13
LTC3729
sn3729 3729fas
inductor ripple current and consequent output voltage
ripple.
Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool M
µ
. Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
controller with the LTC3729: 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 INTV
CC
volt-
age. This voltage is typically 5V during start-up (see
EXTV
CC
Pin Connection). Consequently, logic-level thresh-
old MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
IN
< 5V);
then, sublogic-level threshold MOSFETs (V
GS(TH)
< 3V)
should be used. Pay close attention to the BV
DSS
specifi-
cation 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)
, reverse transfer capacitance C
RSS
,
input voltage, and maximum output current. When the
LTC3729 is operating in continuous mode the duty factors
for the top and bottom MOSFETs of each output stage are
given by:
Main Switch Duty Cycle
V
V
OUT
IN
=
Synchronous Switch Duty Cycle
V
V
V
IN
OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
Kool M
µ
is a registered trademark of Magnetics, Inc.
APPLICATIO S I FOR ATIO
W
U
U
U
Figure 3. Normalized Peak Output 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
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3729
F03
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
∆
I
O(P-P)
V
O
/fL
Accepting larger values of
∆
I
L
allows the use of low
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆
I
L
= 0.4(I
OUT
)/N, where N is the number of channels and
I
OUT
is the total load current. Remember, the maximum
∆
I
L
occurs at the maximum input voltage. The individual
inductor ripple currents are constant determined by the
inductor, input and output voltages.
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 M
µ
®
cores. Actual core
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As induc-
tance increases, core losses go down. Unfortunately,
increased inductance requires more turns of wire and
therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
