MAX17020ETJ+T Maxim Integrated Products, MAX17020ETJ+T Datasheet - Page 27

• Manufacturer Part Number: MAX17020ETJ+T
• Description: IC CTLR PWM DUAL STEP DN 32-TQFN
• Manufacturer: Maxim Integrated Products
• Series: Quick-PWM™r

Specifications of MAX17020ETJ+T

• Applications: Power Supplies
• Current - Supply: 1mA
• Voltage - Supply: 6 V ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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
tions in the Transient Response section). However, low-
capacity filter capacitors typically have high ESR zeros
that could affect the overall stability (see the Output
Capacitor Stability Considerations section).
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 frequen-
cy 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
25mV/1.2A = 20.8mΩ. One 220μF/4V SANYO polymer
(TPE) capacitor provides 15mΩ (max) ESR. This results
in a zero at 48kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly
across the feedback sense point without taking precau-
tions 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
Output Capacitor Stability Considerations
R
f
ESR
ESR
______________________________________________________________________________________
=
≤
f
2π
ESR
I
Dual Quick-PWM Step-Down Controller
LOAD MAX
×
≤
R
with Low-Power LDO, RTC Regulator
V
ESR
RIPPLE
(
f
SW
π
1
×
SAG
)
SAG
C
×
OUT
LIR
and V
and V
SOAR
P-P
SOAR
ripple is
equa-
from
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.
The input capacitor must meet the ripple current
requirement (I
For most applications, nontantalum chemistries (ceramic,
aluminum, or OS-CON) are preferred due to their resis-
tance to power-up surge currents typical of systems
with a mechanical switch or connector in series with the
input. If the MAX17020 is operated as the second stage
of a two-stage power conversion system, tantalum input
capacitors are acceptable. In either configuration,
choose a capacitor that has less than 10°C temperature
rise at the RMS input current for optimal reliability and
lifetime.
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability when
using high-voltage (> 20V) AC adapters. Low-current
applications usually require less attention.
The high-side MOSFET (N
the resistive losses plus the switching losses at both
V
should be roughly equal to the losses at V
lower losses in between. If the losses at V
IN(MIN)
and V
I
RMS
RMS
=
IN(MAX)
I
LOAD
) imposed by the switching currents:
Input Capacitor Selection
×
Power-MOSFET Selection
. Ideally, the losses at V
⎛
⎜
⎝
H
V
) must be able to dissipate
OUT IN
(
V
V
IN
−
V
OUT
IN(MAX)
)
IN(MIN)
⎞
⎟
⎠
IN(MIN)
, with
are
27