Forums

Wanted: Voltage mode PWM controller

Started by powerampfreak March 27, 2009
On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote:
> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak > > > > > > <powerampfr...@hotmail.com> wrote: > >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: > >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wrote: > > >> > 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 elsewhe=
re
> >> > 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 curren=
t.
> >> > 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.electronics.design/browse_thread/t..=
.
> > >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- D=F6lj 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.
On Sat, 28 Mar 2009 04:09:24 -0700 (PDT), powerampfreak
<powerampfreak@hotmail.com> wrote:

>On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote: >> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak >> >> >> >> >> >> <powerampfr...@hotmail.com> wrote: >> >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: >> >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wrote: >> >> >> > 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.electronics.design/browse_thread/t... >> >> >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- D&ouml;lj 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. > >
I had to make my own 1Mhz V mode controller cause I couldn't find one. D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design
On Sat, 28 Mar 2009 04:09:24 -0700 (PDT), powerampfreak
<powerampfreak@hotmail.com> wrote:

>On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote: >> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak >> >> >> >> >> >> <powerampfr...@hotmail.com> wrote: >> >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: >> >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wrote: >> >> >> > 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.electronics.design/browse_thread/t... >> >> >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- D&ouml;lj 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. >
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
On 28 Mar, 17:45, legg <l...@nospam.magma.ca> wrote:
> On Sat, 28 Mar 2009 04:09:24 -0700 (PDT), powerampfreak > > > > > > <powerampfr...@hotmail.com> wrote: > >On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote: > >> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak > > >> <powerampfr...@hotmail.com> wrote: > >> >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: > >> >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wrote: > > >> >> > 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 else=
where
> >> >> > in my circuit. > >> >> > So, a basic Ramp generator and a PWM comparator, and an output st=
age.
> >> >> > An UC3843 would suit if it were voltage controlled instead of cur=
rent.
> >> >> > 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.electronics.design/browse_thread/=
t...
> > >> >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- D=F6lj 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. > > 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- D=F6lj 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?
On Thu, 26 Mar 2009 22:52:36 -0700 (PDT), powerampfreak
<powerampfreak@hotmail.com> wrote:

>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/browse.jsp?N=511+500002+1001865&Ntk=gensearch_001&Ntt=UCC3570&Ntx= or digikey http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556342&keywords=UCC3570
On Mar 28, 10:41=A0am, powerampfreak <powerampfr...@hotmail.com> wrote:
> On 28 Mar, 17:45, legg <l...@nospam.magma.ca> wrote: > > > > > On Sat, 28 Mar 2009 04:09:24 -0700 (PDT), powerampfreak > > > <powerampfr...@hotmail.com> wrote: > > >On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote: > > >> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak > > > >> <powerampfr...@hotmail.com> wrote: > > >> >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: > > >> >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wrot=
e:
> > > >> >> > 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 el=
sewhere
> > >> >> > 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 c=
urrent.
> > >> >> > I've gone through Fairchild, ONsemi websites without finding an=
y
> > >> >> > suitable parts. > > >> >> > Thanks for any hints! > > > >> >> > Regards > > >> >> > PAF > > > >> >> Checkout: > > > >> >>http://groups.google.com/group/sci.electronics.design/browse_threa=
d/t...
> > > >> >The UC3825 has two out of phase outputs. I need one output with ful=
l
> > >> >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 pi=
n
> > >> to produce voltage mode control. Your external error signal is easil=
y
> > >> 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 par=
ts
> > >> 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-pha=
se
> > >> outputs can often be easily summed, paricularly when these outputs a=
re
> > >> 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- D=F6lj 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. > > > 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- D=F6lj 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.
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)
On Mar 28, 6:07=A0pm, gearhead <nos...@billburg.com> wrote:
> On Mar 28, 10:41=A0am, powerampfreak <powerampfr...@hotmail.com> wrote: > > > > > On 28 Mar, 17:45, legg <l...@nospam.magma.ca> wrote: > > > > On Sat, 28 Mar 2009 04:09:24 -0700 (PDT), powerampfreak > > > > <powerampfr...@hotmail.com> wrote: > > > >On 28 Mar, 07:17, legg <l...@nospam.magma.ca> wrote: > > > >> On Fri, 27 Mar 2009 03:07:50 -0700 (PDT), powerampfreak > > > > >> <powerampfr...@hotmail.com> wrote: > > > >> >On 27 Mar, 08:55, Klaus Kragelund <klausk...@hotmail.com> wrote: > > > >> >> On 27 Mar., 06:52, powerampfreak <powerampfr...@hotmail.com> wr=
ote:
> > > > >> >> > Hi all. I need a basic voltage mode PWM controller for a buck > > > >> >> > converter project. I need a wide duty cycle range, like 5 - 9=
5%. 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 outpu=
t 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.electronics.design/browse_thr=
ead/t...
> > > > >> >The UC3825 has two out of phase outputs. I need one output with f=
ull
> > > >> >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 eas=
ily
> > > >> fed into the 'compensation' pin, if the local error amplifier inpu=
t
> > > >> 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 p=
arts
> > > >> in two fairly comprehensive mfr's product lines. Obviously it's no=
t
> > > >> just voltage mode that is your stumbling block, or you would have > > > >> found 78s40, 34060/3, 494/594 etc. Note that parts providing two-p=
hase
> > > >> 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 t=
o
> > > >> 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 fr=
om
> > > >> the device, someone can help you. > > > > >> RL- D=F6lj citerad text - > > > > >> - Visa citerad text - > > > > >We all know that the UC3842/3 devices has a maximum Isense voltage o=
f
> > > >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 usefu=
l
> > > >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 cycl=
e
> > > >output is wrong. The small deadtime between the two outputs will cau=
se
> > > >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 si=
de
> > > >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 am=
p.
> > > >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- D=F6lj 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. > > Take the sg3525 for example. =A0The 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.

"powerampfreak" <powerampfreak@hotmail.com> wrote in message 
news:2913cd68-e5be-4983-8ec8-c38905aada66@z1g2000yqn.googlegroups.com...
> 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
On 29 Mar, 03:27, "Martin Riddle" <martin_...@verizon.net> wrote:
> "powerampfreak" <powerampfr...@hotmail.com> wrote in message > > news:2913cd68-e5be-4983-8ec8-c38905aada66@z1g2000yqn.googlegroups.com... > > > 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, =A0cheap 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