Can this threshold circuit be improved - Automotive Key 15

[Klaus Kragelund]

Hi

The schematic at:

www.microdesign.dk/tmp/dia.pdf

is a reference (TLV431) controlled by the Key signal that is present
in a car when the ignition is on.

Transistor Q5 if turned on will activate Q6 and turn on the reference
for the SMPS and get things running

The trick is that the Key signal is off when below 4V and on when
above 7V. But I have used a zener D30 and the BJT Q5 to set the
threshold level. This is not precise since this threshold will be
influenced by the current in Q5, the B+ voltage (between 9V and 32V),
the current in the zener and the temperature drift of these
components.

The temperature is from -40 to 125 degrees. Can anyone suggest a
better way to do this without using a dedicated opamp/comparator?

Thanks

Klaus

 

[MooseFET]

Hi

The schematic at:

www.microdesign.dk/tmp/dia.pdf

is a reference (TLV431) controlled by the Key signal that is present
in a car when the ignition is on.

Transistor Q5 if turned on will activate Q6 and turn on the reference
for the SMPS and get things running

The trick is that the Key signal is off when below 4V and on when
above 7V. But I have used a zener D30 and the BJT Q5 to set the
threshold level. This is not precise since this threshold will be
influenced by the current in Q5, the B+ voltage (between 9V and 32V),
the current in the zener and the temperature drift of these
components.

The temperature is from -40 to 125 degrees. Can anyone suggest a
better way to do this without using a dedicated opamp/comparator?

Are those F or C degrees? At 125, it matters and since this is
automotive I assume C.

What have you got against a reference and comparitor method? It would
be a lot less parts total.

What is the 74HC14 running on?

 

[Joerg]

Hi

The schematic at:

www.microdesign.dk/tmp/dia.pdf

is a reference (TLV431) controlled by the Key signal that is present
in a car when the ignition is on.

Transistor Q5 if turned on will activate Q6 and turn on the reference
for the SMPS and get things running

The trick is that the Key signal is off when below 4V and on when
above 7V. But I have used a zener D30 and the BJT Q5 to set the
threshold level. This is not precise since this threshold will be
influenced by the current in Q5, the B+ voltage (between 9V and 32V),
the current in the zener and the temperature drift of these
components.

The temperature is from -40 to 125 degrees. Can anyone suggest a
better way to do this without using a dedicated opamp/comparator?

Maybe use another TLV431 instead of D30 and Q5? Its gate could be driven
from a voltage divider that hangs on "Key" and it will draw cathode
current when the gate level exceeds the threshold. Cost-wise it
shouldn't be a huge difference, but it'll be more precise.

 

[Joerg]

The schematic at:

www.microdesign.dk/tmp/dia.pdf

Are those F or C degrees? At 125, it matters and since this is
automotive I assume C.

What have you got against a reference and comparitor method? It would
be a lot less parts total.

What is the 74HC14 running on?

You can eliminate many of those components with this concept, let Klaus
work it out, there are pitfalls here:
View in a fixed-width font such as Courier.

.
.
.
.
. 'HC14
. _____ /|
. KEYON <--------------o |-------------.
. \| |
. |
. |
. |
. pnp |
. | to
. KEY >-[Rs]-+----+-----e c ---------+----> SMPS
. | | b | 4V7
. | | | | input
. | [R1A] | |
. | | | | ---------
. | | --- '431 [RF2] | |
. === +------/ \ | | |
. C| | --- | | |
. | [R1B] | | ---------
. | | | | 4v 7v
. | +-------|------------'
. | | | key->
. | [RF1] |
. | | |
. ----+----+-------+------------------------
.
.

Isn't the quiescent current of the TL431 going to **** up the works?
Place a bleed e-b

Yep, but doesn't have to be much. The cathode current becomes miniscule
when it drops out of regulation.

 

[Klaus Kragelund]

Jim Thompson wrote:

Most definitely over temperature, and then he has that POWER_KEEP
override thing:
           View in a fixed-width font such as Courier.

.
.                          'HC14
.      _____                 /|                    to
.      KEYON <--------------o |-------------+-|>|->SMPS
.                            \|             |      VDD
.                                           |
.PWR_KP>-|>|--.                             |
.             |                    BC857   |
.             |                             |       to
. KEY  >-|>|--+-[Rs]-+----+-----+- e   c ---+---->  SMPS
.             |      |    |     |    b     |       4V7
.             |      |    |   [R2]   |     |       input
.             |      |    |     |    |     |
.             |      |  [R1A]   '----+      |
.             |      |    |          |      |        4v7   -------
.             |      |    |         ---   [RF2]           |    |
.           [RPD]   ===   +---------/ \     |             |    |
.             |     C|    |         ---     |             |    |
.             |      |  [R1B]        |'431  |         0 --------
.             |      |    |          |      |             4v   7v
.             |      |    +----------|------'
.             |      |    |          |                   KEY->
.             |      |  [RF1]        |
.             |      |    |          |
.   ----------+------+----+----------+------------>to SMPS GND
.

