Datasheet

MAX17113
Low-Cost, Multiple-Output
Power Supply for LCD TVs
______________________________________________________________________________________ 25
The output-voltage ripple has two components: varia-
tions in the charge stored in the output capacitor, and
the voltage drop across the capacitor’s ESR caused by
the current into and out of the capacitor:
where I
OUT
_
RIPPLE
is defined in the Inductor Selection
of the Step-Down Regulator section, C
OUT
is the output
capacitance, and R
ESR
_
OUT
is the ESR of the output
capacitor C
OUT
. In Figure 1’s circuit, the inductor ripple
current is 0.8A. If the voltage-ripple requirement of
Figure 1’s circuit is ±1% of the 3.3V output, then the
total peak-to-peak ripple voltage should be less than
66mV. Assuming that the ESR ripple and the capacitive
ripple each should be less than 50% of the total peak-
to-peak ripple, then the ESR should be less than 43mΩ
and the output capacitance should be more than 5μF to
meet the total ripple requirement. A 22μF capacitor with
ESR (including PCB trace resistance) of 10mΩ is select-
ed for the standard application circuit in Figure 1, which
easily meets the voltage-ripple requirement.
The step-down regulator’s output capacitor and ESR can
also affect the voltage undershoot and overshoot when
the load steps up and down abruptly. The step-down
regulator’s transient response is typically dominated by
its loop response and the time constant of its internal
integrator. However, excessive inductance or insufficient
output capacitance can degrade the natural transient
response. Calculating the ideal transient response of
the inductor and capacitor, which assumes an ideal
response from the regulator, can ensure that these
components do not degrade the IC’s natural response.
The ideal undershoot and overshoot have two compo-
nents: the voltage steps caused by ESR, and the voltage
sag and soar due to the finite capacitance and the induc-
tor current slew rate. Use the following formulas to check
if the ESR is low enough and the output capacitance is
large enough to prevent excessive soar and sag.
The amplitude of the ESR step is a function of the load
step and the ESR of the output capacitor:
The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor
value, the input-to-output voltage differential, and the
maximum duty cycle:
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor
value, and the output voltage:
Keeping the full-load overshoot and undershoot less
than 3% ensures that the step-down regulator’s natural
integrator response dominates. Given the component
values in the circuit of Figure 1 and assuming a full 1.5A
step load transient, the voltage step due to capacitor
ESR is negligible. The voltage sag and soar are 76mV
and 73mV, or a little over 1% and 2%, respectively.
Rectifier Diode
The MAX17113’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 2A Schottky diode
works well in the MAX17113’s step-down regulator.
Step-Up Regulator
Inductor Selection
The inductance value, peak current rating, and series
resistance are factors to consider when selecting the
inductor. These factors influence the converter’s effi-
ciency, maximum output load capability, transient
response time, and output-voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
rent ripple, and therefore, reduce the peak current,
which decreases core losses in the inductor and I
2
R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire that increase physical size and can
increase I
2
R losses in the inductor. Low inductance val-
ues decrease the physical size, but increase the cur-
rent ripple and peak current. Finding the best inductor
involves choosing the best compromise among circuit
efficiency, inductor size, and cost.
The equations used here include a constant, LIR, which
is the ratio of the inductor peak-to-peak ripple current to
the average DC inductor current at the full-load current.
The best trade-off between inductor size and circuit effi-
ciency for step-up regulators generally has an LIR
between 0.2 and 0.5. However, depending on the AC
V
LI
CV
OUT SOAR
OUT OUT
OUT OUT
_
()
=
×
××
Δ
2
2
V
LI
CV D
OUT SAG
OUT OUT
OUT IN MIN MAX
_
()
()
=
×
×× ×
Δ
2
2-VV
OUT
()
VIR
OUT ESR STEP OUT ESR OUT__ _
=×Δ
V
I
Cf
OUT RIPPLE C
OUT RIPPLE
OUT SW
_()
_
=
××8
VIR
OUT RIPPLE ESR OUT RIPPLE ESR OUT_() _ _
=×
VV V
OUT RIPPLE OUT RIPPLE ESR OUT RIPPLE C__()_()
=+