Datasheet

LT3742
13
3742fa
operation would reduce the RMS input capacitor current
from ~1.8A
RMS
to ~0.8A
RMS
. While this is an impressive
reduction by itself, remember that power losses are pro-
portional to I
RMS
2
, meaning that the actual power wasted
due to the input capacitor is reduced by a factor of ~4.
Figure 5 shows the reduction in RMS ripple current for a
typical application.
The reduced input ripple current also means that less
power is lost in the input power path. Improvements in
both conducted and radiated EMI also directly accrue as
a result of the reduced RMS input current and voltage.
Significant cost and board footprint savings are also real-
ized by being able to use smaller, less expensive, lower
RMS current-rated input capacitors.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the relative duty cycles of the two
controllers, which in turn, are dependent upon the input
voltage (DC ≈ V
OUT
/V
IN
).
It can be readily seen that the advantages of 2-phase op-
eration are not limited to a narrow operating range, but in
fact extend over a wide region. A good rule of thumb for
most applications is that 2-phase operation will reduce the
input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
Inductor Value Selection
The inductor value directly affects inductor ripple current,
I
RIPPLE
, and maximum output current, I
OUT(MAX)
. Lower
ripple current reduces core losses in the inductor, ESR
losses in the output capacitors and output voltage ripple.
Too large of a value, however, will result in a physically
large inductor. A good trade-off is to choose the inductor
ripple current to be ~30% of the maximum output current.
This will provide a good trade off between the inductor
size, maximum output current, and the amount of ripple
current. Note that the largest ripple current occurs at the
highest the input voltage, so applications with a wide V
IN
range should consider both V
IN(TYP)
and V
IN(MAX)
when
calculating the inductor value:
L ≥
V
IN
– V
OUT
0.3 • I
OUT(MAX)
•
V
OUT
V
IN
•
1
500kHz
This equation provides a good starting point for pick-
ing the inductor value. Most systems can easily tolerate
ripple currents in the range of 10% to 50%, so deviating
slightly from the calculated value is acceptable for most
applications. Pick a standard value inductor close to the
applicaTions inForMaTion
Figure 5. RMS Input Current Comparison
INPUT VOLTAGE (V)
0
INPUT RMS CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0.5
0
10 20 30 40
3742 F05
SINGLE PHASE
DUAL CONTROLLER
2-PHASE
DUAL CONTROLLER
V
O1
= 5V/3A
V
O2
= 3.3V/3A