Boost Converter / LT SPICE

Hi,

The input DC voltage is in the range of 1.2 volts to 40 volts. I need
3.3 volts output and 300mA. So. I decided to use LT3759.
LT3759's data sheet is at the following link
http://cds.linear.com/docs/Datasheet/3759fa.pdf

Plan is to configure the LT2759 to generate 5 volts @ 300mA and then
use LT 1962 to generate 3.3 Volts at 300mA. 300mA is maximum but load
will work between 150mA to 200mA. Please comment that if this plan
will produce less noisy output?

I tried to simulate LT 3759 in LT SPICE but the probem is that it does
not produce 5 volts for input lower than 16 volts. I am using FET
SI7454DP. I do not know much about SEPIC or boost converters. But I
thought that may be its the FET thing. If I can get a FET that can be
turned ON at lower voltages than I might be able to produce 5 volts
when the input voltage is between the range of 1.6 volts to 5 volts. I
do not understand that why this part can not work when the input
voltage is less than 16 volts. The data sheet says other wise. Any
suggestions?

LT1962 data sheet is as follows
http://cds.linear.com/docs/Datasheet/1962fas.pdf

I am also unable to conclude that how much noise this LT3759 will
introduce into the output and how much noise will LT1962 will take it
out. The data sheet says 20microV rms. So does it mean that the noise
will get lower at the output of the LT1962. Can I measure it using
LTSPICE? or oscilloscope is the best choice?

John

On 12/14/2011 8:05 PM, john1987 wrote:
> Hi,
>
> The input DC voltage is in the range of 1.2 volts to 40 volts. I need
> 3.3 volts output and 300mA. So. I decided to use LT3759.
> LT3759's data sheet is at the following link
> http://cds.linear.com/docs/Datasheet/3759fa.pdf
>
> Plan is to configure the LT2759 to generate 5 volts @ 300mA and then
> use LT 1962 to generate 3.3 Volts at 300mA. 300mA is maximum but load
> will work between 150mA to 200mA. Please comment that if this plan
> will produce less noisy output?
>
> I tried to simulate LT 3759 in LT SPICE but the probem is that it does
> not produce 5 volts for input lower than 16 volts. I am using FET
> SI7454DP. I do not know much about SEPIC or boost converters. But I
> thought that may be its the FET thing. If I can get a FET that can be
> turned ON at lower voltages than I might be able to produce 5 volts
> when the input voltage is between the range of 1.6 volts to 5 volts. I
> do not understand that why this part can not work when the input
> voltage is less than 16 volts. The data sheet says other wise. Any
> suggestions?
>
> LT1962 data sheet is as follows
> http://cds.linear.com/docs/Datasheet/1962fas.pdf
>
> I am also unable to conclude that how much noise this LT3759 will
> introduce into the output and how much noise will LT1962 will take it
> out. The data sheet says 20microV rms. So does it mean that the noise
> will get lower at the output of the LT1962. Can I measure it using
> LTSPICE? or oscilloscope is the best choice?
>
> John
>

1.2 to 40 volts input isn't a specification, it's techie bashing.  What 
sort of system would present such a requirement?

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

On Dec 15, 2:32=A0am, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> On 12/14/2011 8:05 PM, john1987 wrote:
>
> > The input DC voltage is in the range of 1.2 volts to 40 volts. I need
> > 3.3 volts output and 300mA. So. I decided to use LT3759.
> > LT3759's data sheet is at the following link
> >http://cds.linear.com/docs/Datasheet/3759fa.pdf
>
> > Plan is to configure the LT2759 to generate 5 volts @ 300mA and then
> > use LT 1962 to generate 3.3 Volts at 300mA. 300mA is maximum but load
> > will work between 150mA to 200mA. Please comment that if this plan
> > will produce less noisy output?
>
> > I tried to simulate LT 3759 in LT SPICE but the probem is that it does
> > not produce 5 volts for input lower than 16 volts. I am using FET
> > SI7454DP. I do not know much about SEPIC or boost converters. But I
> > thought that may be its the FET thing. If I can get a FET that can be
> > turned ON at lower voltages than I might be able to produce 5 volts
> > when the input voltage is between the range of 1.6 volts to 5 volts. I
> > do not understand that why this part can not work when the input
> > voltage is less than 16 volts. The data sheet says other wise. Any
> > suggestions?
>
> > LT1962 data sheet is as follows
> >http://cds.linear.com/docs/Datasheet/1962fas.pdf
>
> > I am also unable to conclude that how much noise this LT3759 will
> > introduce into the output and how much noise will LT1962 will take it
> > out. The data sheet says 20microV rms. So does it mean that the noise
> > will get lower at the output of the LT1962. Can I measure it using
> > LTSPICE? or oscilloscope is the best choice?
>
> > John
>
> 1.2 to 40 volts input isn't a specification, it's techie bashing. =A0What
> sort of system would present such a requirement?

Pointy-headed manager,or a home-work problem set by a pointy-headed
lecturer.

It could be one of those marketing department "specifications/wish
lists" which basically recycle the stupidest and/or most malicious
customer questions that any of the reps have been asked to respond to.

--
Bill Sloman, Nijmegen

Hi,

1) If I have a FET that can operate at lower voltage range than I
might be able to get the LT3759 to work. My input voltage range is 1.2
Volts to 40 volts. If LT3759 can even work with 2.5 V than I will be
happy. Output voltage is 5 V @ between 200 to 300mA.

