Datasheet

10
LT1374
1374fd
APPLICATIONS INFORMATION
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MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
P
) of the LT1374.
This current rating is 4.5A up to 50% duty cycle (DC),
decreasing to 3.7A at 80% duty cycle. This is shown
graphically in Typical Performance Characteristics and as
shown in the formula below:
I
P
= 4.5A for DC ≤ 50%
I
P
= 3.21 + 5.95(DC) – 6.75(DC)
2
for 50% < DC < 90%
DC = Duty cycle = V
OUT
/V
IN
Example: with V
OUT
= 5V, V
IN
= 8V; DC = 5/8 = 0.625, and;
I
SW(MAX)
= 3.21 + 5.95(0.625) – 6.75(0.625)
2
= 4.3A
Current rating decreases with duty cycle because the
LT1374 has internal slope compensation to prevent current
mode subharmonic switching. For more details, read Ap-
plication Note 19. The LT1374 is a little unusual in this regard
because it has nonlinear slope compensation which gives
better compensation with less reduction in current limit.
Maximum load current would be equal to maximum
switch current for an infinitely large inductor , but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current. The following
formula assumes continuous mode operation, implying
that the term on the right is less than one-half of I
P
.
I
OUT(MAX)
=
Continuous Mode
For the conditions above and L = 3.3µH,
At V
IN
= 15V, duty cycle is 33%, so I
P
is just equal to a fixed
4.5A, and I
OUT(MAX)
is equal to:
Note that there is less load current available at the higher
input voltage because inductor ripple current increases.
This is not always the case. Certain combinations of
inductor value and input voltage range may yield lower
available load current at the lowest input voltage due to
reduced peak switch current at high duty cycles. If load
current is close to the maximum available, please check
maximum available current at both input voltage
extremes. To calculate actual peak switch current with a
given set of conditions, use:
For lighter loads where discontinuous operation can be
used, maximum load current is equal to:
I
OUT(MAX)
=
Discontinuous mode
Example: with L = 1.2µH, V
OUT
= 5V, and V
IN(MAX
) = 15V,
The main reason for using such a tiny inductor is that it is
physically very small, but keep in mind that peak-to-peak
inductor current will be very high. This will increase output
ripple voltage. If the output capacitor has to be made larger
to reduce ripple voltage, the overall circuit could actually
wind up larger.
CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR
For most applications the output inductor will fall in the
range of 3µH to 20µH. Lower values are chosen to reduce
physical size of the inductor. Higher values allow more
output current because they reduce peak current seen by
the LT1374 switch, which has a 4.5A limit. Higher values
also reduce output ripple voltage, and reduce core loss.
Graphs in the Typical Performance Characteristics section
show maximum output load current versus inductor size
and input voltage. A second graph shows core loss versus
inductor size for various core materials.