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Topic: Will the I fry the LM239D when used as mosfet driver? (Read 492 times) previous topic - next topic

snoozerman

Hi,
I have designed a circuit with a comparator to switch on/off an p-mosfet to achive a high side switch. I now suspect the design is not very smart  :smiley-eek-blue:.
Question: Is there a risk I will fry the comparator when dischargeing the mosfet gate?

The comparator is an LM239: http://www.datasheetcatalog.org/datasheet/philips/LM239N.pdf
The Mosfet is an IRFR5305: http://www.irf.com/product-info/datasheets/data/irfr5305pbf.pdf

The comparator output sinks the mosfet gate which is pulled up to 12V by a 8k resistor. However I've learned that relatively high currents may be present when turning the mosfet on/off.
In the LM239D datasheet there is a line "Output short-circuit to ground  :  continous" with an additional note "Maximum current is approximately 20 mA independent of the magnitude of V++". Does this mean the current is internally limited to that and I don't have to worry? Or that current must be externally limited to 20mA in order to not destroy the chip?  :~

A circuit-diagram is attached.

DVDdoug

It's standard practice to put a resistor in series with the MOSFET gate.   I don't think the LM239 is internally current limited.    You might not fry the comparator (especially if you are not switching at a high frequency) but a resistor wouldn't hurt.

snoozerman

So I can at least hope the chip won't break down then - good news :)
Not that I was really intending to use it for fast switching but do you think the Arduino 500 Hz may cause damages?

Thanks for your reply!

MarkT


So I can at least hope the chip won't break down then - good news :)
Not that I was really intending to use it for fast switching but do you think the Arduino 500 Hz may cause damages?

Thanks for your reply!


The comparator only sinks current so you will only get fast turn-off - the turn-on time will be dominated
by the RC time constant of the 8k resistor combined with the gate capacitance.  Try something more like 2k ohms,
the comparator guarantees 6mA of output sinking.
[ I won't respond to messages, use the forum please ]

snoozerman


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