8
LTC1874
I
RIPPLE
= 0.4(I
OUT(MAX)
). Remember, the maximum I
RIPPLE
occurs at the maximum input voltage.
In Burst Mode operation on an LTC1874 controller, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current must not exceed:
I
V
R
RIPPLE
SENSE
≤
003.
This implies a minimum inductance of:
L
VV
f
R
VV
VV
MIN
IN OUT
SENSE
OUT D
IN D
=
−
+
+
003.
(Use V
IN(MAX)
= V
IN
)
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value of inductor is known, an off the shelf
inductor can be selected. The inductor should be rated for
the calculated peak current. Some manufacturers specify
both peak saturation current and peak RMS current. Make
sure that the RMS current meets your continuous load
requirements. Also, you may want to compare the DC
resistance of different inductors in order to optimize the
efficiency.
Inductor core losses are usually not specified and you will
need to evaluate them yourself. Usually, the core losses
are not a problem because the inductors operate with
relatively low magnetic flux swings. The best way to
evaluate the core losses is by measuring the converters
efficiency. Converter efficiency will reveal the difference in
both DC current losses and core losses.
Off the shelf inductors are available from numerous manu-
facturers. Some of the most common manufacturers are
Coilcraft, Coiltronics, Panasonic, Toko, Tokin, Murata and
Sumida.
Power MOSFET Selection
The main selection criteria for the power MOSFET are the
threshold voltage V
GS(TH)
, the “on” resistance R
DS(ON)
,
reverse transfer capacitance C
RSS
and total gate charge.
Since the controller is designed for operation down to low
input voltages, a logic 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 controller is less
than the absolute maximum V
GS
rating, typically 8V.
The required minimum R
DS(ON)
of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the controller in dropout, i.e., 100% duty
cycle, at its worst case the required R
DS(ON)
is given by:
R
P
Ip
DS ON
P
OUT MAX
DC
()
()
%=
=
()
+
()
100
2
1 δ
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.
In applications where the maximum duty cycle is less than
100% and the controller is in continuous mode, the
R
DS(ON)
is governed by:
R
P
DC I
p
DS ON
P
OUT
()
≅
()
+
()
2
1 δ
where DC is the maximum operating duty cycle of the
controller.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
MOSFET 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
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