LTC3642 Datasheet, PDF (11/20 Pages) Linear Technology – High Effi ciency, High Voltage 50mA Synchronous Step-Down Converter

LTC3642
APPLICATIONS INFORMATION
700 _{VOUT = 5V}
600 ^{ISET OPEN}
L = 47μH
500
L = 68μH
400
L = 100μH
300
L = 150μH
200
L = 220μH
100
L = 470μH
0
5 10 15 20 25 30 35 40 45
V_IN INPUT VOLTAGE (V)
3642 F02
Figure 2. Switching Frequency for V_OUT = 5V
500
V_OUT = 3.3V
450 _{ISET OPEN}
400
L = 47μH
350
L = 68μH
300
L = 100μH
250
200
L = 150μH
150
L = 220μH
100
L = 470μH
50
0
5 10 15 20 25 30 35 40 45
V_IN INPUT VOLTAGE (V)
3642 F03
Figure 3. Switching Frequency for V_OUT = 3.3V
10000
1000
100
10
100
PEAK INDUCTOR CURRENT (mA)
3642 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 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 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 − 1}
V_OUT
3642f
11