MAX761/MAX762
Selecting the Inductor (L)
In both CCM and DCM, practical inductor values range
from 10µH to 50µH. If the inductor value is too low, the
current in the coil will ramp up to a high level before the
current-limit comparator can turn off the switch. The mini-
mum on-time for the switch (t
ON(min)
) is approximately
2.5µs, so select an inductance that allows the current to
ramp up to I
LIM
/
2
in no less than 2.5µs. Choosing a value
of I
LIM
/
2
allows the half-size pulses to occur, giving high-
er light-load efficiency and minimizing ripple. Hence, cal-
culate the minimum inductance value as:
L ≥
(V
IN(max)
)(t
ON(min)
)
I
LIM
/2
OR
L ≥ (V
IN(max)
)(5)
where V
IN(max)
is in volts and L is in microhenries.
The coil’s inductance need not satisfy this criterion
exactly, as the circuit can tolerate a wide range of val-
ues. Larger inductance values tend to produce physical-
ly larger coils and increase the start-up time, but are oth-
erwise acceptable. Smaller inductance values allow the
coil current to ramp up to higher levels before the switch
can turn off, producing higher ripple at light loads. In
general, an 18µH inductor is sufficient for most applica-
tions (V
IN
≤ 5V). An 18µH inductor is appropriate for
input voltages up to 3.6V, as calculated above. However,
the same 18µH coil can be used with input voltages up
to 5V with only small increases in peak current, as shown
in Figures 4a and 4b.
Inductors with a ferrite core or equivalent are recom-
mended. The inductor’s incremental saturation-current
rating should be greater than the 1A peak current limit. It
is generally acceptable to bias the inductor into satura-
tion by approximately 20% (the point where the induc-
tance is 20% below the nominal value). For highest effi-
ciency, use a coil with low DC resistance, preferably
under 100mΩ. To minimize radiated noise, use a toroid,
a pot core, or a shielded coil.
Table 1 lists inductor types and suppliers for various
applications. The listed surface-mount inductors’ efficien-
cies are nearly equivalent to those of the larger through-
hole inductors.
Diode Selection
The MAX761/MAX762’s high switching frequency
demands a high-speed rectifier. Use a Schottky diode
with a 1A average current rating, such as a 1N5817. For
high-temperature applications, use a high-speed silicon
diode, such as the MUR105 or the EC11FS1. These
diodes have lower high-temperature leakage than
Schottky diodes (Table 1).
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter capac-
itor (C4) is low effective series resistance (ESR). The
product of the inductor current variation and the output
filter capacitor’s ESR determines the amplitude of the
high-frequency ripple seen on the output voltage. A
33µF, 16V Sanyo OS-CON capacitor with 100mΩ ESR
typically provides 100mV ripple when stepping up from
5V to 12V at 150mA.
Because the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. The
smallest low-ESR SMT tantalum capacitors currently
available are the Sprague 595D series. Sanyo OS-CON
organic semiconductor through-hole capacitors and
Nichicon PL series also exhibit very low ESR. Table 1
lists some suppliers of low-ESR capacitors.
Input Bypass Capacitors
The input bypass capacitor, C1, reduces peak currents
drawn from the voltage source, and also reduces noise
at the voltage source caused by the MAX761/MAX762’s
switching action. The input voltage source impedance
determines the size of the capacitor required at the V+
input. As with the output filter capacitor, a low-ESR
capacitor is recommended. For output currents up to
250mA, 33µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable. Bypass the IC sepa-
rately with a 0.1µF ceramic capacitor, C2, placed close
to the V+ and GND pins.
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
10 ______________________________________________________________________________________
V
IN
LX
GND
MAX761
MAX762
SHDN
V+
REF
L1
18µH
LBI
FB
D1
1N5817
C2
C4
V
OUT
C3
R2
8
2
5
4
7
6
3
R2 = R1
( -1)
V
OUT
V
REF
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
C1
R1
V
REF
= 1.5V NOMINAL
Figure 5. Bootstrapped Operation with Adjustable Output
