Wimpy Comparator Open Collector/Drain Current

Take a look at the LM111. It will do about 100mA. It is also easy to add a power NPN to its output. The emitter connection will go up a diode drop very quickly.

That's how I do those things. A BSS123 or 2N7002 can drain the gate capacitance really fast. But mind its max peak current. You can always use more than one. Sure beats finding that expensive boutique comparator and then the purchasing department can't get them anymore some day.

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Regards, Joerg

http://www.analogconsultants.com

On a normal open collector comparitor, a PNP is often a better way to go if fast pull down is needed. You don't have to wait while the pull up charges the gate of a MOSFET.

Slow charge up is ok in my app.. It's for that AC breaker circuit where fast MOS shut down is needed.

Do you mean use the PNP like this for fast turn off?

+12V | R 1 K R | |---|2nF|PowerMosGate---GGGGG | E (PNP)B------sink---->Output open collector comp C | | GGGGGGGGGGGGGGGGGGGGGG

A combination of base and collector current shorts out the gate. ??

D from BC

Yes but don't forget the E-B resistor on the PNP. You need to provide a path for the leakage current of the open collector circuit. The resistor is also what pulls the gate of the MOSFET that last 0.7V

I dunno about that 2N7002.. For shorting out a power mos gate fast the NTR4501N mosfet looks super...

70mohm Rdson Ipulse 10Amps! Qgtot 2.4nC SOT 23 package D from BC

Yeah but it's input capacitance is 200pF and something needs to swing that around. 10:1 is not a lot of a step-down from the 2000pF you've already got there.

--
Regards, Joerg

http://www.analogconsultants.com

True.. The NTR4501 has a 200pf Ciss @Vds10V&1Mhz.. It's tougher to push around compared to the 2N7002 Ciss @ around 20pF. The 2N7002 might be quick to turn on but it's higher on resistance at

7.5ohms discharges a power mosfet gate slower. That's a 107:1 ratio (2N7001:NTR4501).

The drive current (~50mA) has to charge a small mos fast.. This means a low gate charge (or low gate capacitance) is required.. Then it's gotta conduct alot. This means a low Rdson for the most discharge current to short out the charged load(power mos gate) fast. D from BC

Kudos for you D, for looking up the datasheets, etc. But you're barking up the wrong tree. First, comparators are meant for general purpose, of course, and not for driving MOSFETs. Second, choosing an ntr4501 instead of a 2n7000 isn't a solution, with its higher capacitance -- whereas a low-cost honest MOSFET driver IC would be. If you're adverse to such an engineered approach, go with a single 2n4403 PNP follower, that'll do a respectable job on fast turnoff -0.5A drive for small MOSFETs, and beyond that you should really should be using a MOSFET driver.

The truly serious folks use a fast 3A driver, or etc., and then they use that to drive a set of 20 to 30A NMOS + PMOS transistors, which can drive an IGBT module's gates, etc.

I should add, some power MOSFET drivers have internal comparator inputs. These are often parts meant for PWM applications, etc., and may include half-bridge or H-bridge drivers. For example, the HIP4080A types.

On May 20, 7:39 pm, Winfield wrote: [....]

I think you'll find a 2N4403 does more like 700mA when driven by the LM111.

There are transistors like ZXTP25040 from Zetex that do better.

Don't know D's application but those things cost several Dollars. Sometimes a nice (cheap) push pull pair of BJTs after the comparator can be enough.

--
Regards, Joerg

http://www.analogconsultants.com

Let's see, an LM311 costs $0.45 plus a ZXTP2504 pnp costs $0.79 = $1.24, whereas a TC4427CPA dual driver costs $1.05 (single pieces from Digikey). A TC4420CPA driver, with 6A capability, costs $1.35, so we see that while playing with discrete driver circuits is fun, it no longer saves the big bucks.

You might upset the discreterotoes.. :)

Besides... I tend to squish my circuits down as far as possible because I make my own PCB's.. So I gravitate toward IC solutions and minimal parts.

The component reduction caused me to look for general comparators with open drains capable of a tiny 5A to 12A pulse. But.. looks like a bust.. D from BC

Thanks.. I'll look for that.. D from BC