LTC3642 Linear Technology, LTC3642 Datasheet - Page 11

• Manufacturer Part Number: LTC3642
• Description: High Voltage 50mA Synchronous Step-Down Converter
• Manufacturer: Linear Technology

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Figure 4. Recommended Inductor Values for Maximum Effi ciency
APPLICATIONS INFORMATION
Figure 3. Switching Frequency for V
Figure 2. Switching Frequency for V
10000
700
600
400
300
200
500
450
400
350
300
250
200
150
100
1000
500
100
50
100
0
0
5
5
10
V
I
V
I
SET
SET
OUT
OUT
10
10
OPEN
OPEN
= 5V
= 3.3V
PEAK INDUCTOR CURRENT (mA)
15
15
V
V
IN
IN
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
20
20
L = 100μH
L = 150μH
L = 220μH
L = 470μH
L = 100μH
L = 150μH
L = 220μH
L = 470μH
L = 47μH
L = 68μH
L = 47μH
L = 68μH
25
25
30
30
35
35
40
40
OUT
3642 F02
3642 F03
OUT
3642 F04
45
45
= 3.3V
100
= 5V
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 effi ciency 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 fi xed 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 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 fi eld/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko, Sumida and Vishay.
C
The input capacitor, C
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:
IN
I
RMS
and C
=
OUT
I
OUT MAX
Selection
(
)
•
IN
V
, is needed to fi lter the trapezoidal
V
OUT
IN
•
V
V
OUT
IN
−
LTC3642
1
11
3642f