Wanted: Voltage mode PWM controller

[powerampfreak]

Hi all. I need a basic voltage mode PWM controller for a buck
converter project. I need a wide duty cycle range, like 5 - 95%. I
don't need an error-amp in the ic, since this part is placed elsewhere
in my circuit.
So, a basic Ramp generator and a PWM comparator, and an output stage.
An UC3843 would suit if it were voltage controlled instead of current.
I've gone through Fairchild, ONsemi websites without finding any
suitable parts.
Thanks for any hints!

Regards
PAF

 

[Klaus Kragelund]

Hi all. I need a basic voltage mode PWM controller for a buck
converter project. I need a wide duty cycle range, like 5 - 95%. I
don't need an error-amp in the ic, since this part is placed elsewhere
in my circuit.
So, a basic Ramp generator and a PWM comparator, and an output stage.
An UC3843 would suit if it were voltage controlled instead of current.
I've gone through Fairchild, ONsemi websites without finding any
suitable parts.
Thanks for any hints!

Regards
PAF

Checkout:

http://groups.google.com/group/sci....79?hl=da&lnk=gst&q=kragelund#b90a47d3c56c7d79

 

[powerampfreak]

Checkout:

http://groups.google.com/group/sci.electronics.design/browse_thread/t...

The UC3825 has two out of phase outputs. I need one output with full
duty cycle from 5 - 95%

 

[Klaus Kragelund]

The UC3825 has two out of phase outputs. I need one output with full
duty cycle from 5 - 95%- Skjul tekst i anførselstegn -

- Vis tekst i anførselstegn -

Well, you said: "An UC3843 would suit if it were voltage controlled
instead of current". Read the link to the post again´.

 

[powerampfreak]

Well, you said: "An UC3843 would suit if it were voltage controlled
instead of current". Read the link to the post again´.- Dölj citerad text -

- Visa citerad text -

Look, I said I want ONE output with full duty cycle (5-95%). The
UC3843 does have that, but it's intended for current mode operation.
If you take a look at the UC3825 datasheet:
http://focus.ti.com/docs/prod/folders/print/uc3825.html
You'll see it's a two output device most suitable for bridge type
converters like half and full bridge.
Each output provides a maximum of 50% duty cycle and that's too less
for my application.

 

[powerampfreak]

Look, I said I want ONE output with full duty cycle (5-95%). The
UC3843 does have that, but it's intended for current mode operation.
If you take a look at the UC3825 datasheet:http://focus.ti.com/docs/prod/folders/print/uc3825.html
You'll see it's a two output device most suitable for bridge type
converters like half and full bridge.
Each output provides a maximum of 50% duty cycle and that's too less
for my application.- Dölj citerad text -

- Visa citerad text -

The good old NE5561 would do the job perfectly, if it was still
available. Unfortunately it isn't.
There must be a similair ic around.

 

[Hammy]

The good old NE5561 would do the job perfectly, if it was still
available. Unfortunately it isn't.
There must be a similair ic around.

There are lots of controllers like that.

TEXAS INSTRUMENTS - TPS40200DRG4 - Voltage Mode Contoller

http://canada.newark.com/texas-instruments/tps40200drg4/voltage-mode-contoller/dp/59M7211


SEMTECH - SC1103CSTRT - Pulse Width Modulation (PWM) Controller


http://canada.newark.com/semtech/sc1103cstrt/pulse-width-modulation-pwm-controller/dp/63J8418


For that matter whats wrong with using a UC3842 IT HAS 95% DUTY. You
dont have to use it in CMC there are no laws that I know of.From the
datasheet typical application VMC Buck.

http://i41.tinypic.com/1z5uazk.png

UVLO is 16V for the 3842,if this isnt a problem.

from this data sheet.

http://lc.fie.umich.mx/~ifranco/DATASHEET/control_fuentes_conmutadas/slua143.pdf

There is a way to use opto feedback using the comp pin,if you already
have an EA.This seems more trouble then it's worth but it's doable.If
you dont need isolation I wouldnt do it.I think Basso has an example
floating around the net somewhere.

