LTC3630A Datasheet, PDF (12/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

LTC3630A
Applications Information
600
V_OUT = 3.3V
L = 4.2µH
I_SET OPEN
500
400
300
L = 10µH
200
L = 22µH
100
L = 47µH L = 100µH
0
0 10 20 30 40 50 60 70
V_IN INPUT VOLTAGE (V)
3630a F03
Figure 3. Switching Frequency for V_OUT = 3.3V
1000
100
10
100
1000
PEAK INDUCTOR CURRENT (mA)
3630a F04
Figure 4. Recommended Inductor Values for Maximum Efficiency
well-controlled, the inductor value must be chosen so that
it is larger than a minimum value which can be computed
as follows:
L > ^{VIN(MAX) • tON(MIN)} • 1.2
I_PEAK
where V_IN(MAX) is the maximum input supply voltage when
switching is enabled, t_ON(MIN) is 150ns, I_PEAK is the peak
current, and the factor of 1.2 accounts for typical inductor
tolerance and variation over temperature. Inductor values
that violate the above equation will cause the peak current
to overshoot and permanent damage to the part may occur.
Although the above equation provides the minimum in-
ductor value, higher efficiency is generally achieved with
a larger inductor value, which produces a lower switching
frequency. The inductor value chosen should also be large
enough to keep the inductor current from going very nega-
tive which is more of a concern at higher V_OUT (>~12V). For
a given inductor type, however, as inductance is increased,
DC resistance (DCR) also increases. Higher DCR trans-
lates into higher copper losses and lower current rating,
both of which place an upper limit on the inductance. The
recommended range of inductor values for small surface
mount inductors as a function of peak current is shown
in Figure 4. The values in this range are a good compromise
between the trade-offs discussed above. For applications
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 versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
TDK, Toko, and Sumida.
3630af
12
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