MAX8543 MAXIM [Maxim Integrated Products], MAX8543 Datasheet - Page 23

• Manufacturer Part Number: MAX8543
• Description: Step-Down Controllers with Prebias Startup, Lossless Sensing, Synchronization, and OVP
• Manufacturer: MAXIM [Maxim Integrated Products]

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 ∆I
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.
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
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
GND to cancel this ESR zero.
C
Lossless Sensing, Synchronization, and OVP
Step-Down Controllers with Prebias Startup,
and C
C
______________________________________________________________________________________
is all that is needed to have a stable,
Compensation Design
F
) from COMP to
LOAD
.
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
with a pole and zero pair set by R
capacitor (C
that define the power modulator:
where R
frequency, L is the output inductance, and g
1 / (A
rent-sense amplifier and R
the inductor (or current-sense resistor). A
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
parallel, the resulting C
= ESR
allel combination of like capacitors is the same as for an
individual capacitor.
The feedback voltage-divider has a gain of G
V
The transconductance error amplifier has a DC gain,
G
transconductance, which is equal to 110µS, R
output resistance of the error amplifier, which is 10MΩ.
A dominant pole is set by the compensation capacitor
(C
set by the compensation resistor (R
sation capacitor (C
C
occurs near the crossover frequency (f
OUT
F
EA(DC)
C
and R
), the amplifier output resistance (R
f
pMOD
, where V
VCS
(EACH)
OUT
LOAD
= g
C
G
× R
=
to cancel the output-capacitor ESR zero if it
MOD dc
mEA
is composed of “n” identical capacitors in
OUT
f
2π
/ n. Note that the capacitor zero for a par-
pdEA
DC
f
FB
= V
zMOD
×
(
x R
), where A
), and its ESR. Below are equations
is equal to 0.8V.
C
OUT
C
)
OUT
=
O
=
). There is an optional pole set by
=
, where g
2π
g
OUT
2π
/ I
mc
×
×
OUT(MAX)
⎛
⎜
⎝
×
C
×
R
= n x C
DC
R
C
C
VCS
LOAD
R
LOAD
R
OUT
LOAD
×
LOAD
1
mEA
1
1
is the DC resistance of
(
R
is the gain of the cur-
O
+
×
×
OUT(EACH)
, f
C
(
ESR
is the error-amplifier
+
+
×
f
f
) and the compen-
S
S
(
S
LOAD
R
f
f
C
S
S
×
×
O
C
is the switching
). Thus:
L
L
×
×
), and a zero is
)
)
L
L
+
mc
, the output
)
ESR
FB
, and ESR
x R
O
⎞
⎟
⎠
= V
VCS
LOAD
is the
mc
FB
23
is
=
,
/