MAX8543/MAX8544
Step-Down Controllers with Prebias Startup,
Lossless Sensing, Synchronization, and OVP
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The aluminum electrolytic capacitor is the least expen-
sive; however, it has higher ESR. To compensate for this,
use a ceramic capacitor in parallel to reduce the switch-
ing ripple and noise. For reliable and safe operation,
ensure that the capacitor’s voltage and ripple-current rat-
ings exceed the calculated values.
The response to a load transient depends on the
selected output capacitors. After a load transient, the
output voltage instantly changes by ESR x ΔI
LOAD
.
Before the controller can respond, the output voltage
deviates further depending on the inductor and output
capacitor values. After a short period of time (see the
Typical Operating Characteristics), the controller
responds by regulating the output voltage back to its
nominal state. The controller response time depends on
its closed-loop bandwidth. With a higher bandwidth,
the response time is faster, thus preventing the output
voltage from further deviation from its regulation value.
Compensation Design
The MAX8543/MAX8544 use an internal transconduc-
tance error amplifier whose output compensates the
control loop. The external inductor, output capacitor,
compensation resistor, and compensation capacitors
determine the loop stability. The inductor and output
capacitor are chosen based on performance, size, and
cost. Additionally, the compensation resistor and capaci-
tors are selected to optimize control-loop stability. The
component values, shown in the Typical Application
Circuits (Figures 1 and 2), yield stable operation over the
given range of input-to-output voltages.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required cur-
rent through the external inductor, so the MAX8543/
MAX8544 use the voltage drop across the DC resistance
of the inductor or the alternate series current-sense resis-
tor 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 a less elaborate error-amplifier
compensation than voltage-mode control. A simple single
series R
C
and C
C
is all that is needed to have a stable,
high-bandwidth loop in applications where ceramic
capacitors are used for output filtering. 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 fre-
quency. To stabilize a nonceramic output-capacitor loop,
add another compensation capacitor (C
F
) from COMP to
GND to cancel this ESR zero.
The basic regulator loop is modeled as a power modu-
lator, output feedback divider, and an error amplifier.
The power modulator has DC gain set by g
mc
x R
LOAD
,
with a pole and zero pair set by R
LOAD
, the output
capacitor (C
OUT
), and its ESR. Below are equations
that define the power modulator:
where R
LOAD
= V
OUT
/ I
OUT(MAX)
, f
S
is the switching
frequency, L is the output inductance, and g
mc
=
1 / (A
VCS
× R
DC
), where A
VCS
is the gain of the cur-
rent-sense amplifier and R
DC
is the DC resistance of
the inductor (or current-sense resistor). A
VCS
is
dependent on the current-limit selection at ILIM, and
ranges from 3 to 11 (see Current-Sense Amplifier
Voltage Gain in the Electrical Characteristics table).
The frequencies at which the pole and zero created by
the power modulator are determined as follows:
When C
OUT
is composed of “n” identical capacitors in
parallel, the resulting C
OUT
= n x C
OUT(EACH)
, and ESR
= ESR
(EACH)
/ n. Note that the capacitor zero for a par-
allel combination of like capacitors is the same as for an
individual capacitor.
The feedback voltage-divider has a gain of G
FB
= V
FB
/
V
OUT
, where V
FB
is equal to 0.8V.
The transconductance error amplifier has a DC gain,
G
EA(DC)
= g
mEA
x R
O
, where g
mEA
is the error-amplifier
transconductance, which is equal to 110µS, R
O
is the
output resistance of the error amplifier, which is 10MΩ.
A dominant pole is set by the compensation capacitor
(C
C
), the amplifier output resistance (R
O
), and a zero is
set by the compensation resistor (R
C
) and the compen-
sation capacitor (C
C
). There is an optional pole set by
C
F
and R
C
to cancel the output-capacitor ESR zero if it
occurs near the crossover frequency (f
C
). Thus:
