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Well maybe, but I've seen home brew switchers fail in the field, or worse yet, do funny things like whack the battery when starting up. Dell USED to design their own switchers. Note the emphasis on used.

If I don't use a LDO I designed myself, I just buy from TI. TI seems to make cheap stuff that works. It's like National, but with quality. Granted, stuff for your personal use is different than making a product to sell.

I would need a damn good reason to result to using a switcher. Now I do agree that 5W is really too much for a LDO. If the original poster kept it reasonable (say 16V), I'd go for the LDO.

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Note the wording on the 35V requirement "or maybe someone will connect it". The requirement isn't unreasonable because it is a real problem in the intended product. I am very willing to do the needed heat sinking and / or have a thermal shutdown trip to handle the case. I was hoping that someone knew of a regulator that would have a lower drop out than the LM78XX or LM240-XX types of regulators. My current design has to have a P-MOSFET reverse protection circuit because I can't stand the drop of a Schottky. I was hoping to make a simpler design with a different chip.

"Home brew" switchers used to be all that there was. I designed all the off-line and DC-DC switchers for GenRad Portable Products (Omnicomp) from 1977-1987. Never had one fail. And no I/C more complex than an LM339.

...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

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=A0 =A0 ...Jim Thompson

=A0 | =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

I don't know if you shop the electronics flea markets, but most people won't touch an old switcher. They usually end up on the surplus market because they failed. The old linears show up on the surplus market because they are inefficient. In fact new linears show up surplus since somebody deemed the system needs to be more efficient. I don't blame them, i.e. this isn't just marketing. When I build PCs, I go for the greener power supplies, the so-called +80.

One has to know this stuff very well. Or hire a consultant who does ;-)

Could tell you a war story there, too. TPS-something, can look it up if you want to. Turned out it "did not like it" when the input voltage came up too fast. Phsssst ... *BANG*

With the current regulatory trend towards minimum efficiencies and standby power that picture is changing, and fast. Better go with the times.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Wasn't that where your boss came in, rocked the power switch back and forth and it exploded?

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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I would presume one of the PFET's is in the LDO??

See, for example...

...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

That wasn't my boss, it was Ed Greenwood (*) the digital guru for Omnicomp. And I fixed that situation so it could never happen again. (It was the prototype BTW, not production yet.)

(*) Deceased now for some ~12 years, due to liver cancer, from drinking 2-liters per day of a famous "diet" cola. (He weighed 330+ pounds.)

...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

[snip] [snip]

Don't you just hate rate-dependent POR's? My exquisite POR designs aren't ;-)

...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

| 1962 | I love to cook with wine Sometimes I even put it in the food

I don't know what's in there and that is what really irked me. Neither did they want to throw my (simple) circuit on SPICE nor were they willing to release the model, not even on NDA. That's the recipe to get things designed out of the marketplace.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Some of the Spice models the manufacturers disseminate aren't worth the powder to blow 'em to hell... behavioral nightmares :-(

My favorite is the OPA861... you can get hundreds of volts of output, into 50 Ohms, from a 5V supply :-(

...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

Hmm, I can't even get the ideal op-amps to do that (why do I have to hook supplies up to them?).

--
Keith

On Oct 11, 2:04=A0pm, Jim Thompson wrote: [....]

The protection circuit has a resistor and a zener to limit the gate voltage on the MOSFET.

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The protection is just one P-MOSFET. The regulator is a simple LM78XX circuit.

Here ya go:

Version 4 SHEET 1 1244 680 WIRE 96 -144 -16 -144 WIRE 208 -144 96 -144 WIRE 336 -144 208 -144 WIRE 464 -144 432 -144 WIRE 496 -144 464 -144 WIRE 608 -144 576 -144 WIRE 720 -144 608 -144 WIRE 96 -112 96 -144 WIRE 208 -112 208 -144 WIRE 464 -64 464 -144 WIRE 720 -48 720 -144 WIRE -16 -32 -16 -144 WIRE 96 -32 96 -48 WIRE 384 0 384 -80 WIRE 416 0 384 0 WIRE 608 0 608 -144 WIRE 608 0 512 0 WIRE 208 16 208 -32 WIRE 96 64 96 32 WIRE 144 64 96 64 WIRE 96 112 96 64 WIRE 384 112 384 0 WIRE 384 112 208 112 WIRE 608 112 608 0 WIRE -16 128 -16 32 WIRE 384 128 384 112 WIRE 560 160 416 160 WIRE -16 256 -16 208 WIRE 96 256 96 192 WIRE 96 256 -16 256 WIRE 384 256 384 192 WIRE 384 256 96 256 WIRE 608 256 608 192 WIRE 608 256 384 256 WIRE 720 256 720 32 WIRE 720 256 608 256 FLAG -16 256 0 SYMBOL voltage -16 112 R0 WINDOW 123 24 132 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(35 8 0 1 1 100 1k) SYMATTR Value2 AC 1 SYMBOL schottky 0 32 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value MBRS140 SYMATTR Description Diode SYMATTR Type diode SYMBOL current 720 -48 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 116 Left 0 SYMATTR InstName I1 SYMATTR Value 200m SYMATTR SpiceLine load SYMBOL tl431 400 128 M0 SYMATTR InstName U1 SYMBOL potentiometer 624 96 M0 SYMATTR InstName U2 SYMATTR Value Rtot=10K wiper=.3 SYMBOL pnp 144 112 M180 SYMATTR InstName Q2 SYMATTR Value 2N3906 SYMBOL diode 80 -112 R0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 80 -32 R0 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL res 192 -128 R0 SYMATTR InstName R1 SYMATTR Value 82 SYMBOL res 80 96 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL npn 336 -80 R270 SYMATTR InstName Q1 SYMATTR Value 2N3055 SYMBOL npn 416 -64 M90 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL res 592 -160 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 1 TEXT -110 312 Left 0 !.dc V1 35 9 -.1

[.. specs ...]

It isn't a bad regulator but the parts count is higher than my current solution. I need 3 parts for the inverted battery protection but the LM78XX cuts the count on the regulator way down.

BTW: You can improve your solution if you move the current limiting to the collector of the 3055. On over current you cut off the base drive current source.