"josephkk"  wrote in message=20
news:u8vve7dnp8a26rtgrn9gpjek44croe2p86@4ax.com...

> Try using the FET from page 27 of the datasheeet, or maybe the FET
> page 28.  The one page 28 will probably give a 2.4V minimum though.

The LT3759 is only rated to work down to 1.6V. The OP's problem is not =
the=20
MOSFET. I provided an ASCII file with a MOSFET that proves the =
simulation=20
will work down to 1.6 volts. For the 1.2 volt input required by the OP, =
a=20
boost of at least 0.4 volts is needed.

The LT1308A/B will work down to 1.2 V, but the maximum input is 10V. But =

since there is no specification for efficiency, it should work by adding =
an=20
8 volt LDO regulator ahead of the IC. The only problem is that it will =
have=20
to dissipate at least 300mA * 32V or almost 10 watts for 40 volts input. =
Or=20
you could have a voltage sensing relay switch the input from the LT1308 =
to=20
the LT3759 when the voltage exceeds the 10V limit of the 1308.

But the big question is why this application needs such a wide input =
range.

Paul=20

On Sun, 18 Dec 2011 17:49:19 -0800 (PST), john1987
<conphiloso@hotmail.com> wrote:

>Hi,
>
>I am ready to use any good idea. I wanted to test the Linear
>technology claim abt this chip LT3759. I am trying to simulate in LT
>spice. I not know abt d basic inverter circuit. Any help will be
>appreciated> what do you think abt the switcher IC. If I can get the
>right MOSFET, will it work?
>
>john

Try using the FET from page 27 of the datasheeet, or maybe the FET page
28.  The one page 28 will probably give a 2.4V minimum though.

?-)

"john1987"  wrote in message=20
news:b935969f-1c03-4083-bebb-418b90acdb45@u5g2000vbd.googlegroups.com...

> Its not a home work or bar bet, I assure you. An assignment for a
> junior engineer. I do not know how to open your net list in LT spice.
> Any pointers? I need 5 volts with atleast 200mA output with less
> niose as possible. Can you also comment on my questions
> regarding how to measure ripple voltages and noise reduction
> techniques in my OP.

The net list is actually an ASCII file which is the usual default for=20
LTSpice designs. Just create a text file which you may name John3759.asc =
or=20
whatever you wish, and copy/paste the text and save the file. Then use=20
LTSpice to browse to the file and open it. It is almost identical to the =

supplied 3759.asc which comes with LTSpice in the "jigs" folder, and you =

should know how to open them and run simulations. If you are new to =
LTSpice=20
I suggest you take the time to try some of the many tutorials that are=20
available so you become comfortable with the basics. I am by no means an =

expert but I've used the program successfully for many years.

Once you run a simulation (in transient mode) you need to monitor the=20
currents and voltages you are interested in. Then you will see how the=20
circuit starts up and the waveforms over time. To measure ripple =
voltages=20
you just need to select a portion of the plot where the output has=20
stabilized, and then use Ctrl-Click to see the average and RMS value of =
that=20
portion. You can zoom in and use the cursors to read the high and low=20
excursions of the ripple and divide that by the average to get P-P =
ripple as=20
a percentage. You can probably use the math functions of the plot points =
to=20
do this automatically.

I am still a little confused about your needs. I see that you want the =
3759=20
to produce 5.0 VDC and then use an LT1962 to produce 3.3 volts. Do you =
need=20
both voltages for this design? And I still question your need for such a =

wide range of input voltage. And I just saw that the 3759 is rated for =
1.6=20
to 42 volts input, so you will need to boost your 1.2V to at least 1.6V=20
before it has a chance of working. That can be done, and I have made =
some=20
"Joule Thief" circuits using a BJT which produce enough voltage for a =
white=20
LED from as low as 0.8V. If you are really serious about this you can =
search=20
for those, or I can post the LTSpice ASCII text if you can learn how =
that=20
works. Then the rest of the design is up to you, unless you wish to =
contract=20
my services.

We are here to help, but mostly to help you learn how to approach design =

problems. Good luck.

