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-

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.

charging the MOSFET gate. EXT pin rise and fall times

vary with different capacitive loads, as shown in the

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.

LIM MAX

CS MAX

SENSE

I

V

R

)(

)

()

≥=

I

=

V

R

VV

1 s

L

PEAK

CS

CS

OUT

+

+−

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