MAX863
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
12 ______________________________________________________________________________________
Select the Inductor Component
Two essential parameters are required for selecting the
inductor: inductance and current rating.
Inductance should be low enough to allow the MAX863
to reach the peak current limit during each cycle before
the 17.5µs maximum on-time. Conversely, if the induc-
tance is too low, the current will ramp up to a high level
before the current-sense comparator can turn the
switch off. A practical minimum on-time (t
ON(MIN)
) is
1.5µs.
and:
When selecting I
PEAK
using the graphs in Figure 5,
choose inductance values between 1.3 and 1.7 times
the minimum inductance value to provide a good trade-
off between switching frequency and efficiency.
The lower of the inductor saturation current rating or
heating current rating should be greater than I
PEAK
:
I
SATURATION
and I
HEATING
> I
PEAK
The saturation current limit is the current level where
the magnetic field in the inductor has reached the max-
imum the core can sustain, and inductance starts to
fall. The heating current rating is the maximum DC cur-
rent the inductor can sustain without overheating.
Disregarding the inductor’s saturation current rating is
a common mistake that results in poor efficiency, bad
regulation, component overheating, or other problems.
The resistance of the inductor windings should be com-
parable to or less than that of the current-sense
resistor. To minimize radiated noise in sensitive
applications, use a toroid, pot core, or shielded bobbin
core inductor.
Choose the MOSFET Power Transistor
Use N-channel MOSFETs with the MAX863. When
selecting an N-channel MOSFET, five important para-
meters are gate-drive voltage, drain-to-source break-
down voltage, current rating, on-resistance (R
DS(ON)
),
and total gate charge (Q
g
).
The MAX863’s EXT1 and EXT2 outputs swing from
GND to V
DD
. To ensure the external N-channel MOS-
FET is turned on sufficiently, use logic-level MOSFETs
when V
DD
is less than 8V and low-threshold logic-level
MOSFETs when starting from input voltages below 4V.
This also applies in bootstrapped mode to ensure
start-up.
The MOSFET in a simple boost converter must with-
stand the output voltage plus the diode forward volt-
age. Voltage ratings in SEPIC, flyback, and
autotransformer-boost circuits are more stringent.
Choose a MOSFET with a maximum continuous drain-
current rating higher than the current limit set by CS.
The two most significant losses contributing to the
MOSFET’s power dissipation are I
2
R losses and switch-
ing losses. Reduce I
2
R losses by choosing a MOSFET
with low R
DS(ON)
, preferably near the current-sense
resistor value or lower.
A MOSFET with a gate charge (Q
g
) of 50nC or smaller
is recommended for rise and fall times less than 100ns
on the EXT pins. Exceeding this limit results in slower
MOSFET switching speeds and higher switching loss-
es, due to a longer transition time through the linear
region as the MOSFET turns on and off.
Select the Output Diode
Schottky diodes, such as the 1N5817–1N5822 family or
surface-mount equivalents, are recommended. Ultra-
fast silicon rectifiers with reverse recovery times around
60ns or faster, such as the MUR series, are acceptable
but have greater forward voltage drop. Make sure that
the diode’s peak current rating exceeds the current
limit set by R
SENSE
, and that its breakdown voltage
exceeds V
OUT
. Schottky diodes are preferred for heavy
loads, especially in low-voltage applications, due to
their low forward voltage. For high-temperature applica-
tions, some Schottky diodes may be inadequate due to
high leakage currents. In such cases, ultra-fast silicon
rectifiers are recommended, although acceptable per-
formance can often be achieved by using a Schottky
diode with a higher reverse voltage rating.
Determine Input and Output Filter
Capacitors
Low-ESR capacitors are recommended for both input
bypassing and output filtering. Capacitor equivalent
series resistance (ESR) is a major contributor to output
ripple—typically 60% to 90%. Low-ESR tantalum
capacitors offer a good tradeoff between price and
performance. Ceramic and Sanyo OS-CON capacitors
have the lowest ESR. Ceramic capacitors are often a
good choice in high-output-voltage applications where
large capacitor values may not be needed. Low-ESR
aluminum-electrolytic capacitors are tolerable and can
be used when cost is the primary consideration; how-
ever, standard aluminum-electrolytic capacitors should
be avoided.
