switching regulator with mcu

[michael nikolaou]

Hi to newsgroup

I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma @ 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .
The mcu has A/D and PWM controller .Is it possible to use a simple mosfet
switch with
a capacitor to make a dropdown to half voltage if the mcu detects 24 volts
input ?.
Is there an easier way ?.
Any help would be appreciated .


MK

 

[Charlie E.]

Hi to newsgroup

I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma @ 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .
The mcu has A/D and PWM controller .Is it possible to use a simple mosfet
switch with
a capacitor to make a dropdown to half voltage if the mcu detects 24 volts
input ?.
Is there an easier way ?.
Any help would be appreciated .


MK

Michael,
Probably not a good idea. You MCU will take a little while to come up
to speed before it can monitor those voltages, and take action. At
24VDC, you chip is already fried by that time. That is a lot of
voltage to drop with just a linear regulator, esp. if that 24VDC is
not well regulated. It is possible, but it isn't simple...

Charlie

 

[michael nikolaou]

Hi charlie

Thanks fot the reply
If i halve the duty cycle is should get from 6 to 12 volts input .
This can be regulated to 3.3V with no problem .
Is there no simple circuit, switched capacitor possibly ,
to make this voltage halving ????

 

[Hammy]

If your production volumes aren't too terribly high there are some nice
switching regulator modules out there. TI makes a wide variety with a
good size/power ratio, that'll take 12-24V easily. I'm sure that TI
isn't the only one.

For that matter a switcher with an integrated switch and a fixed
output of 3.3V could be had. This would require less real-estate then
either a TO-220 with a heatsink or your switched capacitor circuit.

Search Digikey and you'll find some 3.3V switchers in a dpak only
requiring a few external components and pretty well idiot proof. A
small SOD Schottkey and a tiny smd cap and inductor would be the only
external components you need.

You could build it on a separate PCB and mount it vertically to your
main PCB. I've done that a few times. For 120 mA output you should be
able to get it down around 1/2" x ½".

Mount the DPAK on the ground side put your 3 or 4 other components on
the other side.

 

[Joerg]

Michael,
Probably not a good idea. You MCU will take a little while to come up
to speed before it can monitor those voltages, and take action. At
24VDC, you chip is already fried by that time. That is a lot of
voltage to drop with just a linear regulator, esp. if that 24VDC is
not well regulated. It is possible, but it isn't simple...

You could do it, this way:

Resistor and zener to CPU but much higher zener voltage than the CPU
uses in normal operation, meaning the MCU should have lots of headroom.
Start MCU with bare minimum in code, basically doing nothing other than
the switcher. Now fire up a buck or whatever switcher from 12-24VDC, MCU
controlled hysteretic for example. When that gets into the green zone
let the MCU commence its normal job because now that the switcher has
started it can use all the power it wants.

If you can't spread zener voltage and normal operating voltage that far,
add an extra transistor that turns off the zener after the switcher
start process has run its course. This will cost another port pin.

 

[Nico Coesel]

Hi to newsgroup

I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma @ 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .

Too much circuitry? Look at devices from TI like the TPS5410. Very
small and it will run at a wide variety of input voltages.

 

[Joerg]

Too much circuitry? Look at devices from TI like the TPS5410. Very
small and it will run at a wide variety of input voltages.

But mucho Dolares. I'd try using the MCU if possible if this is a high
volume product. But it'll require lots of nifty engineering. For low
volume, yeah, don't bother and use a chip. Then I'd use the MC34063
which costs under 20 cents. None of those high-faluting newfangled ritzy
ones ;-)

 

[michael nikolaou]

Guys

Thanks for all your replies

My small research has found that switcher solutions are.
1. simple and small sized switcher solutions are expensive.
2. Mc33063 are ok in terms of price but the inductor plus the capacitors are
too much board estate
3. Small size means high frequency and so you start with selective
components etc.
I NEED ONLY 120 ma is there nothing low cost and simple ?.
One switcher i located used 10 uH inductor , 47 uF output capacitor @ 6.3V
and 2.2uF input capacitor
but total cost was 1.8 euros. I's using one ARM7 mcu that costs 3 euro and
i don't want to spend as much for the PSU.
Some lower consumption devices @ 50 mA were using uA78M33 regulator with a
zener if 24 volts was used.
Joerg do you have schematic to study about the idea you are proposing ?
Any other simple ideas ???