If you really want to omit the EA you could search for primary side
controllers or isolated controllers.These dont have or usually dont
have EA's on board the control signal is sent acrross the isolation
boundary by a transformer or optocoupler. Onsemi makes several of
these NCP series comes to mind.

 

[Rich Grise]

Hi all. I need a basic voltage mode PWM controller for a buck
converter project. I need a wide duty cycle range, like 5 - 95%. I
don't need an error-amp in the ic, since this part is placed elsewhere
in my circuit.
So, a basic Ramp generator and a PWM comparator, and an output stage.
An UC3843 would suit if it were voltage controlled instead of current.
I've gone through Fairchild, ONsemi websites without finding any
suitable parts.
Thanks for any hints!

You can turn voltage into current with a resistor. ;-)

Cheers!
Rich

 

[legg]

The UC3825 has two out of phase outputs. I need one output with full
duty cycle from 5 - 95%

The thread points out the ease of converting the CMode circuit to
voltage mode.

For 3842/3 types, you could use the Rt/Ct waveform, suitably
attenuated so as not to exceed 1V peak, as an input to the Csense pin
to produce voltage mode control. Your external error signal is easily
fed into the 'compensation' pin, if the local error amplifier input
pin is grounded, but why not use the integrated part? It's free.

I hesitate to go further, as you claim to have found no suitable parts
in two fairly comprehensive mfr's product lines. Obviously it's not
just voltage mode that is your stumbling block, or you would have
found 78s40, 34060/3, 494/594 etc. Note that parts providing two-phase
outputs can often be easily summed, paricularly when these outputs are
open collector or other simple structures.

PWM can be achieved fairly simply. An integrated device attempts to
provide the bells and whistles that you might find you need in
addition to that simple feature.

Perhaps if you make a list of what actual properties you expect from
the device, someone can help you.

RL

 

[powerampfreak]

The thread points out the ease of converting the CMode circuit to
voltage mode.

For 3842/3 types, you could use the Rt/Ct waveform, suitably
attenuated so as not to exceed 1V peak, as an input to the Csense pin
to produce voltage mode control. Your external error signal is easily
fed into the 'compensation' pin, if the local error amplifier input
pin is grounded, but why not use the integrated part? It's free.

I hesitate to go further, as you claim to have found no suitable parts
in two fairly comprehensive mfr's product lines. Obviously it's not
just voltage mode that is your stumbling block, or you would have
found 78s40, 34060/3, 494/594 etc. Note that parts providing two-phase
outputs can often be easily summed, paricularly when these outputs are
open collector or other simple structures.

PWM can be achieved fairly simply. An integrated device attempts to
provide the bells and whistles that you might find you need in
addition to that simple feature.

Perhaps if you make a list of what actual properties you expect from
the device, someone can help you.

RL- Dölj citerad text -

- Visa citerad text -

We all know that the UC3842/3 devices has a maximum Isense voltage of
1 volt, since the Rt/Ct ramp has a decent 1.8V offset, it requires
level shifting.
If the ramp really was starting at zero (close to) It could be useful
for implementing a compact and cost effective voltage mode control.
The Rt/Ct ramp is often used to implement ramp compensation in peak
current mode converters though.
To those of you who claim that SG3525 would do for a 5-95% duty cycle
output is wrong. The small deadtime between the two outputs will cause
problems.
The MC34060 could eventually suit, I'll take a closer look to it.
I can't use the internal error-amp since the error signal is fed
across a safety barrier, the error-amp is placed on the secondary side
to compensate for opto CTR variations.
Here's a wish list for the ic:

Voltage mode PWM
Duty cycle 5-95%
Directly access to the PWM comparator input, NOT through an error amp.
Undervoltage lock out
Frequency up to 150-200kHz
As low current consumption as possible (max 5-10mA)
Soft-start possibilities

Well, that's all I guess.

