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Lowish DO regulator

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.

Doesn't that take two back to back P-fets due to the parasitic diode?
 
"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
--
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 I love to cook with wine     Sometimes I even put it in the food

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.
 
J

Joerg

Jan 1, 1970
0
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.

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

If I don't use a LDO I designed myself, I just buy from TI. TI seems
to make cheap stuff that works. ...


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*

... 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.


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

Joerg

Jan 1, 1970
0
Jim said:
"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.

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

Joerg

Jan 1, 1970
0
Jim said:
On Sat, 11 Oct 2008 10:20:32 -0700, Joerg

[snip]
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*
[snip]

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

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.
 
K

krw

Jan 1, 1970
0
To-Email- said:
Jim said:
On Sat, 11 Oct 2008 10:20:32 -0700, Joerg

[snip]
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*

[snip]

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

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.

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 :-(

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

MooseFET

Jan 1, 1970
0
Doesn't that take two back to back P-fets due to the parasitic diode?

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

Robert Monsen

Jan 1, 1970
0
I have a 12V lead acid battery or maybe someone will connect this to a
voltage up to 35V. I need to make 8.3V. Some moron may connect the
power backwards.

Currently I am using a circuit with a P channel MOSFET to do the
reverse protection and an LM78XX type regulator. Does someone know of
a 78XX like regulator with a lower overhead number.

The load current is about 200mA max. I have a large heat sink /
chassis available.


BTW: Did you notice that in the debates nobody ever mentions the need
for better voltage regulators and op-amps. They aren't dealling with
any of the issues that really matter. :)

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
 
M

MooseFET

Jan 1, 1970
0
[.. 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.
 
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