Synchronising two voltage regulators

On Sun, 21 Oct 2012 09:03:56 -0700, Simon wrote:

> So, I have a power headache :) 
> 
> One of the chips I want to use in a project has very specific power 
requirements ...
> 
>  - 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)
> 
> ... 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-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.
> 
> 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 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.
> 
> 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 interpreting 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, 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 :)
> 
>  - Any recommendations on which one to use ? 
> 
>  - Is there a better way to do this that I'm missing ? :)
> 
> Cheers
>    Simon.


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

On Mon, 22 Oct 2012 09:50:31 +0000, Allan Herriman wrote:

> On Sun, 21 Oct 2012 09:03:56 -0700, Simon wrote:
> 
>> So, I have a power headache :)
>> 
>> One of the chips I want to use in a project has very specific power
> requirements ...
>> 
>>  - 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)
>> 
>> ... 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-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.
>> 
>> 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 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.
>> 
>> 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 interpreting 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, 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 :)
>> 
>>  - Any recommendations on which one to use ?
>> 
>>  - Is there a better way to do this that I'm missing ? :)
>> 
>> Cheers
>>    Simon.
> 
> 
> 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


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