Well that possibly messes up the KEY_STATUS logic. I think it would be
simpler to use the '431 or a ZD to establish any kind of 5V logic level
and then clean up all this battery level KEY and POWER_KEEPER logic with
a '393 pulled up to the 5V, and route that around to the SMPS and his
control computer...- Skjul tekst i anførselstegn -

- Vis tekst i anførselstegn -

Thanks for the inputs ;-)

I was sick last week with pneumonia and has been to weak to use the
keys on the keyboard (almost)

Well, I did find an alternative way:

www.microdesign.dk/tmp/dia2.pdf

This uses an 100uA LDO to be used both as reference and as the KEY
control. It has a precise input, so the issues of temperature drift
etc are solved. The Flag output clamps the reference in the other
schematics, so this upower LDO is always enabled

Regards

Klaus

 

[Joerg]

Jim Thompson wrote:
Isn't the quiescent current of the TL431 going to **** up the works?
Place a bleed e-b
...Jim Thompson
Most definitely over temperature, and then he has that POWER_KEEP
override thing:
View in a fixed-width font such as Courier.
.
. 'HC14
. _____ /| to
. KEYON <--------------o |-------------+-|>|->SMPS
. \| | VDD
. |
.PWR_KP>-|>|--. |
. | BC857 |
. | | to
. KEY >-|>|--+-[Rs]-+----+-----+- e c ---+----> SMPS
. | | | | b | 4V7
. | | | [R2] | | input
. | | | | | |
. | | [R1A] '----+ |
. | | | | | 4v7 -------
. | | | --- [RF2] | |
. [RPD] === +---------/ \ | | |
. | C| | --- | | |
. | | [R1B] |'431 | 0 --------
. | | | | | 4v 7v
. | | +----------|------'
. | | | | KEY->
. | | [RF1] |
. | | | |
. ----------+------+----+----------+------------>to SMPS GND
.

Well that possibly messes up the KEY_STATUS logic. I think it would be
simpler to use the '431 or a ZD to establish any kind of 5V logic level
and then clean up all this battery level KEY and POWER_KEEPER logic with
a '393 pulled up to the 5V, and route that around to the SMPS and his
control computer...- Skjul tekst i anførselstegn -

- Vis tekst i anførselstegn -

Thanks for the inputs ;-)

I was sick last week with pneumonia and has been to weak to use the
keys on the keyboard (almost)

Sorry to hear that, hope nobody around you got it. Grandma would hang us
over a pot of steaming water with chamomile buds in there and a towel
over the head. It worked ...

Well, I did find an alternative way:

www.microdesign.dk/tmp/dia2.pdf

This uses an 100uA LDO to be used both as reference and as the KEY
control. It has a precise input, so the issues of temperature drift
etc are solved. The Flag output clamps the reference in the other
schematics, so this upower LDO is always enabled

Wow, those are expensive. Looks like tke KEY_STATUS would still need
work since it won't reliably transition at 4V. Can the FLAG output be
used for that as well? Also, you might need a hysteresis back to MON so
the thing doesn't get the shakes when KEY comes a bit sluggish.

Personally I am not a fan of LDOs, seen too much grief with them.
Including stuff where the manufacturer's app engineers said "oh, oh!"
and pretty much gave up.

 

[Joerg]

Fred Bloggs wrote:

Jim Thompson wrote:
Isn't the quiescent current of the TL431 going to **** up the works?
Place a bleed e-b
...Jim Thompson
Most definitely over temperature, and then he has that POWER_KEEP
override thing:
View in a fixed-width font such as Courier.
.
. 'HC14
. _____ /| to
. KEYON <--------------o |-------------+-|>|->SMPS
. \| | VDD
. |
.PWR_KP>-|>|--. |
. | BC857 |
. | | to
. KEY >-|>|--+-[Rs]-+----+-----+- e c ---+----> SMPS
. | | | | b | 4V7
. | | | [R2] | | input
. | | | | | |
. | | [R1A] '----+ |
. | | | | | 4v7 -------
. | | | --- [RF2] | |
. [RPD] === +---------/ \ | | |
. | C| | --- | | |
. | | [R1B] |'431 | 0 --------
. | | | | | 4v 7v
. | | +----------|------'
. | | | | KEY->
. | | [RF1] |
. | | | |
. ----------+------+----+----------+------------>to SMPS GND
.
Well that possibly messes up the KEY_STATUS logic. I think it would be
simpler to use the '431 or a ZD to establish any kind of 5V logic level
and then clean up all this battery level KEY and POWER_KEEPER logic with
a '393 pulled up to the 5V, and route that around to the SMPS and his
control computer...- Skjul tekst i anførselstegn -

- Vis tekst i anførselstegn -
Thanks for the inputs ;-)

I was sick last week with pneumonia and has been to weak to use the
keys on the keyboard (almost)

Sorry to hear that, hope nobody around you got it. Grandma would hang us
over a pot of steaming water with chamomile buds in there and a towel
over the head. It worked ...

Well, I did find an alternative way:

www.microdesign.dk/tmp/dia2.pdf

This uses an 100uA LDO to be used both as reference and as the KEY
control. It has a precise input, so the issues of temperature drift
etc are solved. The Flag output clamps the reference in the other
schematics, so this upower LDO is always enabled

Wow, those are expensive. Looks like tke KEY_STATUS would still need
work since it won't reliably transition at 4V. Can the FLAG output be
used for that as well? Also, you might need a hysteresis back to MON so
the thing doesn't get the shakes when KEY comes a bit sluggish.

Personally I am not a fan of LDOs, seen too much grief with them.
Including stuff where the manufacturer's app engineers said "oh, oh!"
and pretty much gave up.

If Klaus had been visiting me, as planned, he wouldn't have pneumonia,
he'd have sun burn ;-)

When he comes and you don't want to fire up the grill at home maybe take
him to Rustler's Roost at South Mountain. I don't know if they still
have the live bull out front but Europeans usually can't believe that
they are really serving rattle snake as an appetizer. Great view and
good beers, too.