LTC1622
APPLICATIONS INFORMATION
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core materials saturate “hard,” which means that
the inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequently, 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 Mu. 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 mountable designs that do
not increase the height significantly are available.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC1622. The main selection criteria for
the power MOSFET are the threshold voltage V
GS(TH)
and
the “on” resistance R
DS(ON)
,reverse transfer capacitance
C
RSS
and total gate charge.
Since the LTC1622 is designed for operation down to low
input voltages, a sublogic level threshold MOSFET (R
DS(ON)
guaranteed at V
GS
= 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC1622 is less than
the absolute maximum MOSFET V
GS
rating, typically 8V.
The gate drive voltage levels are from ground to V
IN
.
The required minimum R
DS(ON)
of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the LTC1622 in dropout, i.e., 100% duty
cycle, at its worst case the required R
DS(ON)
is given by:
R
DS
(
ON
)
DC
=
100
%
=
In applications where the maximum duty cycle is less than
100% and the LTC1622 is in continuous mode, the R
DS(ON)
is governed by:
(
P
P
I
OUT
(
MAX
)
) (
1
+ δ
p
)
2
where P
P
is the allowable power dissipation and
δp
is the
temperature dependency of R
DS(ON)
. (1 +
δp)
is generally
given for a MOSFET in the form of a normalized R
DS(ON)
vs
temperature curve, but
δp
= 0.005/°C can be used as an
approximation for low voltage MOSFETs.
8
U
W
U
U
R
DS
(
ON
)
≅
( )
P
P
2
DC I
OUT
1
+ δ
p
(
)
where DC is the maximum operating duty cycle of the
LTC1622.
When the LTC1622 is operating in continuous mode, the
MOSFET power dissipation is:
P
MOSFET
=
( ) (
1
+ δ
p
)
R
DS(ON)
2
+
K
(
V
IN
) (
I
OUT
)(
C
RSS
)(
f
)
V
OUT
+
V
D
I
OUT
V
IN
+
V
D
2
where K is a constant inversely related to gate drive
current. Because of the high switching frequency, the
second term relating to switching loss is important not to
overlook. The constant K = 3 can be used to estimate the
contributions of the two terms in the MOSFET dissipation
equation.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches V
OUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short circuited. Under this condition the diode must
safely handle I
PEAK
at close to 100% duty cycle. Therefore,
it is important to adequately specify the diode peak current
and average power dissipation so as not to exceed the
diode ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
V
IN
−
V
OUT
I
D
=
I
OUT
V
IN
+
V
D
LINER [ LINEAR TECHNOLOGY ]