Paul=20

Hi,

Its not a home work or bar bet, I assure you. An assignment for a
junior engineer. I do not know how to open your net list in LT spice.
Any pointers? I need 5 volts with atleast 200mA output with less niose
as possible. Can you also comment on my questions regarding how to
measure ripple voltages and noise reduction techniques in my OP.
Thanks

john

"john1987"  wrote in message=20
news:337f4951-58ff-4c77-8066-d6be2fc1c60e@z19g2000vbe.googlegroups.com...=


> I am ready to use any good idea. I wanted to test the Linear
> technology claim abt this chip LT3759. I am trying to simulate
> in LT spice. I not know abt d basic inverter circuit. Any help
> will be appreciated

> what do you think abt the switcher IC. If I can get the
> right MOSFET, will it work?

I just downloaded the latest from Linear and used their 3759 jig. I =
changed=20
the output resistors for 3.3 volts and I chose a MOSFET that has a Von =
of 1=20
Volt (IRLML6244). I also changed the resistors for the EM/UVLO but it =
was=20
probably not needed. However, I was only able to get it to work down to =
1.6=20
V. If you need less than that, you will have to try some other tricks. =
It=20
certainly seems possible.

But somehow I think this may be either homework or a bar bet. I'll make =
it=20
work if you split your gold star or your winnings with me. I accept =
PayPal!

Circuit follows.

Paul

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Version 4
SHEET 1 1172 680
WIRE -96 -96 -176 -96
WIRE 240 -96 -96 -96
WIRE 320 -96 240 -96
WIRE 560 -96 400 -96
WIRE 576 -96 560 -96
WIRE 672 -96 640 -96
WIRE 752 -96 672 -96
WIRE 848 -96 816 -96
WIRE 912 -96 848 -96
WIRE 992 -96 912 -96
WIRE 1072 -96 992 -96
WIRE -176 -80 -176 -96
WIRE 672 -80 672 -96
WIRE 912 -80 912 -96
WIRE 992 -80 992 -96
WIRE 1072 -80 1072 -96
WIRE -96 -64 -96 -96
WIRE 416 -48 0 -48
WIRE 240 -32 240 -96
WIRE -176 16 -176 0
WIRE 672 16 672 0
WIRE 912 16 912 -16
WIRE 992 16 992 -16
WIRE 1072 16 1072 0
WIRE -96 32 -96 16
WIRE 80 32 -96 32
WIRE 416 32 416 -48
WIRE 416 32 400 32
WIRE 432 32 416 32
WIRE 512 32 496 32
WIRE -96 48 -96 32
WIRE 0 48 0 -48
WIRE 80 128 0 128
WIRE 848 128 848 -96
WIRE 848 128 400 128
WIRE -96 144 -96 128
WIRE 560 144 560 -96
WIRE 0 224 -32 224
WIRE 80 224 64 224
WIRE 512 224 400 224
WIRE 560 256 560 240
WIRE 560 256 432 256
WIRE 560 272 560 256
WIRE 848 288 848 128
WIRE -16 320 -32 320
WIRE 80 320 64 320
WIRE 432 320 432 256
WIRE 432 320 400 320
WIRE 560 368 560 352
WIRE -16 416 -32 416
WIRE 80 416 48 416
WIRE 848 416 848 368
WIRE 848 416 400 416
WIRE 848 432 848 416
WIRE 240 496 240 480
WIRE 848 528 848 512
FLAG 240 496 0
FLAG -32 416 0
FLAG -32 320 0
FLAG -32 224 0
FLAG 512 32 0
FLAG -176 16 0
FLAG -96 144 0
FLAG -176 -96 IN
FLAG 560 368 0
FLAG 672 16 0
FLAG 848 528 0
FLAG 912 16 0
FLAG 992 16 0
FLAG 1072 16 0
FLAG 1072 -96 OUT
SYMBOL PowerProducts\\LT3759 240 224 R0
SYMATTR InstName U1
SYMBOL cap 48 400 R90
WINDOW 39 53 32 VTop 2
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR SpiceLine Rser=3D7.5K
SYMATTR InstName C1
SYMATTR Value 22n
SYMBOL res 80 304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 41.2K
SYMBOL cap 64 208 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value .05=B5
SYMBOL voltage -176 -96 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 1.6
SYMBOL cap 432 48 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C3
SYMATTR Value 4.7=B5
SYMBOL res -112 -80 R0
SYMATTR InstName R2
SYMATTR Value 20k
SYMBOL res -112 32 R0
SYMATTR InstName R3
SYMATTR Value 118K
SYMBOL res -16 32 R0
SYMATTR InstName R4
SYMATTR Value 100K
SYMBOL ind2 416 -80 M270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L1
SYMATTR Value 15=B5
SYMATTR Type ind
SYMATTR SpiceLine Rser=3D50m
SYMBOL ind2 656 -96 R0
SYMATTR InstName L2
SYMATTR Value 15=B5
SYMATTR Type ind
SYMATTR SpiceLine Rser=3D50m
SYMBOL nmos 512 144 R0
WINDOW 0 56 5 Left 2
WINDOW 3 60 79 Left 2
SYMATTR InstName Q1
SYMATTR Value IRLML6244
SYMBOL res 544 256 R0
SYMATTR InstName R5
SYMATTR Value 5m
SYMBOL cap 576 -80 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C4
SYMATTR Value 4.7=B5
SYMBOL schottky 752 -80 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value MBRS360
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 832 272 R0
SYMATTR InstName R6
SYMATTR Value 17.5k
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SYMATTR Value 15.8K
SYMBOL polcap 896 -80 R0
WINDOW 3 25 53 Left 2
WINDOW 123 26 78 Left 2
SYMATTR Value 47=B5
SYMATTR Value2 x4
SYMATTR InstName C5
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=3D20 Irms=3D0 Rser=3D0.15 Lser=3D0 mfg=3D"KEMET"=20
pn=3D"T495X476M020AS" type=3D"Tantalum"
SYMBOL cap 976 -80 R0
SYMATTR InstName C6
SYMATTR Value 10=B5
SYMATTR SpiceLine V=3D10 Irms=3D8.919 Rser=3D0.005 Lser=3D0 =
mfg=3D"KEMET"=20
pn=3D"C1210C106K8PAC" type=3D"X5R"
SYMBOL res 1056 -96 R0
SYMATTR InstName Rload
SYMATTR Value 12
TEXT 496 -104 Bottom 2 !K1 L1 L2 1
TEXT 912 200 Left 2 !.tran 4m startup=20

