Datasheet

= 1 -
K
1
dl
0
dl
1
Steady-State
Inductor
Current
dI
0
dI
1
t
ON
Inductor Current with
Initial Perturbation
K - 0.5
f
SW
Z
p_HF
=
LM25117
LM25117-Q1
SNVS714E –APRIL 2011–REVISED MARCH 2013 www.ti.com
For higher crossover frequency, R
COMP
can be increased, while proportionally decreasing C
COMP
. Conversely,
decreasing R
COMP
while proportionally increasing C
COMP
, results in lower bandwidth while keeping the same zero
frequency in the feedback transfer function.
The sampled gain inductor pole is inversely proportional to the K factor, which is defined as:
(22)
The maximum achievable loop bandwidth, in fact, is limited by this sampled gain inductor pole. In traditional
current mode control, the maximum achievable loop bandwidth varies with input voltage. With the LM25117’s
unique slope compensation scheme, the sampled gain inductor pole is independent of changes to the input
voltage. This frees the user from additional concerns in wide varying input range applications and is an
advantage of the LM25117.
If the sampled gain inductor pole or the ESR zero is close to the crossover frequency, it is recommended that the
comprehensive formulas in Table 1 be used and the stability should be checked by a network analyzer. The
modulator transfer function can be measured and the feedback transfer function can be configured for the
desired open loop transfer function. If a network analyzer is not available, step load transient tests can be
performed to verify acceptable performance. The step load goal is minimum overshoot/undershoot with a
damped response.
SUB-HARMONIC OSCILLATION
Peak current mode regulators can exhibit unstable behavior when operating above 50% duty cycle. This
behavior is known as sub-harmonic oscillation and is characterized by alternating wide and narrow pulses at the
SW pin. Sub-harmonic oscillation can be prevented by adding an additional voltage ramp (slope compensation)
on top of the sensed inductor current shown in Figure 23. By choosing K≥1, the regulator will not be subject to
sub-harmonic oscillation caused by a varying input voltage.
In time-domain analysis, the steady-state inductor current starts and ends at the same value during one clock
cycle. If the magnitude of the end-of-cycle current error, dI
1
, caused by an initial perturbation, dI
0
, is less than the
magnitude of dI
0
or dI
1
/dI
0
> -1, the perturbation naturally disappears after a few cycles. When dI
1
/dI
0
< -1, the
initial perturbation does not disappear, resulting in sub-harmonic oscillation in steady-state operation.
Figure 32. Effect of Initial Perturbation when dl
1
/dl
0
< -1
dI
1
/dI
0
can be calculated by:
(23)
The relationship between dI
1
/dI
0
and K factor is illustrated graphically in Figure 33.
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