Datasheet

9
LTC1149
LTC1149-3.3/LTC1149-5
APPLICATIO S I FOR ATIO
WUU U
Inductor Core Selection
Once the minimum 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 more expensive
ferrite, molypermalloy, or Kool Mµ
®
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 increase.
Ferrite designs have very low core loss, 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
inductor ripple current and consequent output voltage
ripple which can cause Burst Mode operation to be falsely
triggered in the LTC1149 series. 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 Mµ. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, new surface mount designs available
from Coiltronics do not increase the height significantly.
P-Channel MOSFET Selection
Two external power MOSFETs must be selected for use
with the LTC1149 series: a P-channel MOSFET for the
main switch, and an N-channel MOSFET for the synchro-
nous switch.
The minimum input voltage determines whether standard
threshold or logic-level threshold MOSFETs must be used.
For V
IN
> 8V, standard threshold MOSFETs (V
GS(TH)
< 4V)
may be used. If V
IN
is expected to drop below 8V, logic-
level threshold MOSFETs (V
GS(TH)
< 2.5V) are strongly
recommended. When logic-level MOSFETs are used, the
absolute maximum V
GS
rating for the MOSFETs must be
greater than the LTC1149 series internal regulator
voltage V
CC
.
Selection criteria for the P-channel MOSFET include the
on-resistance R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1149 is operating in continuous mode, the duty cycle
for the P-channel MOSFET is given by:
P-Ch Duty Cycle =
V
OUT

V
IN
The P-channel MOSFET dissipation at maximum output
current is given by:
P-Ch P
D
=
V
OUT
V
IN
+ K(V
IN
)
2
(I
MAX
)(C
RSS
)(f)
(I
MAX
)
2
(1 + ∂
P
) R
DS(ON)
where ∂ is the temperature dependency of R
DS(ON)
and K
is a constant related to the gate drive current. Note the two
distinct terms in the equation. The first gives the I
2
R
losses, which are highest at low input voltages, while the
second gives the transition losses, which are highest at
high input voltages. For V
IN
< 24V, the high current
efficiency generally improves with larger MOSFETs
(although gate charge losses begin eating into the gains.
See Efficiency Considerations). For V
IN
> 24V, the transi-
tion losses rapidly increase to the point that the use of a
higher R
DS(ON)
device with lower C
RSS
actually provides
higher efficiency. This is illustrated in the Design Example
section.
The term (1 + ∂) is generally given for a MOSFET in the
form of a normalized R
DS(ON)
vs temperature curve, but
∂ = 0.007/°C can be used as an approximation for low
voltage MOSFETs. C
RSS
is usually specified in the MOSFET
electrical characteristics. The constant K is much harder to
pin down, but K = 5 can be used for the LTC1149 series to
estimate the relative contributions of the two terms in the
P-channel dissipation equation.
N-Channel MOSFET and D1 Selection
The same input voltage constraints apply to the N-channel
MOSFET as to the P-channel with regard to when logic-
level devices are required. However, the dissipation calcu-
lation is quite different. The duty cycle and dissipation for
Kool Mµ is a registered trademark of Magnetics, Inc.