Datasheet

MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
19
Maxim Integrated
where:
and:
where:
Output Capacitor
The output filter capacitor must have low enough ESR
to meet output ripple and load-transient requirements,
yet have high enough ESR to satisfy stability require-
ments. The output capacitance must be high enough to
absorb the inductor energy while transitioning from full-
load to no-load conditions without tripping the overvolt-
age fault protection. When using high-capacitance,
low-ESR capacitors, the filter capacitor’s ESR dominates
the output-voltage ripple. The size of the output capaci-
tor depends on the maximum ESR required to meet the
output-voltage ripple (V
RIPPLE(P-P)
) specifications:
In skip mode, the inductor current becomes discontinu-
ous, with the peak current set by the skip-mode cur-
rent-sense threshold (forced-peak I
L
). In skip mode, the
no-load output ripple can be determined as follows:
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.
When using low-value 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 overshoot
requirement, undershoot at the rising load edge is no
longer a problem (see the V
SAG
and V
SOAR
equations
in the Transient Response section). However, low-value
filter capacitors typically have high-ESR zeros that can
affect the overall stability.
Compensation Design
The MAX16955 uses an internal transconductance error
amplifier with its inverting input and its output available
to the user for external frequency compensation. The
output capacitor and compensation network determine
the loop stability. The inductor and the output capacitor
are chosen based on performance, size, and cost.
Additionally, the compensation network optimizes the
control-loop stability.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required
current through the external inductor. The MAX16955
uses the voltage drop across the DC resistance of the
inductor or the alternate series current-sense resistor to
measure the inductor current. Current-mode control
eliminates the double pole in the feedback loop caused
by the inductor and output capacitor, resulting in a
smaller phase shift and requiring less elaborate error-
amplifier compensation than voltage-mode control. A
simple single-series resistor (R
C
) and capacitor (C
C
)
are required to have a stable, high-bandwidth loop in
applications where ceramic capacitors are used for
output filtering (Figure 5). For other types of capacitors,
due to the higher capacitance and ESR, the frequency
of the zero created by the capacitance and ESR is
lower than the desired closed-loop crossover frequen-
cy. To stabilize a nonceramic output capacitor loop,
add another compensation capacitor (C
F
) from COMP
to SGND to cancel this ESR zero.
V
V ESR
R
RIPPLE P P
SKIP
SENSE
()−
=
×
V ESR I LIR
RIPPLE P P LOAD MAX() ( )−
=× ×
D
V
V
OUT
SUP
=
C
IDD
Vf
IN
OUT
QSW
=
×−
()
Δ×
1
Δ=
−
()
×
××
I
VV V
VfL
L
SUP OUT OUT
SUP SW
Figure 5. Compensation Network
R1
R
C
R2
C
C
C
F
V
OUT
V
REF
g
m
COMP