MAX1626 Datasheet, PDF (10/16 Pages) Maxim Integrated Products – 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers

5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
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.
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.
( ) _IPEAK
= ^VCS +
R _CS
V + − V_OUT × 1µs
L
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
are not recommended for use with high switching
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,
KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.
or shielded-core inductor to minimize radiated noise.
Table 1 lists inductor types and suppliers for various
applications.
External Switching Transistor
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.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Qg), and RDS(ON). The
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:
I_D(MAX) ≥ I_LIM(MAX)
=
V_CS(MAX)
R_SENSE
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.
Qg specifies all of the capacitances associated with
charging the MOSFET gate. EXT pin rise and fall times
vary with different capacitive loads, as shown in the
Typical Operating Characteristics. RDS(ON) should be
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.
Table 1. Component Selection Guide
PRODUCTION
METHOD
Surface Mount
Miniature
Through-Hole
Low-Cost
Through-Hole
INDUCTORS
CAPACITORS
DIODES
Sumida
CDRH125-470 (1.8A) AVX
CDRH125-220 (2.2A) TPS series
Motorola
MBRS340T3
CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Sprague
595D series
Nihon
NSQ series
Sumida
RCH875-470M (1.3A)
Sanyo
OS-CON series
low-ESR organic
semiconductor
CoilCraft
PCH-45-473 (3.4A)
Nichicon
PL series
low-ESR electrolytics
United Chemi-Con
LXF series
Motorola
1N5817 to
1N5823
CURRENT-SENSE
RESISTORS
MOSFETS
Dale
WSL series
IRC
LRC series
Siliconix
Little Foot series
Motorola
medium-power
surface-mount products
IRC
OAR series
Motorola
Motorola
TMOS power MOSFETs
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