Datasheet

LTC3642
11
3642fc
APPLICATIONS INFORMATION
Figure 3. Switching Frequency for V
OUT
= 3.3V
Figure 2. Switching Frequency for V
OUT
= 5V
Figure 4. Recommended Inductor Values for Maximum Efficiency
where board area is not a limiting factor, inductors with
larger cores can be used, which extends the recommended
range of Figure 4 to larger values.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
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.
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
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently output voltage ripple. Do
not allow the core to saturate!
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 materials
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.
C
IN
and C
OUT
Selection
The input capacitor, C
IN
, is needed to filter the trapezoidal
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:
I
RMS
= I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
− 1
INPUT VOLTAGE (V)
5
SWITCHING FREQUENCY (kHz)
400
500
600
35
3642 F02
300
200
15 25 45
30
10 20 40
100
0
700
L = 47µH
L = 68µH
L = 100µH
L = 150µH
L = 220µH
L = 470µH
V
OUT
= 5V
I
SET
OPEN
INPUT VOLTAGE (V)
5
0
SWITCHING FREQUENCY (kHz)
50
150
200
250
500
350
15
25
30
3642 F03
100
400
450
300
10 20
35
40
45
L = 470µH
L = 220µH
L = 150µH
L = 100µH
L = 68µH
L = 47µH
V
OUT
= 3.3V
I
SET
OPEN
PEAK INDUCTOR CURRENT (mA)
100
INDUCTOR VALUE (µH)
1000
10000
10 100
3642 F04