Digitally Adjustable LCD Bias Supply
Choosing an Inductor
MAXIMUM OUTPUT CURRENT (mA)
MAX749
Practical inductor values range from 22µH to 100µH,
and 47µH is normally a good choice. Inductors with a
ferrite core or equivalent are recommended. The induc-
tor’s saturation current rating – the current at which the
core begins to saturate and the inductance falls to 80%
or 90% of its nominal value – should ideally equal the
current limit (see
Current-Sense Resistor
section).
However, because the current is limited by the
MAX749, the inductor can safely be driven into satura-
tion with only a slight impact on efficiency.
For highest efficiency, use a coil with low resistance,
preferably under 300mΩ. To minimize radiated noise,
use a toroid, pot-core, or shielded inductor.
100
80
60
40
0.2
MAX749-Fig 11
V
OUT
= -15V
L = 47µH
0.25
0.3
R
SENSE
(Ω)
R
SENSE
(Ω)
0.5
20
1.0
0
2
3
4
INPUT VOLTAGE (V)
5
6
Figure 9c. Maximum Output Current vs. Input Voltage,
V
OUT
= -15V
250
MAX749-Fig 9
0.2
MAXIMUM OUTPUT CURRENT (mA)
V
OUT
= -5V
L = 47µH
60
0.2
MAX749-Fig 12
MAXIMUM OUTPUT CURRENT (mA)
200
0.25
R
SENSE
(Ω)
0.3
50
40
30
20
10
0
2
V
OUT
= -24V
L = 47µH
150
0.25
0.3
100
0.5
0.5
1.0
50
1.0
0
2
3
4
INPUT VOLTAGE (V)
5
6
3
4
INPUT VOLTAGE (V)
5
6
Figure 9a. Maximum Output Current vs. Input Voltage,
V
OUT
= -5V
140
MAXIMUM OUTPUT CURRENT (mA)
120
100
80
60
40
20
0
2
3
4
INPUT VOLTAGE (V)
5
6
0.5
1.0
MAX-749-Fig10
Figure 9d. Maximum Output Current vs. Input Voltage,
V
OUT
= -24V
25
0.2
V
OUT
= -48V
L = 47µH
MAX749-Fig 13
V
OUT
= -12V
L = 47µH
MAXIMUM OUTPUT CURRENT (mA)
0.2
20
0.25
0.3
0.25
R
SENSE
(Ω)
0.3
10
0.5
5
1.0
0
2
3
4
INPUT VOLTAGE (V)
5
6
Figure 9b. Maximum Output Current vs. Input Voltage,
V
OUT
= -12V
Figure 9e. Maximum Output Current vs. Input Voltage,
V
OUT
= -48V
9
_______________________________________________________________________________________
R
SENSE
(Ω)
15
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
