TPS75003 Buck feedback network and feedforward capacitor

[Greg Neff]

I am designing in this TPS75003:

http://www-s.ti.com/sc/ds/tps75003.pdf

This includes two bucks and an LDO. The buck controller is intended
to be used with output capacitors that have enough ESR to allow some
ripple back into the feedback pin. Naturally, we have to use ceramic
capacitors, so TI suggests we use the feedback network described in
figure 27 of this data sheet:

http://www-s.ti.com/sc/ds/tps64200.pdf

The resistors are no problem, except that I am inclined to reduce the
suggested values by an order of magnitude so that there will be less
effects from parasitics. No problem so far, but now I need to figure
out the feedforward capacitor Cff. There is no guidance in either
datasheet regarding selection of this value. I contacted TI tech
support regarding this, and they said plug in a 100pf cap, and then
tweak it based on the transient response. I was looking for something
a little more scientific.

I thought that the feedforward capacitor is used to cancel a pole
caused by the parasitic capacitance of the feedback pin. I have
looked at some datasheets from other buck controller manufacturers,
and there seems to be as many methods to calculate Cff as there are
buck controllers. Does anyone have any opinion on this?


================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]

 

[John Popelish]

I am designing in this TPS75003:

http://www-s.ti.com/sc/ds/tps75003.pdf

This includes two bucks and an LDO. The buck controller is intended
to be used with output capacitors that have enough ESR to allow some
ripple back into the feedback pin. Naturally, we have to use ceramic
capacitors, so TI suggests we use the feedback network described in
figure 27 of this data sheet:

http://www-s.ti.com/sc/ds/tps64200.pdf

The resistors are no problem, except that I am inclined to reduce the
suggested values by an order of magnitude so that there will be less
effects from parasitics. No problem so far, but now I need to figure
out the feedforward capacitor Cff. There is no guidance in either
datasheet regarding selection of this value. I contacted TI tech
support regarding this, and they said plug in a 100pf cap, and then
tweak it based on the transient response. I was looking for something
a little more scientific.

I thought that the feedforward capacitor is used to cancel a pole
caused by the parasitic capacitance of the feedback pin. I have
looked at some datasheets from other buck controller manufacturers,
and there seems to be as many methods to calculate Cff as there are
buck controllers. Does anyone have any opinion on this?


================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]

(my take on a quick read through the data sheet)
The minimum on/off time aspects of this control scheme mean that the
feedback amplifier is not integrating (or acting as a PI controller)
on the output voltage but is acting essentially as a comparator to
detect the output voltage passing through the setpoint. The Cff cap
is altering the shape of the ripple voltage to tweak the point in the
ripple that switches the comparator. The normal lead lag compensation
for PI controller tuning is not really applicable to this design.
There is a big interaction between the ESR of the output cap (which
determines the form of the ripple voltage) and the optimum value of
the Cff capacitor. The entire effect of Cff takes place in
essentially a single operation cycle as it phase shifts the switching
points of the comparator with respect to the ripple voltage.

Am I stating the painfully obvious, (or am I completely off the beam)
or does this overflight give you any new insight?

 

[Greg Neff]

(my take on a quick read through the data sheet)
The minimum on/off time aspects of this control scheme mean that the
feedback amplifier is not integrating (or acting as a PI controller)
on the output voltage but is acting essentially as a comparator to
detect the output voltage passing through the setpoint. The Cff cap
is altering the shape of the ripple voltage to tweak the point in the
ripple that switches the comparator. The normal lead lag compensation
for PI controller tuning is not really applicable to this design.
There is a big interaction between the ESR of the output cap (which
determines the form of the ripple voltage) and the optimum value of
the Cff capacitor. The entire effect of Cff takes place in
essentially a single operation cycle as it phase shifts the switching
points of the comparator with respect to the ripple voltage.

Am I stating the painfully obvious, (or am I completely off the beam)
or does this overflight give you any new insight?

Right. I knew this, but I let myself get confused by looking at other
buck controllers.

In this case (see TPS64002 datasheet figure 27) we are using a very
low ESR ceramic capacitor that leaves very little ripple. If the
output capacitor ESR is low, then Cff will have questionable benefit
regarding ripple sense timing, hence the need for R1b. This is why I
started to question the function of Cff around R1a in this
application. It seems like Cff will act more like an integrator,
slowing the rise time of the feedback pin as current flows through R1b
during Ton. Does this sound right?

================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
[email protected]

 

[John Popelish]

Greg Neff wrote:
(snip)

In this case (see TPS64002 datasheet figure 27) we are using a very
low ESR ceramic capacitor that leaves very little ripple. If the
output capacitor ESR is low, then Cff will have questionable benefit
regarding ripple sense timing, hence the need for R1b.

Agreed. The filter inductor has a phase lead effect somewhat
analogous to that of Cff.

This is why I
started to question the function of Cff around R1a in this
application. It seems like Cff will act more like an integrator,
slowing the rise time of the feedback pin as current flows through R1b
during Ton. Does this sound right?

Yes. If the output is well filtered at the switching frequency,
connecting Cff to it is just about the same as connecting Cff to
ground, but ground is more predictable.

Once you have the full 90 degrees lead produced by R1a, I would think
you could tune it by moderating the effect with Cff between the
feedback node and ground, rather than to the output. This gives you
essentially two knobs to turn, R1a for magnitude and Cff for phase
adjustment. The only advantage I can see to connecting Cff to the
output is to adapt the tuning to cover unit to unit variations in
output cap ESR. I would want to test any selected values for R1a and
Cff with a pair of output capacitors selected as examples of high and
low ESR extremes (in the final layout), to make sure the supply
doesn't misbehave in some strange way over this range.