G
Greg Neff
Guest
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.
greg@guesswhichwordgoeshere.com
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.
greg@guesswhichwordgoeshere.com