LTC3630
APPLICATIONS INFORMATION
600
L = 4.2μH
SWITCHING FREQUENCY (kHz)
500
400
300
200
L = 22μH
100
0
L = 47μH
L = 100μH
10
100
PEAK INDUCTOR CURRENT (mA)
3630 F03
3630 F04
V
OUT
= 3.3V
I
SET
OPEN
1000
INDUCTOR VALUE (μH)
40
50
60
L = 10μH
100
0
10
20
30
1000
V
IN
INPUT VOLTAGE (V)
Figure 3. Switching Frequency for V
OUT
= 3.3V
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
>
V
IN(MAX)
• t
ON(MIN)
• 1.2
I
PEAK
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!
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
3630fb
12
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
