LTC3735
13
3735fa
APPLICATIONS INFORMATION
In a 2-phase converter, the net ripple current seen by
the output capacitor is much smaller than the individual
inductor ripple currents due to ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Linear Technology Application Note 77.
Figure 3 shows the net ripple current seen by the output
capacitors for 1- and 2-phase configurations. The output
ripple current is plotted for a fixed output voltage as the
duty factor is varied between 10% and 90% on the x-axis.
The graph can be used in place of tedious calculations,
simplifying the design process.
Accepting larger values of ∆I
L
allows the use of low in-
ductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆I
L
= 0.4(I
OUT
)/2, where I
OUT
is the total load current.
Remember, the maximum ∆I
L
occurs at the maximum
input voltage. The individual inductor ripple currents
are determined by the frequency, inductance, input and
output voltages.
is very dependent on inductor type selected. As inductance
increases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore cop-
per losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
A variety of inductors designed for high current, low volt-
age applications are available from manufacturers such
as Sumida, Coilcraft, Coiltronics, Toko and Panasonic.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
output stage with the LTC3735: one N-channel MOSFET
for the top (main) switch, and one N-channel MOSFET for
the bottom (synchronous) switch.
The peak-to-peak drive levels are set by the PV
CC
volt-
age. This voltage typically ranges from 4.5V to 7V
. Con-
sequently, logic-level threshold MOSFETs must be used
in most applications. Pay close attention to the BV
DSS
specification for the MOSFETs as well; most of the logic-
level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, gate charge Q
G
, reverse transfer ca-
pacitance C
RSS
, breakdown voltage BV
DSS
and maximum
continuous drain current I
D(MAX)
.
When the LTC3735 is operating at continuous mode in a
step-down configuration, the duty cycles for the top and
bottom MOSFETs of each power stage are approximately:
Top MOSFET Duty Cycle =
OUT
V
IN
(1)
Bottom MOSFET Duty Cycle =
V
IN
– V
OUT
V
IN
(2)
Figure 3. Normalized Output Ripple Current
vs Duty Factor [I
RMS
≈ 0.3 (∆I
O(P-P)
]
DUTY FACTOR (V
OUT
/V
IN
)
0.1 0.2 0.3 0.4
0.5 0.6 0.7 0.8 0.9
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3735 F03
2-PHASE
1-PHASE
O(P-P)
V
O
/fL
Inductor Core Selection
Once the values for L1 and L2 are 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 more expensive
ferrite, molypermalloy, or Kool Mµ cores. Actual core loss
is independent of core size for a fixed inductor value, but it
