MAX649 Maxim, MAX649 Datasheet - Page 11

• Manufacturer Part Number: MAX649
• Description: 5V/3.3V/3V or Adjustable / High-Efficiency /
• Manufacturer: Maxim

Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little induc-
tance and are well suited for use as current-sense
resistors. A wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5mΩ and 0.1Ω
are available (see Table 1).
Practical inductor values range from 10µH to 50µH or more.
The circuit operates in discontinuous-conduction mode if:
R, the switch on-time/off-time ratio, equals 6.7. V
diode’s drop, and V
P-channel FET. To get the full output capability in
discontinuous-conduction mode, choose an inductor
value no larger than:
where V
In both the continuous and discontinuous modes, the
lower limit of the inductor is more important. With a
small inductor value, the current rises faster and over-
shoots the desired peak current limit because the cur-
rent-limit comparator cannot respond fast enough. This
reduces efficiency slightly and, more importantly, could
cause the current rating of the external components
to be exceeded. Calculate the minimum inductor value
as follows:
where I is the percentage of inductor-current over-
shoot, where I
it takes the comparator to switch. An overshoot of 10%
is usually not a problem. Inductance values above the
minimum work well if the maximum value defined above
is not exceeded. Smaller inductance values cause
higher output ripple because of overshoot. Larger val-
ues tend to produce physically larger coils.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/I
minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc-
L(max) = —————————————————
L(min) = ————————————––——
CS
V + ≤ ———————— + —— + V
is the current-sense voltage.
LIM
R
5V/3.3V/3V or Adjustable, High-Efficiency,
V
SENSE
OUT
(V+(max) - V
______________________________________________________________________________________
= V
SW
x (R + 1)
R
CS
x 12µs x (V+ - V
is the voltage drop across the
/R
Low I
I x I
SENSE
SW
V
LIM
Inductor Selection
CS
- V
(min)
and 0.3µs is the time
V
OUT
LIM
R
D
Q
SW
) x 0.3µs
works best. To
, Step-Down DC-DC Controllers
SW
- V
OUT
D
)
is the
tor’s saturation-current rating is greater than I
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where the
inductance is 20% below its nominal value).
The peak current of Figure 1 is 2.35A for a 1.5A output.
The inductor used in this circuit is specified to drop by
10% at 2.2A (worst case); a curve provided by the
manufacturer shows that the inductance typically drops
by 20% at 3.1A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency. The MAX649/MAX651/MAX652
current limit prevents any damage from an underrated
inductor’s low inductance at high currents.
Table 1 lists inductor types and suppliers for various
applications. The efficiencies of the listed surface-
mount inductors are nearly equivalent to those of the
larger size through-hole versions.
The MAX649/MAX651/MAX652’s high switching fre-
quency demands a high-speed rectifier (commonly
called a catch diode when used in switching-regulator
circuits). Schottky diodes, such as the 1N5817 through
1N5822 families (and their surface-mount equivalents),
are recommended. Choose a diode with an average
current rating equal to or greater than I
voltage rating higher than V+(max). For high-tempera-
ture applications, where Schottky diodes can be
inadequate because of high leakage currents, use
high-speed silicon diodes instead. At heavy loads and
high temperatures, the disadvantages of a Schottky
diode’s high leakage current may outweigh the benefits
of its low forward voltage. Table 1 lists diode types and
suppliers for various applications.
The MAX649/MAX651/MAX652 drive P-channel
enhancement-mode MOSFET transistors only. The
choice of power transistor is primarily dictated by the
input voltage and the peak current. The transistor's
on-resistance, gate-source threshold, and gate
capacitance must also be appropriately chosen. The
drain-to-source and gate-to-source breakdown voltage
ratings must be greater than V+. The total gate-charge
specification is normally not critical, but values should
be less than 100nC for best efficiency. The MOSFET
should be capable of handling the peak current and,
for maximum efficiency, have a very low on-resistance
at that current. Also, the on-resistance must be low for
the minimum available V
Select a transistor with an on-resistance between 50%
and 100% of the current-sense resistor. The Si9430
transistor chosen for the Typical Operating Circuit has
External Switching Transistor
GS
, which equals V+(min).
Diode Selection
LIM
(max) and a
LIM
(max).
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