LTC3631_15 Datasheet, PDF (11/22 Pages) Linear Technology – High Efficiency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
Applications Information
700 _{VOUT = 5V}
600 ^{ISET OPEN}
L = 22µH
500
L = 47µH
400
300
L = 100µH
200
L = 220µH
100
0
5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
3631 F02
Figure 2. Switching Frequency for V_OUT = 5V
500
450
L = 22µH
400
350
V_OUT = 3.3V
I_SET OPEN
300
L = 47µH
250
200
150
L = 100µH
100
L = 220µH
50
0
5 10 15 20 25 30 35 40 45
INPUT VOLTAGE (V)
3631 F03
Figure 3. Switching Frequency for V_OUT = 3.3V
10000
1000
100
10
10
100
1000
PEAK INDUCTOR CURRENT (mA)
3631 F04
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 satura-
tion. 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 charac-
teristics. 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, TDK,
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
For more information www.linear.com/LTC3631
3631fe
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