscillator and the cathode to pins 2 & 6 of the 40kHz oscillator.
I figured out a way that you might be able to eliminate the diodes, but it would require p-channel mosfets, because the idle state of the 40kHz oscillator was at VCC. I can show you that if you're interested.
 
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