Synchronising two voltage regulators

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.

Reply to
Simon
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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 
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Reply to
Tim Wescott

[...]

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

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Reply to
Vladimir Vassilevsky

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.

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

Hmm. All I'm seeing in the datasheets (eg:

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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

Reply to
Simon

rements ...

nsure correct startup.

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.

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.

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.

m, hence this long plea for help :) A few questions:

the-world, and gate to the AVR to switch 5A ?

, 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 :)

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

-Lasse

Reply to
langwadt

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

Reply to
Simon

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 
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Reply to
Tim Wescott

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.

Reply to
Simon

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).

Reply to
David Brown

requirements ...

correct startup.

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..

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.

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.

hence this long plea for help :) A few questions:

rest-of-the-world, and gate to the AVR to switch 5A ?

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 :)

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

Reply to
rickman

requirements ...

ensure correct startup.

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.

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.

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.

them, hence this long plea for help :) A few questions:

the-world, and gate to the AVR to switch 5A ?

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 :)

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

Reply to
Allan Herriman

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

Reply to
Allan Herriman

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