2. Inductor Selection (L)

A. Preliminary Calculations

To select the inductor, the calculation of the following

three parameters is necessary:

Dmax, the maximum switch duty cycle (0

≤ D ≤ 0.9):

=

0.8V for fast recovery diodes.

E

• T, the product of volts • time that charges the induc-

tor:

E

• T =

52,000Hz

(V• µs)

=

1.05

1

B. Identify Inductor Value:

1. From Figure 4, identify the inductor code for the region

indicated by the intersection of E

code gives the inductor value in microhenries. The L or H

prefix signifies whether the inductor is rated for a maxi-

mum E

• T of 90Vµs (L) or 250Vµs (H).

2. If D < 0.85, go to step C. If D

≥ 0.85, calculate the

minimum inductance needed to ensure the switching

regulator’s stability:

0.3

0.4 0.45

0.35

0.5 0.6 0.7 0.8 0.9 1.0

1.5

2.0

2.5

3.0

20

30

35

25

40

45

50

60

70

80

90

100

200

150

L47

L68

L100

L150

L220

L330

H2200

L680

H1500

H1000

H680

H470

H330

H220

H150

L470

Figure 4. Inductor Selection Graph

B1, go on to step C. Otherwise, the inductor value found

in step B1 is too low; an appropriate inductor code

should be obtained from the graph as follows:

1.

Find the lowest value inductor that is greater than

2.

Find where E

• T intersects this inductor value to

determine if it has an L or H prefix. If E

• T intersects

both the L and H regions, select the inductor with an

H prefix.

C.

Inductor Selection

Select an inductor from the table of Figure 5 which cross

references the inductor codes to the part numbers of the

three different manufacturers. The inductors listed in this

table have the following characteristics:

are low etectromagnetic interference (EMI), small

physical size, and very low power dissipation (core

loss).

fits are low EMI and ability to withstand E

• T and

peak current above rated value better than ferrite

cores.

low cost and best ability to withstand E

• T and peak

current above rated value. Be aware that these in-

ductors generate more EMI than the other types, and

this may interfere with signals sensitive to noise.

UC2577-ADJ

Note: This chart assumes that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current

(when the regulator is under full load). Greater ripple current causes higher peak switch currents and greater output ripple volt-

age. Lower ripple current is achieved with larger value inductors. The factor of 20% to 30% is chosen as a convenient balance

between the two extremes.

APPLICATIONS INFORMATION (cont.)

5