10
LTC1143/LTC1143L
LTC1143L-ADJ
Once the frequency has been set by C
T
, the inductor L must
be chosen to provide no more than 25mV/R
SENSE
of peak-
to-peak inductor ripple current. This results in a minimum
required inductor value of:
L
MIN
= 5.1(10
5
)(R
SENSE
)(C
T
)V
REG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
eased at the expense of efficiency. If too small an inductor
is used, the inductor current will become discontinuous
before the LTC1143 series enters Burst Mode operation. A
consequence of this is that the LTC1143 series will delay
entering Burst Mode operation and efficiency will be
degraded at low currents.
Inductor Core Selection
Once the minimum value for L is known, the type of
inductor must be selected. The highest efficiency will be
obtained using Ferrite, Kool Mµ
®
or Molypermalloy (MPP)
cores. Lower cost powdered iron cores provide suitable
performance, but cut efficiency by 3% to 7%. Actual core
loss is independent of core size for a fixed inductor value,
but it is very dependent on inductance selected. As
inductance 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, 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 current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple that
can cause Burst Mode operation to be falsely triggered. Do
not allow the core to saturate!
Kool Mµ (from Magnetics, Inc.) is a very good, low loss core
material for toroids with a “soft” saturation characteristic.
Molypermalloy is slightly more efficient at high ( > 200 kHz)
switching frequencies but quite a bit more expensive.
Toroids are very space efficient, especially when you can
use several layers of wire, while inductors wound on
bobbins are generally easier to surface mount. New designs
for surface mount are available from Coiltronics, Coilcraft
and Sumida.
APPLICATIONS INFORMATION
WUU
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Power MOSFET Selection
An external power MOSFET must be selected for use with
each section of the LTC1143 series. The main selection
criteria for the power MOSFETs are the threshold voltage
V
GS(TH)
, maximum V
GS
rating and on resistance R
DS(ON)
.
Surface mount P-channel power MOSFETs are widely
available in both single and dual configurations. Logic
level MOSFETs are specified for operation up to 20V
maximum V
GS
and guarantee a maximum R
DS(ON)
with
V
GS
= 4.5V. Newer ‘sub’ logic level MOSFETs allow only 8V
maximum V
GS
but guarantee R
DS(ON)
with V
GS
= 2.7V. If
V
IN
will exceed 8V, logic level MOSFETs must be used; if
conservatively specified, they are generally usable down
to the 3.5V minimum V
IN
rating of the LTC1143L and
LTC1143L-ADJ.
The maximum output current I
MAX
determines the R
DS(ON)
requirement for the two MOSFETs. When the LTC1143
series is operating in continuous mode, the simplifying
assumption can be made that either the MOSFET or
Schottky diode is always conducting the average load
current. The duty cycles for the MOSFET and diode are
given by:
PCh
V
Schottky
VV
V
IN
OUT D
IN
- Duty Cycle
V
Diode Duty Cycle =
V
OUT
IN
≈
−+
()
From the duty cycles the required R
DS(ON)
for each MOSFET
can be derived:
PCh
P
V
I
P
OUT MAX
P
- R =
V
DS(ON)
IN
()
+
()
2
1δ
where P
P
is the allowable power dissipation and δ
P
is the
temperature dependencies of R
DS(ON)
. P
P
will be determined
by efficiency and/or thermal requirements (see Efficiency
Considerations). (1+ δ
P
) is generally given for a MOSFET in
the form of a normalized R
DS(ON)
vs temperature curve,
Kool Mµ is a registered trademark of Magnetics, Inc.
