Low ESR caps for LDO regulator

[John Popelish]

The datasheet for REG1117 specifies only the upper boundary on the ESR
and the lower boundary on the capacity. Does it mean that one can add
the ceramic caps on the output rail without a stability problem, once
the base 10uF ESR<0.5 Ohm tantalum capacitor is there?

I don't think so. If the other parallel capacitors added up to much
more than the lumped capacitance at the regulator, they would begin to
dominate the phase of the current at high frequencies.

 

[Andy]

Andy wrote:




I don't think so. If the other parallel capacitors added up to much
more than the lumped capacitance at the regulator, they would begin to
dominate the phase of the current at high frequencies.

Okay, how is one supposed to evaluate the stability / dynamic
characteristics of a real circuit powered by a LM/REG/LD/LT/NCP/...1117?
No model seems to be available from the manufacturers. Only through test
and trial??

Suppose I am powering by an LM1117 an IC, with the tantalum as required
at the output of the regulator. The IC has 10 ceramic 0.1uF caps spread
around, from the power rail to ground. Is there a minimum trace
inductance to observe between the regulator and the first 0.1uF, that
would preserve good dynamic response of the reg.?

I understand that there may be no response easily available. If this is
a really grey area, and everyone has to make his own tests, why not
simply go with some AnyCAP from AD?

Thank you.

-- Andy

 

[John Popelish]

Okay, how is one supposed to evaluate the stability / dynamic
characteristics of a real circuit powered by a LM/REG/LD/LT/NCP/...1117?
No model seems to be available from the manufacturers. Only through test
and trial??

Suppose I am powering by an LM1117 an IC, with the tantalum as required
at the output of the regulator. The IC has 10 ceramic 0.1uF caps spread
around, from the power rail to ground. Is there a minimum trace
inductance to observe between the regulator and the first 0.1uF, that
would preserve good dynamic response of the reg.?

I understand that there may be no response easily available. If this is
a really grey area, and everyone has to make his own tests, why not
simply go with some AnyCAP from AD?

I think that this is the big problem with LDO regulators. Their
stability is highly load dependent. When I have used them, I
performed stability tests (injected disturbances), varying all
relevant component values over a wider range than tolerances would
account for, in the actual layout, and using several different
regulator units from different production batches. This means that
the board layout had to include provisions for alternatives.

But I always worry about the generality of the solution I arrive at,
especially if the regulator or load undergoes any significant
revision.

 

[Anders F]

Not so paranoid. Supply rail mechanical-intermittent short is the kind
of fault that should cause no damage to a power supply. No small smd
film resistors are really appropriate in series with larger caps - but
below 220uF and at low voltage, is less than 100mJ per pop.

How would one determine the "mJ" capacity of standard 0603/0805/1210
resistors?

One could hope for a decent ramp-up of the LDO and put it in series with the
load (?). Of course it will see the entire DC-load and drop the voltage a
bit at high loads... But it won't get the "AC-kicks" and limit the caps.

/Anders