Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

ussing a 555 as a mosfet driver (reliability related)

Status
Not open for further replies.
hello, i'm doing a buck-boost for some high current application , and i cant find a mosfet driver ic (at least in my country) that has no lag and can deliver the rise time
i used a totem pole till now but now the lag it has just renders it unusable (150µS), so i tried a 555 directly and the rise and fall time are pretty good (100nS) but as the data sheet says 200mA max and in short circuit i measured 410mA @12V, but for the rise time (100nS) it's delivering about 140mA (14nC/100nS = 0.14A, i'm not sure if i got it right, so correct me if it's wrong)
i know the 555/556 are like "generic" devices so how long will it last under that load? it doesnt get hot or anything and it's working at 33khz (but the idea is 500Khz)
and btw, any better ic to drive mosfet? i think that i'll have to make it synchronous so if i could get to about 25nS that would be optimal
any sugestion? thanks in advance

note: i'm really noob to this topic so i might have missed something obvious
 
My guess is that it is ok since the pulse is very short, but since your gate charge is only nC14 why not limit the current to 200 ma? You would still be pretty fast and the gate resistor would help damp any oscillations.

Ton=nC/Igate
 
i'll give it a try, i don't see any other solution either.
i'll see if i get a cmos version to see if it's better or not, but the cmos version says 100mA maximum.
in any case i'll have to let them some time to test so ill probabli di some without the resistor.
 
The power required to charge and discharge a capacitor is

P=F*Q*V

In your case, P=5e5 Hz * 14e-9 coul *12 volts

P=84mW

Actually, the voltage swing for a bipolar 555 is only about 10.5V, so the power would be slightly less.
A current limiter will not reduce the total power required to charge and discharge the cap, but it will transfer some of the power dissipation to the current limiter (resistor).

This is in addition to the dissipation of the 555 due to quiescent current and shoot-through current.
Regarding shoot-through current, on-line lore has it at about 400mA, but the only comment I could find regarding pulse width of this spike is that it "is measured in less than 10's of nanoseconds". I question this, because the rise and fall times are spec'ed at 100nS (typ) each. I could measure it, except my lab is still in boxes since I moved.:(
 
Last edited:
A Cmos 555 has a very low output high current. Its output current depends on the supply voltage.
With a 5V supply the output high current is about 2mA and with a 12V supply it is about 10mA.
Its output low current is much more (8mA and 50mA) but also depends on the supply voltage.
 
A Cmos 555 has a very low output high current. Its output current depends on the supply voltage.
With a 5V supply the output high current is about 2mA and with a 12V supply it is about 10mA.
Its output low current is much more (8mA and 50mA) but also depends on the supply voltage.

But Audio, as far as I can see, he was taking about the 555 not the 7555.
 
But Audio, as far as I can see, he was talking about the 555 not the 7555.
There is the ICM7555, the LMC555 and the TLC555 Cmos 555s. Only the ICM7555 shows graphs of the output current.

The OP said a wrong statement so I corrected him:
QUOTE: "i'll see if i get a cmos version to see if it's better or not, but the cmos version says 100mA maximum."
He wants the output to go high then a Cmos 555 has low output current.
 
Last edited:
There is the ICM7555, the LMC555 and the TLC555 Cmos 555s. Only the ICM7555 shows graphs of the output current.

The OP said a wrong statement so I corrected him:
QUOTE: "i'll see if i get a cmos version to see if it's better or not, but the cmos version says 100mA maximum."
He wants the output to go high then a Cmos 555 has low output current.
Yes, I should have read the whole thread.

Still no scrolling, this time I'm using Internet Explorer.

I'll waffle a little more to see if it starts.

So far so good.

For anyone wondering about this, see Audio's other post about scrolling when writing a post with a quote.

See "During a quoted reply the quote keeps scrolling to the beginning"

Still not scrolling, so I'll give up.
 
Last edited:
I got a blank (no text) Reply box again.
Keep typing about 10 or 12 lines like this:
qqq
www
eee
rrr
ttt
yyy
uuu
iii
Then click outside the reply box and scrolling will begin whenever you move your mouse.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top