ML4890
DESIGN CONSIDERATIONS
INDUCTOR
Selecting the proper inductor for a specific application
usually involves a trade-off between efficiency and
maximum output current. Choosing too high a value will
keep the regulator from delivering the required output
current under worst case conditions. Choosing too low a
value causes efficiency to suffer. It is necessary to know
the maximum required output current and the input
voltage range to select the proper inductor value. The
maximum inductor value can be estimated using the
following formula:
where
η
is the efficiency, typically between 0.75 and
0.85, and V
OS
is the dropout voltage at I
OUT(MAX)
taken
from Figure 3. Note that this is the value of inductance
that just barely delivers the required output current under
worst case conditions. A lower value may be required to
cover inductor tolerance, the effect of lower peak inductor
currents caused by resistive losses, and minimum dead
time between pulses.
Another method of determining the appropriate inductor
value is to make an estimate based on the typical
performance curves given in Figures 5 and 6. Figure 5
shows maximum output current as a function of input
voltage for several inductor values. These are typical
performance curves and leave no margin for inductance
and ON-time variations. To accommodate worst case
conditions, it is necessary to derate these curves by at
least 10% in addition to inductor tolerance.
V
IN(MIN)
×
T
ON(MIN)
× η
L
MAX
=
2
×
(V
OUT
+
V
OS
)
×
I
OUT(MAX)
2
(1)
80
70
ML4890-T
µ
H
µ
H
µ
H
100
ML4890-3
µ
H
L
=
L=
L=
=
90
80
70
L=
L=
L
L
=
µ
H
22
33
µ
H
µ
H
H
15
22
I
OUT
MAX (mA)
I
OUT
MAX (mA)
50
40
30
20
10
0
1.0
L
=
L=
60
68
µ
H
15
L
=
33
10
47
µ
H
10
µ
L
=
=
47
µ
H
60
50
40
30
20
10
L
µ
H
68
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
0
1.0
1.5
2.0
V
IN
(V)
2.5
3.0
V
IN
(V)
200
180
160
140
I
OUT
MAX (mA)
ML4890-5
L=
22
µ
H
L
µ
H
15
L=
=
120
100
µ
H
L
=
80
60
40
20
0
1.0
L
8
=6
L=
1.5
10
2.0
2.5
3.0
3.5
4.0
33
47
µ
H
µ
H
µ
H
4.5
V
IN
(V)
Figure 5. Output Current versus Input Voltage.
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
