# 555 Timer - How many FETs?

#### hydromagnet

Joined Aug 31, 2011
5
Hello, How do I determine how many FETs can be driven from one 555 timer?

I'm trying to drive multiple IRFP260N (50A -200V) Nfets...

with Rds of .04 ohms onstate resistance.

Thanks you for any Help!
marty

#### colinb

Joined Jun 15, 2011
351
I think it mostly depends on how fast you need to switch the FETs. The FET gate capacitance is what the 555 needs to be able to drive. Unlike bipolar junction transistors, virtually no current flows** into the FET gate, so the 555's output can drive a virtually unlimited number of FET gates.

**In fact, some current flows: a minuscule amount as leakage, but primarily you would be concerned with the need to charge the gate (based on its capacitance), so you can control the gate charge current by putting a series resistor between the 555 output and the FET gate.

I may be wrong on this, but this is my current understanding.

#### JingleJoe

Joined Jul 23, 2011
186
I may be wrong on this, but this is my current understanding.
I see what you did there

It may be useful to know that the 555 can output (sink or source) 200mA.

#### #12

Joined Nov 30, 2010
18,224
colin is right. It's all about switching time, and fast is good.

Vgate = Vsource times [e to the negative time over (Resistance x capacitance)], where little e is the root of natural logarythms. (Is there a spell chequer on this website?)

#### CraigHB

Joined Aug 12, 2011
127
As already mentioned, you have to look at total gate charge and frequency to determine how many FETs can be driven with a 555. For larger FETs at higher frequencies, AC gate currents can be considerable. In those cases, you need a FET driver with single digit impedance.

The outputs on a 555 are not particularly low impedance so you'll definitley have to consider those factors if you plan to do any high speed switching. For very low frequency or DC, you should be able to drive a good number of FETs as long as you have liberal tolerance for transients. If you do need to tighten things up and drive the FETs hard, you can always add a FET driver chip.

#### Wendy

Joined Mar 24, 2008
23,281
Actually, at 200ma drive, the 555 is pretty good. It is also fast. The grounding impedance is better than the Vcc side, but even then the chip will burn itself out trying to provide more current through either the Darlington pair or the grounding transistor. I suspect it can handle much higher short term surges, though there is nothing in the datasheet to back up this assertion.

A CMOS 555 doesn't have much drive, but it does go rail to rail.