LTC3642
APPLICATIONS INFORMATION
700
where board area is not a limiting factor, inductors with
largercorescanbeused,whichextendstherecommended
range of Figure 4 to larger values.
V
SET
= 5V
OUT
I
OPEN
L = 47ꢀH
600
500
L = 68ꢀH
Inductor Core Selection
400
300
200
100
L = 100ꢀH
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
affordthecorelossfoundinlowcostpowderedironcores,
forcing the use of the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
L = 150ꢀH
L = 220ꢀH
L = 470ꢀH
0
10 15 20 25
INPUT VOLTAGE (V)
40 45
5
30 35
V
IN
3642 F02
Figure 2. Switching Frequency for VOUT = 5V
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductancecollapsesabruptlywhenthepeakdesigncurrent
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently output voltage ripple. Do
not allow the core to saturate!
500
V
SET
= 3.3V
OUT
450
400
350
300
250
200
150
100
50
L = 47ꢀH
I
OPEN
L = 68ꢀH
L = 100ꢀH
L = 150ꢀH
L = 220ꢀH
L = 470ꢀH
Different core materials and shapes will change the
size/current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy ma-
terials are small and do not radiate energy but generally
cost more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko, Sumida and Vishay.
0
5
25
35 40
10 15 20
30
45
V
INPUT VOLTAGE (V)
IN
3642 F03
Figure 3. Switching Frequency for VOUT = 3.3V
10000
C and C
Selection
IN
OUT
1000
100
The input capacitor, C , is needed to filter the trapezoidal
IN
current at the source of the top high side MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used.
Approximate RMS current is given by:
10
100
PEAK INDUCTOR CURRENT (mA)
VOUT
V
VOUT
3642 F04
IN
IRMS = IOUT(MAX)
•
•
− 1
V
IN
Figure 4. Recommended Inductor Values for Maximum Efficiency
3642f
11
Linear [ Linear ]