 

[legg]

We all know that the UC3842/3 devices has a maximum Isense voltage of
1 volt, since the Rt/Ct ramp has a decent 1.8V offset, it requires
level shifting.
If the ramp really was starting at zero (close to) It could be useful
for implementing a compact and cost effective voltage mode control.
The Rt/Ct ramp is often used to implement ramp compensation in peak
current mode converters though.
To those of you who claim that SG3525 would do for a 5-95% duty cycle
output is wrong. The small deadtime between the two outputs will cause
problems.
The MC34060 could eventually suit, I'll take a closer look to it.
I can't use the internal error-amp since the error signal is fed
across a safety barrier, the error-amp is placed on the secondary side
to compensate for opto CTR variations.
Here's a wish list for the ic:

Voltage mode PWM
Duty cycle 5-95%
Directly access to the PWM comparator input, NOT through an error amp.
Undervoltage lock out
Frequency up to 150-200kHz
As low current consumption as possible (max 5-10mA)
Soft-start possibilities

Well, that's all I guess.

You are being disingenuous and possibly obtuse.

Internal error amplifiers in a great range of controllers feed high
impedance low-current pull-up nodes. When internal error amps are
disabled by biasing their inputs appropriately, they leave this node
free for external manipulation.

Dead-time in two phase outputs simply affects maximum duty, when
summed. A 95% duty requirement is not prevented.

Bite the bullet, pick a good, fast chip and go.

RL

 

[powerampfreak]

You are being disingenuous and possibly obtuse.

Internal error amplifiers in a great range of controllers feed high
impedance low-current pull-up nodes. When internal error amps are
disabled by biasing their inputs appropriately, they leave this node
free for external manipulation.

Dead-time in two phase outputs simply affects maximum duty, when
summed. A 95% duty requirement is not prevented.

Bite the bullet, pick a good, fast chip and go.

RL- Dölj citerad text -

- Visa citerad text -

Yes - I know. And my requirement included access directly to the PWM
comparator, which is the same point as the error amp output node.
BUT, many devices doesn't put the error amp output (and pwm comparator
input) to a separate pin on the chip. THEN, it's a problem. You're
forced to use the internal ea.
So, what do you mean by I'm disingenuous, really?

 

[Hammy]

Hi all. I need a basic voltage mode PWM controller for a buck
converter project. I need a wide duty cycle range, like 5 - 95%. I
don't need an error-amp in the ic, since this part is placed elsewhere
in my circuit.
So, a basic Ramp generator and a PWM comparator, and an output stage.
An UC3843 would suit if it were voltage controlled instead of current.
I've gone through Fairchild, ONsemi websites without finding any
suitable parts.
Thanks for any hints!

Regards
PAF

UCC3570 series

http://focus.ti.com/lit/ds/symlink/ucc35705.pdf


http://canada.newark.com/jsp/search...02+1001865&Ntk=gensearch_001&Ntt=UCC3570&Ntx=

or digikey

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556342&keywords=UCC3570

 

[gearhead]

Yes - I know. And my requirement included access directly to the PWM
comparator, which is the same point as the error amp output node.

Take the sg3525 for example. The error amp output has a pullup
resistor and a current sink, so you can toggle the error amp low and
control the pwm from the compensation pin.

(snip)

 

[gearhead]

Take the sg3525 for example.  The error amp output has a pullup
resistor and a current sink, so you can toggle the error amp low and
control the pwm from the compensation pin.

(snip)

sorry, the error amp has an active pull-up, not a pull-up resistor.
The rest of my post was correct.

 

[Martin Riddle]

Hi all. I need a basic voltage mode PWM controller for a buck
converter project. I need a wide duty cycle range, like 5 - 95%. I
don't need an error-amp in the ic, since this part is placed elsewhere
in my circuit.
So, a basic Ramp generator and a PWM comparator, and an output stage.
An UC3843 would suit if it were voltage controlled instead of current.
I've gone through Fairchild, ONsemi websites without finding any
suitable parts.
Thanks for any hints!

Regards
PAF

TL494, cheap part.

Cheers

 

[powerampfreak]

TL494,  cheap part.

Cheers

I appreciate all your tips, unfortunately it seems some of you didn't
read all the requirements.
It should be a ONE output 90% duty cycle pwm. I've used the '3525 chip
many times for converters like half bridges.
I got two interesting tips here, that is MC34060 which appears to be
exactly what I'm looking for.
Also the UCC3570 series seems great.
Thanks a lot!

Cheers