Synchronising two voltage regulators

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
So, I have a power headache :)  

One of the chips I want to use in a project has very specific power require
ments ...

 - The VDD supply must ramp from 0V to its final value within 10ms to ensur
e correct startup.
 - The IO VDD supply must ramp to its final value before VDD reaches 0.4 V.
 - The power supplies must be brought up monotonically
 - (other stuff that's not so relevant)

... and at the same time, I need a fair amount of power (in this case about
 5A on the 3.3v supply and about 2A on the 1V supply to cover the worst-cas
e scenarios). As far as I'm aware, there isn't a single chip solution that  
can handle this, so I'm trying to figure out how to guarantee all the above
.

One way I came up with was to sample the 3.3v and 1v lines via an AVR chip'
s built-in ADC's. There's already an AVR on-board so it doesn't need any mo
re components, just a bit of wiring. Once the voltages have stabilised to t
heir correct final values, I would just have the AVR allow the supply of vo
ltage to the rest of the board.

The question is how to do that ? I thought of using solid-state relays, but
 looking at the ones at digikey that can handle the power, the variance in  
on-time switching is such that I'm not sure the monotonicity (which I'm int
erpreting to mean 'at the same time') would be preserved.

So, a vague thought about MOSFETs surfaced - thing is I've never used them,
 hence this long plea for help :) A few questions:

 - Can I just connect the source to v-reg output, drain to the rest-of-the-
world, and gate to the AVR to switch 5A ?

 - Does a MOSFET care about the voltage it's switching ? In other words, wi
ll it work if Vds = 1v ? In the datasheets, I only see maxima specified,  
so I'm assuming there is no minimum value but it's worth asking :)

 - Any recommendations on which one to use ?  

 - Is there a better way to do this that I'm missing ? :)

Cheers
   Simon.


Re: Synchronising two voltage regulators
On Sun, 21 Oct 2012 09:03:56 -0700, Simon wrote:

Quoted text here. Click to load it

Are there any app notes from the manufacturer on how to do this?

If you're going to do the power supply sequencing with an AVR, and you  
turn on "the rest of the board" last, and the AVR is on "the rest of the  
board", will it work?

The MOSFET does not care about minimum voltages.  It does care about gate-
source voltage (it needs enough).  The rest of your circuit cares about  
voltage drops.  So you may have a challenge finding FETs with low enough  
drain-source resistance for you, at the gate-source voltages that you can  
achieve.

--  
Tim Wescott
Control system and signal processing consulting
We've slightly trimmed the long signature. Click to see the full one.
Re: Synchronising two voltage regulators
On Sunday, October 21, 2012 9:30:17 AM UTC-7, Tim Wescott wrote:
Quoted text here. Click to load it

Not at this power level. Their (XMOS) reference design uses an LTC3417 which
supplies 1.5A/1.5A. This is actually underpowered (by my reckoning) for the chip
they're using anyway, but its possible it's fine in the specific case of that
design.  

I'm trying to power a video decoder, a JPEG2k encoder, the XMOS chip, an AVR and
a USB fifo, as well as the ancillary support components and my power budget is a
little higher.
  
Quoted text here. Click to load it

[grin]. No, the AVR has its own 5v voltage regulator and is sufficiently
isolated from the rest of the board to work :)

Quoted text here. Click to load it

Hmm. All I'm seeing in the datasheets (eg:
http://www.vishay.com/docs/69940/si2305ad.pdf )  are limits, eg:

Parameter                               Symbol            Limit          
Drain-Source voltage                 Vds                -8v
Gate-Source voltage                  Vgs                +/-8v
Gate-Source threshold voltage  Vgs(th)           -0.45v < typical < -0.8v

Is there something I ought to be looking for that would let me derive whether a
particular MOSFET was suitable ?

Or, does anyone have a 'Noddies guide' to how to start using these things ? :)

Cheers
    Simon

Re: Synchronising two voltage regulators
On Sun, 21 Oct 2012 11:06:14 -0700, Simon wrote:

Quoted text here. Click to load it

In your case the two most important parameters are the Rds(ON), which is  
the effective resistance of the part at the specified gate-source  
voltage, and the power dissipation.

The Rds(ON) affects both the voltage drop across the part (which affects  
the quality of your power supply), and the power dissipated in the part.

If you're a beginner, be very careful of the current and power  
dissipation ratings: they're probably based on some unrealistic amounts  
of heat sinking, and possibly unrealistic ambient temperatures.  You  
probably need to do the thermal calculations yourself.

That P-channel part will probably work for your 3.3V supply, but it's not  
rated for a 1V g-s voltage, which is what you'd have available pulling  
the gate low with a pin on the processor.  For the 1-V supply, if you  
connect an N-channel part as a source follower, then when you raise the  
gate to +5V you'll have a 4V g-s voltage, which would be plenty in a  
comparable N-channel part.

