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From: nospam on 19 Dec 2008 09:48
Raveninghorde <raveninghorde(a)invalid> wrote:

>For the benefit of the OP.

>This circuit is not suitable for your application as drawn.

The first question is is it acceptable to shut down during a load dump. If
it is you can buy LDO regulators with reverse battery and 60v load dump
protection built in.

I presume 60v is enough for 12v automotive systems else they wouldn't
bother trying to sell them?
--
From: Jim Thompson on 19 Dec 2008 10:04
On Fri, 19 Dec 2008 11:03:59 +0000, Raveninghorde
<raveninghorde(a)invalid> wrote:

>On Thu, 18 Dec 2008 09:22:00 -0700, Jim Thompson
><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote:
>
>>On Thu, 18 Dec 2008 08:13:19 -0800 (PST), gearhead
>><nospam(a)billburg.com> wrote:
>>
>>>On Dec 18, 5:58 am, eaglecbr <Christopher.B.Robe...(a)gmail.com> wrote:
>>>> I'm designing a Automotive Regulator Circuit for a system that runs at
>>>> about 100mA at 7V with a max current draw of around 600mA for 10secs.
>>>> The problem i'm having is with the protection circuit.  I'm using the
>>>> RBO40-40GT(http://www.stmicroelectronics.com/stonline/products/
>>>> literature/ds/5484.pdf).  This is the best chip I have found that can
>>>> easily suppress voltages up to 120V(highest that I've tested it
>>>> against).  The problem is that this chip only clamps the voltage to a
>>>> max of 40V.  I'd like to use a linear regulator if possible, due to
>>>> various load changes.  I've been using an LT1129 from Linear Tech, but
>>>> the problem is that it has a max input of 30V.  Most linear regulators
>>>> that can handle this current usually have a max input of 30V.  I'd
>>>> also like to keep the circuit small and surface mount if possible.
>>>>
>>>> Any idea what I can do to clamp the voltage to 30V, while still using
>>>> the RBO40-40GT?  If I add another Zener diode between the regulator
>>>> and the RBO, clamping the voltage to 30V - will this work?  Any ideas
>>>> would be great.  Thanks
>>>
>>>How about using a circuit that goes open circuit instead of clamping.
>>>You might have to do it with discretes.
>>>Actually Jim Thompson posted a link here to just such a circuit on his
>>>website a few months ago.
>>
>>http://analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf
>>
>>>Goes something like this:
>>>A p-channel mosfet in the top rail with source connected to the
>>>positive power supply and base tied to ground provides reverse
>>>protection.
>>>To get overvoltage protection you use a TL431, a transistor and
>>>another mosfet in the rail. With a voltage divider to the reference
>>>terminal of the TL431, an overvoltage condition will cause the TL431
>>>"cathode" to sink current, turning on a pnp transistor that connects
>>>the source and gate of the mosfet to turn it off.
>>>Plus the various resistors you would need in a circuit such as this.
>>>Remember the TL431 may sink some bit of current even when the
>>>reference pin sees low voltage, so you should put a resistor from the
>>>cathode to the top rail.
>>
>> ...Jim Thompson
>
>For the benefit of the OP.
>
>This circuit is not suitable for your application as drawn. The FETs
>don't have the drain to source voltage rating and if you put in
>suitable FETs M1will go phut when you exceed its gate source voltage
>rating.

The posted circuit was to meet a specific SED functional request.
Scale voltage ratings and take appropriate additional protective
measures as necessary for your application.

In other words, act like a designer ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
From: Jim Thompson on 19 Dec 2008 10:32
On Fri, 19 Dec 2008 14:48:21 +0000, nospam <nospam(a)please.invalid>
wrote:

>Raveninghorde <raveninghorde(a)invalid> wrote:
>
>>For the benefit of the OP.
>
>>This circuit is not suitable for your application as drawn.
>
>The first question is is it acceptable to shut down during a load dump.

Depends on the circuit configuration. Many of my ignition system
designs protected themselves by turning off appropriate devices
_during_ the load dump pulse.

>If
>it is you can buy LDO regulators with reverse battery and 60v load dump
>protection built in.
>
>I presume 60v is enough for 12v automotive systems else they wouldn't
>bother trying to sell them?

I haven't looked "under the hood" with a 'scope in years, so I'm not
familiar with what load-dump controls that might have been added.

But I can assure (scare) you that I ran storage 'scope investigations
in the '60's that showed load dump pulses as high as 400V :-(

And I have chips turned to beads of silicon to prove it ;-)
(Inductance-driven arc discharge across the chip.)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
From: Jim Thompson on 19 Dec 2008 10:39
On Fri, 19 Dec 2008 08:32:02 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote:

>On Fri, 19 Dec 2008 14:48:21 +0000, nospam <nospam(a)please.invalid>
>wrote:
>
>>Raveninghorde <raveninghorde(a)invalid> wrote:
>>
>>>For the benefit of the OP.
>>
>>>This circuit is not suitable for your application as drawn.
>>
>>The first question is is it acceptable to shut down during a load dump.
>
>Depends on the circuit configuration. Many of my ignition system
>designs protected themselves by turning off appropriate devices
>_during_ the load dump pulse.
>
[snip]

I should have noted here that BVCER (or even better BVCEX) transistor
ratings are generally substantially higher than BVCEO or even BVCBO
standoffs.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
From: Raveninghorde on 19 Dec 2008 13:01
On Fri, 19 Dec 2008 08:32:02 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote:

>On Fri, 19 Dec 2008 14:48:21 +0000, nospam <nospam(a)please.invalid>
>wrote:
>
>>Raveninghorde <raveninghorde(a)invalid> wrote:
>>
>>>For the benefit of the OP.
>>
>>>This circuit is not suitable for your application as drawn.
>>
>>The first question is is it acceptable to shut down during a load dump.
>
>Depends on the circuit configuration. Many of my ignition system
>designs protected themselves by turning off appropriate devices
>_during_ the load dump pulse.
>
>>If
>>it is you can buy LDO regulators with reverse battery and 60v load dump
>>protection built in.
>>
>>I presume 60v is enough for 12v automotive systems else they wouldn't
>>bother trying to sell them?
>
>I haven't looked "under the hood" with a 'scope in years, so I'm not
>familiar with what load-dump controls that might have been added.
>
>But I can assure (scare) you that I ran storage 'scope investigations
>in the '60's that showed load dump pulses as high as 400V :-(
>
>And I have chips turned to beads of silicon to prove it ;-)
>(Inductance-driven arc discharge across the chip.)
>
> ...Jim Thompson

MIL-STD-1275 for 24V vehicles specifies a single fault spike of 250V
for 70us with 100V surges of 50ms. So 251V is allowed to fry the
electronics.
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