 

[Joerg]

Guys

Thanks for all your replies

My small research has found that switcher solutions are.
1. simple and small sized switcher solutions are expensive.
2. Mc33063 are ok in terms of price but the inductor plus the capacitors are
too much board estate
3. Small size means high frequency and so you start with selective
components etc.

Not really, the passive parts are easy. But the challenge will be to
find a mainstream (meaning inexpensive) small switcher chip. The cheap
ones are all old and slow, 300kHz or less. This will take some time
because you must always check pricing. You could start by looking at
chips for the Power over Ethernet (PoE) market.

I NEED ONLY 120 ma is there nothing low cost and simple ?.
One switcher i located used 10 uH inductor , 47 uF output capacitor @ 6.3V
and 2.2uF input capacitor
but total cost was 1.8 euros. I's using one ARM7 mcu that costs 3 euro and
i don't want to spend as much for the PSU.
Some lower consumption devices @ 50 mA were using uA78M33 regulator with a
zener if 24 volts was used.
Joerg do you have schematic to study about the idea you are proposing ?

No, that would be a little R&D project. Requires tight reigns on the
firmware because the switcher must never skip a beat. But if you have a
free timer in your ARM MCU it can be done.

Any other simple ideas ???

Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long as
doesn't cost much. Pipe Vref out of your MCU (hoping it has that ...)
and use a cheap opamp to pull the Schmitt oscillator input "to the side"
when the target voltage is reached. That reduces the duty cycle as much
as needed to maintain regulation, pretty much like the throttle on a
gasoline-powered generator. If the target voltage doesn't have to be
very precise you could also use a NPN plus zener for that, without a
reference source. Probably a TL431-type device would work as well and
those are quite cheap, in the penny range.

 

[Jon Kirwan]

Not really, the passive parts are easy. But the challenge will be to
find a mainstream (meaning inexpensive) small switcher chip. The cheap
ones are all old and slow, 300kHz or less. This will take some time
because you must always check pricing. You could start by looking at
chips for the Power over Ethernet (PoE) market.


No, that would be a little R&D project. Requires tight reigns on the
firmware because the switcher must never skip a beat. But if you have a
free timer in your ARM MCU it can be done.


Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long as
doesn't cost much. Pipe Vref out of your MCU (hoping it has that ...)
and use a cheap opamp to pull the Schmitt oscillator input "to the side"
when the target voltage is reached. That reduces the duty cycle as much
as needed to maintain regulation, pretty much like the throttle on a
gasoline-powered generator. If the target voltage doesn't have to be
very precise you could also use a NPN plus zener for that, without a
reference source. Probably a TL431-type device would work as well and
those are quite cheap, in the penny range.

This has been an interesting discussion. What is bothering me a lot,
in reading it, is the "ONLY 120mA" thing! 120mA? Only? Cripes. If
I were imagining being as space-constrained as the OP suggests, I'd IN
THE FIRST PLACE start asking myself why I actually need 120mA! Is it
the processor, itself? If so, look to get rid of it and find
something else. Doing so may put constraints on the application
itself (doing logarithms on a PIC16 is quite different in speed than
doing them on an ARM9, for example), but it may greatly relax the
power supply design issues. Everything is trade-off. But I'm
bothered by the casual acceptance of a 120mA spec as gospel when there
is a serious space and cost issue here.

What is sucking that power? Can it be changed? If not, why not?

Jon

 

[Joerg]

This has been an interesting discussion. What is bothering me a lot,
in reading it, is the "ONLY 120mA" thing! 120mA? Only? Cripes. If
I were imagining being as space-constrained as the OP suggests, I'd IN
THE FIRST PLACE start asking myself why I actually need 120mA! Is it
the processor, itself? If so, look to get rid of it and find
something else. Doing so may put constraints on the application
itself (doing logarithms on a PIC16 is quite different in speed than
doing them on an ARM9, for example), but it may greatly relax the
power supply design issues. Everything is trade-off. But I'm
bothered by the casual acceptance of a 120mA spec as gospel when there
is a serious space and cost issue here.

