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4hv.org :: Forums :: General Science and Electronics
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TC4427A Problems

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EDY19
Sun Apr 23 2006, 01:16AM Print
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
I have several of these drivers I need to get working, however, theres a problem- They seem to work fine for a (very) short time after applying power (the outputs are low when the inputs are low) but after about 4 seconds, the outputs automatically go high- I really don't know why. There is nothing connected to the inputs of the chips, and the only thing i can think of is that the pins are acting like capacitors and gain enough charge after 4 seconds to turn on the driver. Any suggestions? Would putting a resistor between the input pins and ground help? When i short the input pins to ground, the outputs do go low for about another four seconds. If so, what sort of value? On the datasheet, it says to ground unused inputs, but it should't just turn on when the input is absent...
Thanks!
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Marko
Sun Apr 23 2006, 01:39AM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
If inputs are hanging in the air it is normal that IC goes wild, especially if input is CMOS.
You must put a pullup resistor to bring the input high and you turn it off by grounding it, like all logic IC's.
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EDY19
Sun Apr 23 2006, 02:05AM
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Does that consist of putting a resistor from the inputs to ground? How exactly would one do that?
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Marko
Sun Apr 23 2006, 02:17AM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Not to ground, but to logic supply voltage (18V max).
This driver has a mosfet at input that has no internall pullups, and it is very sensitive to noise, static charges, fingers, etc.

WIth resistor you charge its gate and turn it on, and you turn the IC off simply by connecting it directly to ground.
This way you can actually test any logic circuit.

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EDY19
Sun Apr 23 2006, 02:31AM
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
I feel like a complete moron, because I still do not understand what you are saying. Do you mean a resistor from the input of the driver to +5V or whatever the logic supply is? This will just make the output high all the time- sorry for not understanding a potentially simple solution tongue

Edit: Oops... I get it, I admit, its getting a bit late and i am tired. I know how to do this :) Ill let you know if it works.
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rupidust
Sun Apr 23 2006, 03:10AM
rupidust Banned
Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85
[quote]
There is nothing connected to the inputs of the chips, and the only thing i can think of is that the pins are acting like capacitors and gain enough charge after 4 seconds to turn on the driver. Would putting a resistor between the input pins and ground help?
[/quote1145761416]

Did you attempt to use a pull-down resistor? Actually you did but did not go all the way, just keep the resistor connected instead of removing it. If you have a scope, you can view how the outputs momentarily toggle from low-high-low on power up of the TC4427 IC. Firkragg already explained why this happens, so no need for me to repeat it. What pull-down resistor value? The one that does not cause too much of a voltage divider and current sink for what ever will be connected to the TC4427's inputs. 1K is fine and you can go higher.

I encountered this problem before and did not want to use pull-downs because of board space limitations. So I used an equivalent chip from On Semiconductor, MC34152D. On board power up, I did not get that annoying single false pulse output as with the (free sampled) TC4427. However, the TC4427 is operable at +5v Vcc unlike the MC34152D, forcing me to go back to TC4427 and extra pull-down resistors.
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EDY19
Sun Apr 23 2006, 10:39AM
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
You mean keep a resistor connected between the input and ground, rupidust?
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Marko
Sun Apr 23 2006, 12:28PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
You mean keep a resistor connected between the input and ground, rupidust?


Since input of TC4427 is actually just a mosfet (with some clamp diodes, see datasheet) you basically turn it on and off by connecting it to + or - by any way.

In this case pulldown will also work, I just said what is the right way to turn on and off inputs of any logic IC's.

Most of logic IC's (especially TTTL) are internally-pulled up and they actually consider their inputs high when hanging in the air. You turn them off by simply grounding the input.

When you really run MOSFET gate rivers you don't need pullups as IC that drives them (any, NE555, TL494, 4046, 5v logic...) already swings from 0 to supply voltage (its output is designed to give or sink the current charging and discharging input mosfet's gate).


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EDY19
Mon Apr 24 2006, 11:28AM
EDY19 Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
Yup- everything works fine now with a 1k pulldown resistor from the input to the gate. At first, things weren't working very well- the motors would *just* turn on- but then i realized that the ground of the supply for the driver chips wasn't connected to the h bridge negative supply- im not even sure why the drivers even thought about conducting :) Anyway, its all good now!
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