Hi,

I am ready to use any good idea. I wanted to test the Linear
technology claim abt this chip LT3759. I am trying to simulate in LT
spice. I not know abt d basic inverter circuit. Any help will be
appreciated> what do you think abt the switcher IC. If I can get the
right MOSFET, will it work?

john

john1987 wrote:

> Hi,
> 
> How about just use a BJT and get rid of MOSFET alltogether. Can you
> give me some schematic example or some application note.
> 
> 
> John

  Is there is reason you must use a switcher IC ? I mean if memory
serves, you're trying to get 5V@300ma from 1..40 volt input?
   I was thinking of d basic inverter circuit with feed back regulation
to throttle back the input side.


  Jamie

Hi,

How about just use a BJT and get rid of MOSFET alltogether. Can you
give me some schematic example or some application note.


John

john1987 wrote:
> Hi,
> 
> Would you explain briefly that what exactly did you mean by hybrid?
> 
> Thanks
> 
> john
  You need to put in some context of prior messages so that us old
folks can remember what the thread was about !

   When I said hybrid I meant using a combination of a bipolar
transistor with a power mosfet..

    If that circuit you are dealing with can in deed operate at a
much lower level than needed drive for a mosfet, you can couple
a NPN transistor around MOSFET circuit for it to perform the same
work at the lower levels.

   The bipolar will start conducting < 1 volts between Base and emitter.

   When the operating drive point gets to ta level where the mosfet gate
can turn on, then the bipolar will just sit idle around it.

   You would need to study the circuit that is doing the driving to see 
if you can incorporate this into the circuit. I am sure you can.

  Jamie

Hi,

Would you explain briefly that what exactly did you mean by hybrid?

Thanks

john