What is sucking that power? Can it be changed? If not, why not?

I sure hope it's not the MCU alone because that would spell trouble.
Often there are other things like ADCs, analog stuff etc. 120mA is
really a piece of cake from a power converter point of view. After all,
that's only 600mW if his VCC is 5V. Low enough for an energy star

Michael may have to go with a SEPIC if he wants the FET to switch to GND
but that's not a big deal either, just two parts more. Since the advent
of PoE he's got plenty of options.

 

[Jon Kirwan]

I sure hope it's not the MCU alone because that would spell trouble.

It sounds familiar. I've looked at these 'low power' ARM chips,
noticed that many seem to average 20-50mA, with the lower figure only
on a good day and many requiring more. It was NOT a shock to me to
see 120mA with ARMs. In fact, I'm just fine with that. But when the
OP writes, "I'm making a mcu based device which i want to be very
small and low cost" and then out the other side of the mouth says
"only 120mA" then I'm truly wondering.

One of the BIG tradeoffs is power __AND__ heat. And by the time you
get anywhere near 120mA, you've often got both problems in spades.
It's a fundamentally different domain.

I guess that's why I just went _white_ when I read those figures and
the OP's language in the same context. The only way 120mA is a
little, these days, is if you are used to x86 processors running at
GHz and requiring multiple power supply rails to help contain heat
problems better.

If you live in that world, I can see it. But that sure isn't MY
embedded world perspective. I consider moving into the 120mA domain
as being akin to a "damn-the-torpedoes, devil-may-care" world. At
that point, you are already spending dollars, not pennies, and have
board room to spare. And if you are chugging 120mA, you NEED space,
anyway.

Often there are other things like ADCs, analog stuff etc. 120mA is
really a piece of cake from a power converter point of view. After all,
that's only 600mW if his VCC is 5V. Low enough for an energy star

Hehe. Yeah. If we are talking washing machines, 120mA is no problem.
But room isn't a problem, then, either. There's always a corner, plus
one HUGE heat sink, too.

Michael may have to go with a SEPIC if he wants the FET to switch to GND
but that's not a big deal either, just two parts more. Since the advent
of PoE he's got plenty of options.

I'd recommend that Michael rethink 120mA. I mean, jeeez! If you
nearing a watt already, with overhead, you need space and you expect
to spend something on the power supply, too. Or some serious, crafted
time. Or both.

Almost two decades ago, I was worrying over a thermal cooling stack
with two Peltier stages and the bottom stage was consuming half a
watt. I was worried about that much heat. And the tiny micro device
plus analog circuits at the top was burning some 30mW. Now that's the
kind of thing you get with 120mA! 2-stage Peltier coolers AND a
micro. 20 years ago.

This is crazy-making to think about 120mA! Yeah, if you are making an
iPhone or internet interface device with RF and all. I mean, you need
to actually broadcast maybe 1/4 watt or so. So yeah. But "I'm making
a mcu based device which i want to be very small and low cost?"

The OP and I must come from different universes!

Jon

 

[Joerg]

Jon Kirwan wrote:

[...]

I sure hope it's not the MCU alone because that would spell trouble.

It sounds familiar. I've looked at these 'low power' ARM chips,
noticed that many seem to average 20-50mA, with the lower figure only
on a good day and many requiring more. It was NOT a shock to me to
see 120mA with ARMs. In fact, I'm just fine with that. But when the
OP writes, "I'm making a mcu based device which i want to be very
small and low cost" and then out the other side of the mouth says
"only 120mA" then I'm truly wondering.

One of the BIG tradeoffs is power __AND__ heat. And by the time you
get anywhere near 120mA, you've often got both problems in spades.
It's a fundamentally different domain.

I guess that's why I just went _white_ when I read those figures and
the OP's language in the same context. The only way 120mA is a
little, these days, is if you are used to x86 processors running at
GHz and requiring multiple power supply rails to help contain heat
problems better.

