5 V/3 .3 V o r Ad ju s t a b le , 1 0 0 % Du t y-Cyc le ,
Hig h -Effic ie n c y, S t e p -Do w n DC-DC Co n t ro lle rs
With high inductor values, the MAX1626/MAX1627 will
begin continuous-conduction operation at a lower frac-
tion of the full load (see Detailed Description). Low-value
inductors may be smaller and less expensive, but they
result in greater peak current overshoot due to current-
sense comparator propagation delay. Peak-current
overshoot reduces efficiency and could cause the exter-
nal components’ current ratings to be exceeded.
or shielded-core inductor to minimize radiated noise.
Table 1 lists inductor types and suppliers for various
applications.
Ex t e rn a l S w it c h in g Tra n s is t o r
The MAX1626/MAX1627 drive P-channel enhancement-
mode MOSFETs. The EXT output swings from GND to
the voltage at V+. To ensure the MOSFET is fully on,
use logic-level or low-threshold MOSFETs when the
input voltage is less than 8V. Tables 1 and 2 list recom-
mended suppliers of switching transistors.
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to pre-
vent overheating and core saturation. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and inductance
starts to fall. The heating current rating is the maximum
DC current the inductor can sustain without overheating.
The peak switching current is the sum of the current limit
set by the current-sense resistor and overshoot during
current-sense comparator propagation delay.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Q ), and R
. The
g
DS(ON)
drain-to-source breakdown voltage rating should be at
least a few volts higher than V+. Choose a MOSFET
with a maximum continuous drain current rating higher
than the peak current limit:
/MAX1627
V
CS(MAX)
V+ − V
×1µs
(
+
)
V
OUT
I
≥ I
=
CS
D(MAX)
LIM(MAX)
I
=
PEAK
R
SENSE
R
L
CS
The Qg specification should be less than 100nC to
ensure fast drain voltage rise and fall times, and reduce
power losses during transition through the linear region.
1µs is the worst-case current-sense comparator propa-
gation delay.
Inductors with a core of ferrite, Kool Mu™, METGLAS™,
or equivalent, are recommended. Powder iron cores
a re not re c omme nd e d for us e with hig h s witc hing
frequencies. For optimum efficiency, the inductor wind-
ings’ resistance should be on the order of the current-
sense resistance. If necessary, use a toroid, pot-core,
Q
specifies all of the capacitances associated with
g
charging the MOSFET gate. EXT pin rise and fall times
vary with different capacitive loads, as shown in the
Typical Operating Characteristics. R
should be
DS(ON)
as low as practical to reduce power losses while the
MOSFET is on. It should be equal to or less than the
current-sense resistor.
KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.
Table 1. Component Selection Guide
PRODUCTION
METHOD
CURRENT-SENSE
RESISTORS
INDUCTORS
Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)
CAPACITORS
DIODES
MOSFETS
Siliconix
AVX
TPS series
Motorola
MBRS340T3
Dale
WSL series
Little Foot 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
Motorola
TMOS power MOSFETs
CoilCraft
PCH-45-473 (3.4A)
United Chemi-Con
LXF series
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
