5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
Table 1. Component Selection Guide
PRODUCTION
METHOD
CURRENT-SENSE
RESISTORS
INDUCTORS
CAPACITORS
DIODES
MOSFETS
Sumida
CDRH125-470 (1.8A) AVX
CDRH125-220 (2.2A) TPS series
Siliconix
Little Foot series
Motorola
MBRS340T3
Dale
WSL Series
Surface Mount
Motorola
medium-power
surface-mount products
CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Sprague
595D series
Nihon
NSQ series
IRC
LRC series
Sanyo
Miniature
Through-Hole
Sumida
OS-CON series
IRC
OAR series
Motorola
RCH875-470M (1.3A) low-ESR organic
semiconductor
Nichicon
PL series
low-ESR electrolytics
Motorola
1N5817 to
1N5823
Low-Cost
Through-Hole
CoilCraft
PCH-45-473 (3.4A)
Motorola
TMOS power MOSFETs
49/MAX651
United Chemi-Con
LXF series
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 2.7A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency.
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
,
GS
which equals V+(min). Select a transistor with an on-
re s is ta nc e b e twe e n 50% a nd 100% of the c urre nt-
sense resistor. The Si9430 transistor chosen for the
Typical Operating Circuit has a drain-to-source rating
of -20V and a typical on-resistance of 0.070Ω at 2A with
VGS = -4.5V. Tables 1 and 2 list suppliers of switching
transistors suitable for use with these devices.
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.
Dio d e S e le c t io n
The MAX1649/MAX1651’s high switching frequency
demands a high-speed rectifier. Schottky diodes, such
as the 1N5817 through 1N5823 (and their surface-
mount e q uiva le nts ), a re re c omme nd e d . Choos e a
diode with an average current rating equal to or greater
Ca p a c it o r S e le c t io n
Output Filter Capacitor
The p rima ry c rite rion for s e le c ting the outp ut filte r
capacitor is low equivalent series resistance (ESR),
rather than high capacitance. An electrolytic capacitor
with low e noug h ESR will a utoma tic a lly ha ve hig h
enough capacitance. The product of the inductor-cur-
re nt va ria tion a nd the outp ut filte r c a p a c itor’s ESR
determines the amplitude of the high-frequency ripple
s e e n on the outp ut volta g e . Whe n a 330µF, 10V
Sprague surface-mount capacitor (595D series) with
ESR = 0.15Ω is used, 40mV of output ripple is typically
observed when stepping down from 10V to 5V at 1A.
The output filter capacitor's ESR also affects efficiency.
Again, low-ESR capacitors perform best. Table 1 lists
some suppliers of low-ESR capacitors.
tha n I
(ma x) a nd a volta g e ra ting hig he r tha n
LIM
V+(max).
Ex t e rn a l S w it c h in g Tra n s is t o r
The MAX1649/MAX1651 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 charge must also be appro-
p ria te ly c hos e n. The d ra in-to-s ourc e a nd g a te -to-
source breakdown voltage ratings must be greater than
V+. The total gate-charge specification is normally not
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
