is used to compensate the MAX1627 for even switch-

ing. Values between 0pF and 330pF work for many

applications. See the

Stability and MAX1627 Feedback

Compensation section for details.

Current-Sense-Resistor Selection

The current-sense comparator limits the peak switching

85mV to 115mV. Minimizing the peak switching current

will increase efficiency and reduce the size and cost of

external components. However, since available output

current is a function of the peak switching current, the

peak current limit must not be set too low.

Set the peak current limit above 1.3 times the maximum

load current by setting the current-sense resistor to:

Alternatively, select the current-sense resistor for 5V

and 3.3V output applications using the current-sense

resistor graphs in Figures 6a and 6b. The current-sense

resistor’s power rating should be 20% higher than:

Standard wire-wound resistors have an inductance

high enough to degrade performance, and are not rec-

ommended. Surface-mount (chip) resistors have very

little inductance and are well suited for use as current-

sense resistors. Power metal-strip resistors feature

1/2W and 1W power dissipation, 1% tolerance, and

inductance below 5nH. Resistance values between

10m

Ω and 500mΩ are available.

Inductor Selection

The essential parameters for inductor selection are

inductance and current rating. The MAX1626/MAX1627

operate with a wide range of inductance values. In many

applications, values between 10µH and 68µH take best

advantage of the controller’s high switching frequency.

Calculate the minimum inductance value as follows:

where 2µs is the minimum on-time. Inductor values

L

=

V+

- V

(MIN)

(MAX)

OUT

()

()

xs

V

R

CS MIN

CS

2

µ

R

=

V

R

POWER RATING (W)

2

CS

CS MAX

()

R

=

V

1.3 x I

CS

CS(MIN)

OUT(MAX)

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

_______________________________________________________________________________________

9

R3

R2

FROM

OUTPUT

TO FB

Figure 5. Adjustable-Output Operation Using the MAX1627

4.5

5.5

5.0

6.0

12

10

14

16

INPUT VOLTAGE (V)

3.0

3.5

2.5

2.0

1.5

1.0

0

0.5

Figure 6a. MAX1626 5V-Operation Current-Sense Resistor

Graph

3.0

4.0

3.5

4.5

12

10

14

16

INPUT VOLTAGE (V)

3.0

3.5

2.5

2.0

1.5

1.0

0

0.5

Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor

Graph