If you live in that world, I can see it. But that sure isn't MY
embedded world perspective. I consider moving into the 120mA domain
as being akin to a "damn-the-torpedoes, devil-may-care" world. At
that point, you are already spending dollars, not pennies, and have
board room to spare. And if you are chugging 120mA, you NEED space,
anyway.

Oh, I don't know. Last week I finished a project for a client. The board
takes in 3W, of which half goes on to another part that this baord
controls. Peak power is above 10W but brief and gets sunk into a metal
buffer. The whole thing is slightly above a square inch and mounted to a
metal flange. Works.

Hehe. Yeah. If we are talking washing machines, 120mA is no problem.
But room isn't a problem, then, either. There's always a corner, plus
one HUGE heat sink, too.

600mW is pretty easy to dissipate. A couple of SO8 chips could do it in air.

I'd recommend that Michael rethink 120mA. I mean, jeeez! If you
nearing a watt already, with overhead, you need space and you expect
to spend something on the power supply, too. Or some serious, crafted
time. Or both.

Almost two decades ago, I was worrying over a thermal cooling stack
with two Peltier stages and the bottom stage was consuming half a
watt. I was worried about that much heat. And the tiny micro device
plus analog circuits at the top was burning some 30mW. Now that's the
kind of thing you get with 120mA! 2-stage Peltier coolers AND a
micro. 20 years ago.

This is crazy-making to think about 120mA! Yeah, if you are making an
iPhone or internet interface device with RF and all. I mean, you need
to actually broadcast maybe 1/4 watt or so. So yeah. But "I'm making
a mcu based device which i want to be very small and low cost?"

The OP and I must come from different universes!

Maybe he grew up in the days of the Plymouth Fury

 

[Joerg]

It would appear that you are new to this engineering thing.

Microcontrollers are so cheap because the manufactures figured you would
spend the money to make it work.

Power devices cost more then CPU silicon, always have.

Nah, I wouldn't agree. Look at the MC34063. Costs about 15c in qties and
can swing more than an amp. It's quite big inside. Same for FETs.

Problem is that only the slowpokes among switchers are cheap, for
anything above 500kHz they want top Dollar. So guys like me often do
those discrete. Until the prices come down to less than 50c.

[...]

 

[Martin Riddle]

Guys

Thanks for all your replies

My small research has found that switcher solutions are.
1. simple and small sized switcher solutions are expensive.
2. Mc33063 are ok in terms of price but the inductor plus the
capacitors are too much board estate
3. Small size means high frequency and so you start with selective
components etc.
I NEED ONLY 120 ma is there nothing low cost and simple ?.
One switcher i located used 10 uH inductor , 47 uF output capacitor @
6.3V and 2.2uF input capacitor
but total cost was 1.8 euros. I's using one ARM7 mcu that costs 3
euro and i don't want to spend as much for the PSU.
Some lower consumption devices @ 50 mA were using uA78M33 regulator
with a zener if 24 volts was used.
Joerg do you have schematic to study about the idea you are proposing
?
Any other simple ideas ???

What about a LDO? National, LP2952. 30V input. Dissapation isnt that
much at 120ma.
You could always stick some resistance in the input pin to lower Vin,
and device dissipation.

Cheers

 

[Joerg]

What about a LDO? National, LP2952. 30V input. Dissapation isnt that
much at 120ma.
You could always stick some resistance in the input pin to lower Vin,
and device dissipation.

Be careful. I don't know the LP2952 but I had one of the 29-series go
berserk when the source impedance became too large. It wasn't mentioned
in the datasheet ...

Bottomline is I stay away from LDOs. Besides the above there are also
ESR-related stability issues.

 

[John Devereux]

Not really, the passive parts are easy. But the challenge will be to
find a mainstream (meaning inexpensive) small switcher chip. The cheap
ones are all old and slow, 300kHz or less. This will take some time
because you must always check pricing. You could start by looking at
chips for the Power over Ethernet (PoE) market.




No, that would be a little R&D project. Requires tight reigns on the
firmware because the switcher must never skip a beat. But if you have
a free timer in your ARM MCU it can be done.



