One of the wonderful things about SMD stuff is trying to proto type it, and
gets even more fun when it involves RF, feedback loops, good ground planes
etc...
So, I have 'bread boarded' up the LTC1934 step down regulator to give it a
go.. Twice..
I have to admit that both times it is not the best layout as can only do so
much with vero board, so grounding could be the problem..
What I am trying to achieve is a buck regulator that can operated with Vout
of 3.6V @ 60 - 150mA range in normal operation, and around 5mA in 'sleep'
mode.
It has to do this with an input voltage of up to 30Volts and lower than 6V.
To add to this the main circuit has a GSM modem on it, which when it TXs has
some nice 2A current spikes - so I have to have 1000uf x2 low ESR electro
caps to keep it happy..
Well, it does work but only with an input voltage of 6 to 12Volts.
efficiency at 60ma is horrible, around 20%, and at 100ma only slightly less
horrible at 30%.. Was expecting at least 50% given poor layout and not the
preferred schotky diodes..
Next problem is when I get to above 12V the switch frequency starts
'missing' pulses, during this time the output will vary like a large square
wave from 3.6V to 4V - getting worse the higher I take the input...
Has anyone out there had experience with this chip, or had behaviour like
this?
Or am I asking to much from it?
I don't want to waste A$600+ dollars getting a prototype 4 layer board done
if I find it still does the same!!
Any help is appreciated - losing enough hair as it is!
> Hi All,
> I am having some odd behaviour happening from this device, but it
> might just be me :)
> SMPS is not exactly my preferred area of engineering ;)
>
This has happened before with me ;)
> One of the wonderful things about SMD stuff is trying to proto type
> it, and gets even more fun when it involves RF, feedback loops, good
> ground planes etc...
They sell SMT prototyping boards I believe :) {Quote hidden}
>
> So, I have 'bread boarded' up the LTC1934 step down regulator to give
> it a go.. Twice..
> I have to admit that both times it is not the best layout as can only
> do so much with vero board, so grounding could be the problem..
> What I am trying to achieve is a buck regulator that can operated
> with Vout of 3.6V @ 60 - 150mA range in normal operation, and
> around 5mA in 'sleep' mode.
> It has to do this with an input voltage of up to 30Volts and lower
> than 6V.
Hmmm... I think you should check the variant of the 1934 you are using
the 1034-1 is only rated for 60ma. The normal one is rated for 250ma.
>
> To add to this the main circuit has a GSM modem on it, which when it
> TXs has some nice 2A current spikes - so I have to have 1000uf x2
> low ESR electro caps to keep it happy..
Hmmmm this is a problem you might need a device that can handle 500ma
or more.
>
> Well, it does work but only with an input voltage of 6 to 12Volts.
> efficiency at 60ma is horrible, around 20%, and at 100ma only
> slightly less horrible at 30%.. Was expecting at least 50% given poor
> layout and not the preferred schotky diodes..
Check your inductor it needs to be the RIGHT one. Check page 7 and
examine the thing saying 'Inductor Selection'. I suggest using an
inductor with lower DCR than there 0.3 ohm. The lower the better. This
sounds like the inductor is acting as a power resistor. Is it getting
hot?
>
> Next problem is when I get to above 12V the switch frequency starts
> 'missing' pulses, during this time the output will vary like a large
> square wave from 3.6V to 4V - getting worse the higher I take the
> input...
No it's NOT missing pulses at all, try page 6 of the data sheet,
entitle 'Operation'
"The LT1934 uses Burst Mode control, combining both low quiescent
current operation and high switching frequency, which result in high
efficiency across a wide range of load currents and a small total
circuit size." <-- straight from the data sheet.
"A comparator monitors the voltage at the FB pin of the LT1934. If this
voltage is higher than the internal 1.25V reference, the comparator
disables the oscillator and power switch."
So basically it stops sending pulses when it doesn't need to send them.
>
> Has anyone out there had experience with this chip, or had behaviour
> like this? Or am I asking to much from it?
I think you need to analyze your power needs more carefully expecting a
250ma regulator to charge a cap after it being hit by a 2A current
surge is a little bit on the hanging by a thread situations. I suggest
a small inductor to go between the bulk decoupling cap for the modem so
it doesn't kill the power supply.
>
> I don't want to waste A$600+ dollars getting a prototype 4 layer
> board done if I find it still does the same!!
>
> Any help is appreciated - losing enough hair as it is!
>
> Cheers,
> Peter Mcalpine
>
I've lost most of mine so I no longer worry about these things :)
However my suggestions are the following:
1) Be sure you are using the 1934 and not the 1934-1
2) Check your inductor's sizing and DCR so it's not acting like a
resistor in your circuit. (Page 7 of the data sheet)
3) Check all your components and go through the design proceedure laid
out in the data sheet. You should be taking into account you are
abusing this regulator with the GSM modem. Either give the GMS it's
own regulator or find a way you aren't abusing the power supply. It
should be getting REALLY hot with all that you are doing to it. You may
need two 1934's or a second higher current switching regulator that you
can shut off when not using the GSM.
=====
Stephen R. Phillips was here
Please be advised what was said my be absolutely wrong, and hereby this disclaimer follows. I reserve the right to be wrong and admit it as well. :)
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At 09:48 PM 2/21/2005, .....petemca1KILLspam@spam@bigpond.net.au wrote:
>Hi All,
>I am having some odd behaviour happening from this device, but it might just
>be me :)
>SMPS is not exactly my preferred area of engineering ;)
>
>Well, it does work but only with an input voltage of 6 to 12Volts.
>efficiency at 60ma is horrible, around 20%, and at 100ma only slightly less
>horrible at 30%.. Was expecting at least 50% given poor layout and not the
>preferred schotky diodes..
What diodes are you using? If they are garden variety 1n400x series, you
need to change them.
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For the schotky I am using a 1n5819 diode, which should be ok. Final design
is using a MBR0540
These problems I am experiencing is prior to adding the GSM to it, so only a
micro and a GPS receiver.. But with the GSM attached and transmitting the
current spikes are not appearing back at the regulator output so the 2000uf
of low esr caps seem to be doing the business - but will take on board the
inductor in series and see how that goes..
Inductor is the one suggested on the first page 47uh Sumida..
And not getting hot at all so think it is all fine. Will buy some more
samples to make sure it is the correct version - but everything points to it
being ok..
Biting the bullet and will get a proper PCB made - to hard to predict what
will happen unless I do it on the final 4 layer board. What works for my
dodgy layout may not work on the real thing...
bigpond.net.au wrote:
