Maker Pro
Maker Pro

Wimpy Comparator Open Collector/Drain Current

D

D from BC

Jan 1, 1970
0
I want to discharge a mosfet gate really fast!
Cg~2nF at worst.
Vg=12V

I'd like to have a open collector/open drain output comparator do the
job.

For example.

The LMV393 open collector sink output acts like a 12 ohm resistor..
The graph stops at 50mA sink..

The LMV7235 open drain levels off at 60mA @ a Vlow of 2V!!!
Built in current limit?
Wtf? There are mosfets with milliohm conduction...Why not put'm in the
comparator..

Is it safe to assume all common comparators have wimpy internal open
collector/drain output transistors?

Can I get a 12 amp spike!! :) Arrrffff Arrrfff...

I suspect I'll have to resort an external transistor to short out the
charge.
D from BC
 
M

MooseFET

Jan 1, 1970
0
I want to discharge a mosfet gate really fast!
Cg~2nF at worst.
Vg=12V

I'd like to have a open collector/open drain output comparator do the
job.

For example.

The LMV393 open collector sink output acts like a 12 ohm resistor..
The graph stops at 50mA sink..

The LMV7235 open drain levels off at 60mA @ a Vlow of 2V!!!
Built in current limit?
Wtf? There are mosfets with milliohm conduction...Why not put'm in the
comparator..

Is it safe to assume all common comparators have wimpy internal open
collector/drain output transistors?

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.
 
J

Joerg

Jan 1, 1970
0
D said:
I want to discharge a mosfet gate really fast!
Cg~2nF at worst.
Vg=12V

I'd like to have a open collector/open drain output comparator do the
job.

For example.

The LMV393 open collector sink output acts like a 12 ohm resistor..
The graph stops at 50mA sink..

The LMV7235 open drain levels off at 60mA @ a Vlow of 2V!!!
Built in current limit?
Wtf? There are mosfets with milliohm conduction...Why not put'm in the
comparator..

Is it safe to assume all common comparators have wimpy internal open
collector/drain output transistors?

Can I get a 12 amp spike!! :) Arrrffff Arrrfff...

I suspect I'll have to resort an external transistor to short out the
charge.


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.
 
M

MooseFET

Jan 1, 1970
0
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.

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.
 
D

D from BC

Jan 1, 1970
0
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
 
M

MooseFET

Jan 1, 1970
0
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. ??

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
 
D

D from BC

Jan 1, 1970
0
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.

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
 
J

Joerg

Jan 1, 1970
0
D said:
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.
 
D

D from BC

Jan 1, 1970
0
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.

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
 
W

Winfield

Jan 1, 1970
0
D said:
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.

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.
 
W

Winfield

Jan 1, 1970
0
Winfield said:
First, comparators are meant for general purpose,
of course, and not for driving MOSFETs.

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.
 
M

MooseFET

Jan 1, 1970
0
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.

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.
 
J

Joerg

Jan 1, 1970
0
Winfield said:
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.

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.
 
W

Winfield Hill

Jan 1, 1970
0
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.

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.
 
D

D from BC

Jan 1, 1970
0
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
 
D

D from BC

Jan 1, 1970
0
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.


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