Another option would be to use a CD40106 or something similar as a
Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can
drive a little FET, a simple logic level device like a 2N7002 as long
as doesn't cost much. Pipe Vref out of your MCU (hoping it has that
...) and use a cheap opamp to pull the Schmitt oscillator input "to
the side" when the target voltage is reached. That reduces the duty
cycle as much as needed to maintain regulation, pretty much like the
throttle on a gasoline-powered generator. If the target voltage
doesn't have to be very precise you could also use a NPN plus zener
for that, without a reference source. Probably a TL431-type device
would work as well and those are quite cheap, in the penny range.

Hi Jeorg,

Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001
drives a transformer? How do you stop the opamp railing during startup
or a load step? The schmitt osc would stop and .. how does it go
.... Phut!

Also of course by the time he's actually bought a SEPIC transformer it
could all approach his 1.8 Euros. Or are there some super-cheap ones
now?

 

[michael nikolaou]

I need to make clear so we avoid confusions

The device uses a 2.2" tft lcd . This thing has a backlight and no matter
what
draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption
i end up to 120 ma.
Now as i see whether i have a small switcher with huge inductor and
capacitor
so my board space is too large .
The other solution is super fast switcher that has everything small but
needs 2 euros
at least .
I imagine every engineer designing devices has faced this question .
I'm sure though that a simple cpu controlled switcher could cope with the
large dropout
from 24V to 4-5 volts and then a simple linear regulator could take you to
3.3V so you play
it safe.If things go wrong (cpu malfunction ) thermal shutdown from the
linear regulator would
reset the cpu and that would restart the system
As i see it a simple fet switch with cpu pwm control should satisfy that .
Any opinions or experience on that subject???




The 120

 

[John Devereux]

The 120
[...]
[...]

Be careful. I don't know the LP2952 but I had one of the 29-series go
berserk when the source impedance became too large. It wasn't mentioned in
the datasheet ...

Bottomline is I stay away from LDOs. Besides the above there are also
ESR-related stability issues.

(top post corrected)

I need to make clear so we avoid confusions

The device uses a 2.2" tft lcd . This thing has a backlight and no matter
what
draws 60 ma @ 3.3V .That i can't avoid . So with 60 ma Arm7 consumption
i end up to 120 ma.
Now as i see whether i have a small switcher with huge inductor and
capacitor
so my board space is too large .
The other solution is super fast switcher that has everything small but
needs 2 euros
at least .
I imagine every engineer designing devices has faced this question .
I'm sure though that a simple cpu controlled switcher could cope with the
large dropout
from 24V to 4-5 volts and then a simple linear regulator could take you to
3.3V so you play
it safe.If things go wrong (cpu malfunction ) thermal shutdown from the
linear regulator would
reset the cpu and that would restart the system
As i see it a simple fet switch with cpu pwm control should satisfy that .
Any opinions or experience on that subject???

I have never had the nerve to try a CPU-controlled switcher - but I
never thought of a linear regulator as a "safety cutout" either. I'd say
go for it! You have a couple of issues:

* startup - maybe just a trickle of current if the circuit has a very
low initial requirement

* Configuration - either you need a high-side driver with a buck
configuration or an easier low-side one that needs a transformer
(SEPIC or flyback). Or I guess the simplest is inverting but that's
nasty if you have to connect to anything else since incoming "ground"
ends up being your positive rail.

Also your "12-24V" sounds a bit automotive. If so be aware that there
can be some very nasty spikes and voltage surges, meaning that a strict
interpretation of the standards can require withstanding >>70V, not just
24.

 

[Jasen Betts]

Hi to newsgroup

I'm making a mcu based device which i want to be very small and low cost
The design consumes 100..120 ma @ 3.3V
The problem is the installation that requires 12 or 24 volts to operate.
I don't want to use a switching regulator to make the voltage drop since its
to much
circuit involved and also not a zener since its to much heat involved .
The mcu has A/D and PWM controller .Is it possible to use a simple mosfet
switch with
a capacitor to make a dropdown to half voltage if the mcu detects 24 volts
input ?.

with just a cap and a mosfet?
doiesn't seem possible,

Is there an easier way ?.

linear regulator or buck regulator.