If you have a supply that goes above 5V, you might want to consider using  
N-channel parts for both FETs.  Connect the 1V switch to a processor pin  
through an RC per Vladimir's suggestion, and connect the 3.3V switch  
through a level shifter.

Better yet -- do you have to control the 3.3V supply at all?  Reading  
your constraints, I don't see anything that says the 3.3V supply can't be  
on for a day before you switch on the 1V: am I getting it right?

--  
Tim Wescott
Control system and signal processing consulting
We've slightly trimmed the long signature. Click to see the full one.
Re: Synchronising two voltage regulators
On Sunday, October 21, 2012 11:29:48 AM UTC-7, Tim Wescott wrote:
Quoted text here. Click to load it
  
Quoted text here. Click to load it

In fact, I think you may be right. I was interpreting 'monotonically' incor
rectly to be 'at one time' rather than 'always increasing', I think. I'm go
ing to raise a support ticket with XMOS to be sure, but on a different chip
 they have a FAQ saying that VDDIO must be fully operational before VDD is  
powered, which is a far cry from the tight specifications I had in mind abo
ve.  

In which case, the whole MOSFET thing is not required - I can power up both
 5v and 3.3v supplies at boot and once the AVR reads 3.3v is available, it  
can enable the 1v regulator via an enable-pin. Job done.

Still, it was good to find out about MOSFETs a little, thanks all :)

Simon.

Re: Synchronising two voltage regulators


Quoted text here. Click to load it
 > The VDD supply must ramp from 0V to its final value within 10ms to ensure  
correct startup.
 >The IO VDD supply must ramp to its final value before VDD reaches 0.4 V.
 ->The power supplies must be brought up monotonically
 >(other stuff that's not so relevant)
Quoted text here. Click to load it

[...]

Quoted text here. Click to load it

You can use MOSFETs to turn  the voltages on. There are many FETs with low  
Rds and logic level gate drive voltage; check with IRF, Siliconix, NXP and  
other usual providers. To avoid problems with huge inrush current, slow down  
the gate drive by RC circuit.

Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com




Re: Synchronising two voltage regulators
Quoted text here. Click to load it
rements ...
nsure correct startup.
Quoted text here. Click to load it
4 V.
ut 5A on the 3.3v supply and about 2A on the 1V supply to cover the worst-c
ase scenarios). As far as I'm aware, there isn't a single chip solution tha
t can handle this, so I'm trying to figure out how to guarantee all the abo
ve.
Quoted text here. Click to load it
p's built-in ADC's. There's already an AVR on-board so it doesn't need any  
more components, just a bit of wiring. Once the voltages have stabilised to
 their correct final values, I would just have the AVR allow the supply of  
voltage to the rest of the board.
Quoted text here. Click to load it
ut looking at the ones at digikey that can handle the power, the variance i
n on-time switching is such that I'm not sure the monotonicity (which I'm i
nterpreting to mean 'at the same time') would be preserved.
Quoted text here. Click to load it
m, hence this long plea for help :) A few questions:
Quoted text here. Click to load it
the-world, and gate to the AVR to switch 5A ?
Quoted text here. Click to load it
, will it work if Vds = 1v ? In the datasheets, I only see maxima specifi
ed, so I'm assuming there is no minimum value but it's worth asking :)
Quoted text here. Click to load it

use regulators with enable and powergood, use powergood on IOVDD to
enable VDD ?

-Lasse

Re: Synchronising two voltage regulators
On Sunday, October 21, 2012 11:22:10 AM UTC-7, snipped-for-privacy@fonz.dk wrote:
Quoted text here. Click to load it

Yeah, I thought of that, but I think it might not match the 'monotonicity'
clause. I don't know how much slack I have between IOVDD and VDD being brought
up, but if they're stating it as a design requirement, I'm assuming it's not
much...

Simon



Re: Synchronising two voltage regulators
On 21/10/12 18:03, Simon wrote:
Quoted text here. Click to load it


You need a fair amount of power - but that doesn't necessarily imply  
that you need maximal efficiency.

A simple way to ensure that all your power supplies rise and fall  
together is to have one switch-mode (or linear, if you want) power  
supply for the highest voltage supply.  All your lower supplies are  
generated by LDO linear regulators from that supply.  When your 3.3V  
supply rises, your 1V supply will follow it within perhaps 0.2V until it  
hits 1V.

For safe power-off, put a schottkey going from each lower voltage supply  
up to the next higher voltage supply - then you will never get the 3.3V  
line more than about 0.1V below the 1V line (if you skip this, you can  
get odd effects if the capacitance on the lower line holds 1V longer  
than the capacitance on the 3.3V line).