 I found the following FET from digikey with Vgs voltage is 2.5 volts
minimum to turn it ON ( Comment needed here).

FET Data sheet link: http://www.fairchildsemi.com/ds/ND/NDS7002A.pdf

Any advice. I am looking for a FET that can be turned ON completely
between the Vgs values of 2 to 3 volts. Vds = 60 Volts.


2)  I do not have ANY  experience with measuring the ripple at the
output of the DC to DC converter and LD0. I am using LT3970. Input
voltage is 9 Volts , output voltage 5 Volts, switching frequency is
600KHz , output current = 300mA. with output capacitor of 22uF. This
output goes to LM1117IMP-3.3.
I used a LM1117IMP-3.3 LDO to generate 3.3 V output with output
capacitor value of 20uF.  My load is drawing 60mA.
Data Sheet linnk for LT3970 and LM1117IMP are as follows

http://cds.linear.com/docs/Datasheet/3970fa.pdf

http://www.ti.com/lit/ds/symlink/lm1117-n.pdf

What I did so far is that I connect the oscilloscope probe at the
output of the DC to DC converter, and the second probe to output of
the LM1117IMP with 200mV / DIV and 5usec / DIV settings for both
channels . I can see that both outputs look like same actually they
are same with some noise and ripples but I am unable to correlate with
the data sheet. For example LT3970 data sheet says at page 1 that the
output ripple will be 5mV peak to peak and page 12 says that with the
capacitor value of 20uF the ripple will be 2mV peak to peak.

LM1117 's data sheet shows at page 8 the graph between ripple
rejection vs current that at current draw less than 200mA the ripple
rejection is higher number @ 20Khz, but LT 3970's switching frequency
s 600KHz.

My hope was to see much cleaner signal at the output of the LDO but I
see same signal at the input of the LDO and at its output.
Oscilloscope probes are 10Mohm inputs and 13pF capacitance. I  might
be doing somethong wrong in measuring the input and outputs.

The other thing is that part LT1962 has 20uV rms output ripple. How
can I compare this with LM 1117IMP's output ripple voltage and how can
see the 20uV rms on the scope?


Any suggestions will be appreciated.

John

john1987 wrote:

> Hi,
> 
> 1) If I have a FET that can operate at lower voltage range than I
> might be able to get the LT3759 to work. My input voltage range is 1.2
> Volts to 40 volts. If LT3759 can even work with 2.5 V than I will be
> happy. Output voltage is 5 V @ between 200 to 300mA.
> 
>  I found the following FET from digikey with Vgs voltage is 2.5 volts
> minimum to turn it ON ( Comment needed here).
> 
> FET Data sheet link: http://www.fairchildsemi.com/ds/ND/NDS7002A.pdf
> 
> Any advice. I am looking for a FET that can be turned ON completely
> between the Vgs values of 2 to 3 volts. Vds = 60 Volts.
> 
> 
> 2)  I do not have ANY  experience with measuring the ripple at the
> output of the DC to DC converter and LD0. I am using LT3970. Input
> voltage is 9 Volts , output voltage 5 Volts, switching frequency is
> 600KHz , output current = 300mA. with output capacitor of 22uF. This
> output goes to LM1117IMP-3.3.
> I used a LM1117IMP-3.3 LDO to generate 3.3 V output with output
> capacitor value of 20uF.  My load is drawing 60mA.
> Data Sheet linnk for LT3970 and LM1117IMP are as follows
> 
> http://cds.linear.com/docs/Datasheet/3970fa.pdf
> 
> http://www.ti.com/lit/ds/symlink/lm1117-n.pdf
> 
> What I did so far is that I connect the oscilloscope probe at the
> output of the DC to DC converter, and the second probe to output of
> the LM1117IMP with 200mV / DIV and 5usec / DIV settings for both
> channels . I can see that both outputs look like same actually they
> are same with some noise and ripples but I am unable to correlate with
> the data sheet. For example LT3970 data sheet says at page 1 that the
> output ripple will be 5mV peak to peak and page 12 says that with the
> capacitor value of 20uF the ripple will be 2mV peak to peak.
> 
> LM1117 's data sheet shows at page 8 the graph between ripple
> rejection vs current that at current draw less than 200mA the ripple
> rejection is higher number @ 20Khz, but LT 3970's switching frequency
> s 600KHz.
> 
> My hope was to see much cleaner signal at the output of the LDO but I
> see same signal at the input of the LDO and at its output.
> Oscilloscope probes are 10Mohm inputs and 13pF capacitance. I  might
> be doing somethong wrong in measuring the input and outputs.
> 
> The other thing is that part LT1962 has 20uV rms output ripple. How
> can I compare this with LM 1117IMP's output ripple voltage and how can
> see the 20uV rms on the scope?
> 
> 
> Any suggestions will be appreciated.
> 
> John
> 
> 
> 
  make yourself a hybrid circuit. a bipolar around the fet. The bipolar
can work nicely at those low voltages.

  Actually, you could most likely do it all with a bipolar..


  Jamie

Hi,

Any recommendation regarding the bipolar part number ! Can I skip FET
altogether. Any comments on measuring noise at the outputs of LDO.
JOHN

Hi,

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

Thanks

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,

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,
> 
> 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