Rainbow Electronics MAX1627 - Manual (Page 10)

MAX1626/MAX1627
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.
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,
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 (Q
g ), and R DS(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:
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.
Q
g 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. R
DS(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.
I
D(MAX LIM MAX
CS MAX
SENSE
I
V
R
) ( )
( ) ≥ =
I =
V
R
V V 1 s
L
PEAK
CS
CS
OUT +
+ −
( )
× µ
5V/3.3V or Adjustable, 100% Duty-Cycle,
High-Efficiency, Step-Down DC-DC Controllers
10 ______________________________________________________________________________________
KOOL Mu is a trademark of Magnetics.
METGLAS is a trademark of Allied Signal.
PRODUCTION
METHOD
INDUCTORS CAPACITORS DIODES
CURRENT-SENSE
RESISTORS
MOSFETS
Surface Mount
AVX
TPS series
Sprague
595D series
Motorola
MBRS340T3
Nihon
NSQ series
Dale
WSL series
IRC
LRC series
Miniature
Through-Hole Sumida
RCH875-470M (1.3A)
Sanyo
OS-CON series
low-ESR organic
semiconductor
IRC
OAR series
Motorola
Low-Cost
Through-Hole CoilCraft
PCH-45-473 (3.4A)
Motorola
1N5817 to
1N5823
Motorola
TMOS power MOSFETs
Sumida
CDRH125-470 (1.8A)
CDRH125-220 (2.2A)
CoilCraft
DO3316-473 (1.6A)
DO3340-473 (3.8A)
Siliconix
Little Foot series
Motorola
medium-power
surface-mount products
Nichicon
PL series
low-ESR electrolytics
United Chemi-Con
LXF series
Table 1. Component Selection Guide