Re: Synchronising two voltage regulators
On 10/21/2012 12:03 PM, Simon wrote:
Quoted text here. Click to load it
requirements ...
Quoted text here. Click to load it
correct startup.
Quoted text here. Click to load it
5A on the 3.3v supply and about 2A on the 1V supply to cover the worst-case
scenarios). As far as I'm aware, there isn't a single chip solution that can
handle this, so I'm trying to figure out how to guarantee all the above..
Quoted text here. Click to load it
built-in ADC's. There's already an AVR on-board so it doesn't need any more
components, just a bit of wiring. Once the voltages have stabilised to their
correct final values, I would just have the AVR allow the supply of voltage to
the rest of the board.
Quoted text here. Click to load it
looking at the ones at digikey that can handle the power, the variance in
on-time switching is such that I'm not sure the monotonicity (which I'm
interpreting to mean 'at the same time') would be preserved.
Quoted text here. Click to load it
hence this long plea for help :) A few questions:
Quoted text here. Click to load it
rest-of-the-world, and gate to the AVR to switch 5A ?
Quoted text here. Click to load it
it work if Vds = 1v ? In the datasheets, I only see maxima specified, so I'm
assuming there is no minimum value but it's worth asking :)
Quoted text here. Click to load it


Monotonic means that the voltages ramp up without reversing.  It has  
nothing to do with the synchronization.  Each supply has to increase  
voltage to the final value without the voltage lowering at any point.

The rest of your problem remains.

Rick

Re: Synchronising two voltage regulators
On Sun, 21 Oct 2012 09:03:56 -0700, Simon wrote:

Quoted text here. Click to load it
requirements ...
ensure correct startup.
Quoted text here. Click to load it
0.4 V.
about 5A on the 3.3v supply and about 2A on the 1V supply to cover the  
worst-case scenarios). As far as I'm aware, there isn't a single chip  
solution that can handle this, so I'm trying to figure out how to  
guarantee all the above.
Quoted text here. Click to load it
chip's built-in ADC's. There's already an AVR on-board so it doesn't need  
any more components, just a bit of wiring. Once the voltages have  
stabilised to their correct final values, I would just have the AVR allow  
the supply of voltage to the rest of the board.
Quoted text here. Click to load it
but looking at the ones at digikey that can handle the power, the  
variance in on-time switching is such that I'm not sure the monotonicity  
(which I'm interpreting to mean 'at the same time') would be preserved.
Quoted text here. Click to load it
them, hence this long plea for help :) A few questions:
Quoted text here. Click to load it
the-world, and gate to the AVR to switch 5A ?
Quoted text here. Click to load it
words, will it work if Vds = 1v ? In the datasheets, I only see maxima  
specified, so I'm assuming there is no minimum value but it's worth  
asking :)
Quoted text here. Click to load it


I did something like that in a recent design.

In my case, I used multiple LTM4614 DC/DC converters.  These have open  
drain power power_good outputs and track inputs.  The internal Vref is  
0.8V and it will use the track input instead if it is < 0.8V, switching  
to the fixed Vref once the track input exceeds 0.8V.  This allows direct  
control over the output voltage.

The "synchronisation" (usually called "sequencing" in the context of  
power supply design) was achieved by connecting the power good output of  
one stage to the track input of the next.  This would hold the output of  
one DC/DC converter at 0V until the previous one reached about 90% of its  
final value.  An RC network on the track input made the output voltage  
rise at a predictable, monotonic rate.

The LTM4614 isn't cheap, but it did what I wanted.  (I was more  
interested in its small size for this particular project.)

Many DC/DC controllers have track inputs.  Often this function is  
overloaded on the soft-start pin, so you have to read the datasheet  
carefully to work out whether it will do the job for you.

Except for the 5A requirement, a single LTM4614 could do everything you  
ask.

Regards,
Allan

Re: Synchronising two voltage regulators
On Mon, 22 Oct 2012 09:50:31 +0000, Allan Herriman wrote:

Quoted text here. Click to load it


Here is is in ASCII art:


  stage N      vbias    stage N+1
+-----------+    |    +-----------+
|           |    R    |           |
|           |    R    |           |
|           |    |    |           |
|       Pgd |----+-+--|Track      |
|           |      |  |           |
+-----------+      |  +-----------+
                  ===
                   |
                  gnd


The output of stage N+1 is held at 0V and doesn't start to ramp until the  
output of stage N has reached its Power_good threshold.

Vbias must be greater than 0.8V (or whatever the cutover voltage for the  
track input is).

Regards,
Allan

Site Timeline