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I need something faster than a 555 timer.

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Ash Small

Thu May 23 2013, 10:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

According to the datasheet, the LM555 has a maximum frequency of 100kHz when used as an astable multivibrator.

I'm pretty sure I need something faster, maybe 1MHz or faster. Any suggestions?

Ben Solon

Thu May 23 2013, 11:28AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Does it have to be adjustable, or can it be a fixed frequency oscillator?

Avalanche

Thu May 23 2013, 11:31AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Might be worth having a look at the 4047 timer from the 4000 series logic. I like it because it has a built in divide by 2 stage, so you get complimentary outputs with an exact 50/50 duty cycle. I'm not sure what it's frequency spec is, but with it being in the 4000 series I'm pretty sure it will be good into the low Mhz.

Ash Small

Thu May 23 2013, 11:39AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

ben123324 wrote ...

Does it have to be adjustable, or can it be a fixed frequency oscillator?

Preferably adjustable, I think, but fixed might be ok.

Avalanche wrote ...

Might be worth having a look at the 4047 timer from the 4000 series logic. I like it because it has a built in divide by 2 stage, so you get complimentary outputs with an exact 50/50 duty cycle. I'm not sure what it's frequency spec is, but with it being in the 4000 series I'm pretty sure it will be good into the low Mhz.

Thanks, I'll have a look.

EDIT: I'm not sure if I'm reading the datasheet correctly, but the switching times still look a bit slow (~100nS, or am I reading it wrong?)

Avalanche

Thu May 23 2013, 01:17PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Just had a quick look, yeah it's not that fast. The 100nS looks like propagation delays, to get the actual switching speeds you have to look in the 4000 series family specs - the switching speeds are about 30-60nS. You could probably push the 4047 to 1 - 1.5Mhz as a guess, but it might change frequency as it gets warm!

I suppose you could build an RC oscillator around 74HC logic, i.e. a couple of NAND gates followed by a divider to get your 50/50 duty cycle outputs. I have a circuit in front of me now that is doing exactly that.

Ben Solon

Thu May 23 2013, 02:30PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Anything over 1 Mhz I wouldn't use an rc based oscillator. A vco would work, but the supply needs to be incredibly stable. One of those programmable clocks would work, but I'd go with a fixed crystal and make my circuit work with the nearest clock frequency or integer division of one of these frequencies.

Patrick

Thu May 23 2013, 03:03PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
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i am almost certain the CMOS versions of the 555 go to 2.1 MHz.

EDIT: the TI cmos 555, rev j, can do 3 MHz in astable mode... I really try to avoid the transistor RC type circuits, they get weird quickly and don't have the certainty of a buffered clock or 555 timer.

Ash are you needing a specific rise/fall time to be much less than a fraction of the level period?

Ash Small

Thu May 23 2013, 03:21PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the replies. It seems the 4000 series (or some of them) can do 10MHz (from a quick look at the application notes).

I need to switch a MOSFET (or MOSFET totem pole, driving a MOSFET, or bank of paralleled MOSFETs) to drive a flyback, so I need the MOSFET to switch off as fast as possible.

(Am I correct in thinking a P channel MOSFET will stop conducting faster than an N channel one?)

I doubt I'll get up to 1MHz, but I do need fast switching times. This is for driving a high power flyback circuit (CCM), and after reading up on volt-seconds, etc. I realise I need a much faster switching speed in order to keep the number of turns as low as possible (ideally, I want a one turn primary @ 300V, but I realise this is probably too ambitious. This won't be running continuously, only for less than a millisecond at a time- see my thread on ignition systems).

Ash Small

Thu May 23 2013, 03:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

Ash are you needing a specific rise/fall time to be much less than a fraction of the level period?

Thanks for the 3MHz info, Patrick. I assume I need the fastest switching speeds I can get, within reason. I don't mind spending a bit if this has any chance of actually working.

(MODS: I intended to edit the above post, but clicked on the wrong button by mistake. Feel free to delete this post, and append to the above post if you wish.)

Ben Solon

Thu May 23 2013, 04:38PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

just a question for my benefit- what happens if you don't have a perfect fall time? the arc is sustained during the cycle where the exhaust is being vented? That does no harm though right?

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