Rainbow Electronics MAX17031 User Manual
Page 21
In applications without large and fast load transients,
the output capacitor’s size often depends on how much
ESR is needed to maintain an acceptable level of out-
put voltage ripple. The output ripple voltage of a step-
down controller equals the total inductor ripple current
multiplied by the output capacitor’s ESR. Therefore, the
maximum ESR required to meet ripple specifications is:
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as to
the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value (this is true of tanta-
lums, OS-CONs, polymers, and other electrolytics).
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V
SAG
and V
SOAR
from
causing problems during load transients. Generally,
once enough capacitance is added to meet the over-
shoot requirement, undershoot at the rising load edge
is no longer a problem (see the V
SAG
and V
SOAR
equa-
tions in the
Transient Response
section). However, low-
capacity filter capacitors typically have high ESR zeros
that might affect the overall stability (see the
Output
Capacitor Stability Considerations
section).
Output Capacitor Stability Considerations
For Quick-PWM controllers, stability is determined by
the value of the ESR zero relative to the switching fre-
quency. The boundary of instability is given by the fol-
lowing equation:
where:
For a typical 300kHz application, the ESR zero frequency
must be well below 95kHz, preferably below 50kHz.
Tantalum and OS-CON capacitors in widespread use at
the time of publication have typical ESR zero frequen-
cies of 25kHz. In the design example used for inductor
selection, the ESR needed to support 25mV
P-P
ripple is
25mV/1.2A = 20.8m
Ω. One 220µF/4V SANYO polymer
(TPE) capacitor provides 15m
Ω (max) ESR. This results
in a zero at 48kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly on
OUT1 and OUT2 pins to ensure stability. Large ceramic
capacitors can have a high-ESR zero frequency and
cause erratic, unstable operation. However, it is easy to
add enough series resistance by placing the capacitors
a couple of inches downstream from the feedback
sense point, which should be as close as possible to
the inductor.
Unstable operation manifests itself in two related but
distinctly different ways: double-pulsing and fast-feed-
back loop instability. Double-pulsing occurs due to
noise on the output or because the ESR is so low that
there is not enough voltage ramp in the output-voltage
signal. This “fools” the error comparator into triggering
a new cycle immediately after the 400ns minimum off-
time period has expired. Double-pulsing is more annoy-
ing than harmful, resulting in nothing worse than
increased output ripple. However, it can indicate the
possible presence of loop instability due to insufficient
ESR. Loop instability results in oscillations at the output
after line or load steps. Such perturbations are usually
damped, but can cause the output voltage to rise
above or fall below the tolerance limits.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully
observe the output-voltage ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Do not
allow more than one cycle of ringing after the initial
step-response under/overshoot.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents:
For most applications, nontantalum chemistries (ceram-
ic, aluminum, or OS-CON) are preferred due to their
resistance to power-up surge currents typical of sys-
tems with a mechanical switch or connector in series
with the input. If the MAX17031 is operated as the sec-
ond stage of a two-stage power conversion system,
tantalum input capacitors are acceptable. In either con-
figuration, choose a capacitor that has less than 10°C
temperature rise at the RMS input current for optimal
reliability and lifetime.
I
I
V
V
V
V
RMS
LOAD
OUT
IN
OUT
IN
=
−
(
)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
f
R
C
ESR
ESR
OUT
=
Ч
Ч
1
2
π
f
f
ESR
SW
≤
π
R
V
I
LIR
ESR
RIPPLE
LOAD MAX
≤
(
)
R
V
I
ESR
STEP
LOAD MAX
≤
∆
(
)
MAX17031
Dual Quick-PWM Step-Down Controller with Low-
Power LDO and RTC Regulator for MAIN Supplies
______________________________________________________________________________________
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