7
LTC1265/LTC1265-3.3/LTC1265-5
Kool Mµ is a registered trademark of Magnetics, Inc.
Under short-circuit condition, the peak inductor current is
determined by:
I
SC(PK)
=
150mV
R
SENSE
(Amps)
In this condition, the LTC1265 automatically extends the
off time of the P-channel MOSFET to allow the inductor
current to decay far enough to prevent any current build-
up. The resulting ripple current causes the average short-
circuit current to be approximately I
OUT(MAX)
.
C
T
and L Selection for Operating Frequency
The LTC1265 uses a constant off-time architecture with
t
OFF
determined by an external capacitor C
T
. Each time the
P-channel MOSFET turns on, the voltage on C
T
is reset to
approximately 3.3V. During the off time, C
T
is discharged
by a current that is proportional to V
OUT
. The voltage on C
T
is analogous to the current in inductor L, which likewise,
decays at a rate proportional to V
OUT
. Thus the inductor
value must track the timing capacitor value.
The value of C
T
is calculated from the desired continuous
mode operating frequency:
C
T
=
1
1.3(10
4
)f
V
IN
– V
OUT
V
IN
+ V
D
(Farads)
)
)
where V
D
is the drop across the Schottky diode.
As the operating frequency is increased, the gate charge
losses will reduce efficiency. The complete expression for
operating frequency is given by:
f ≈
1
t
OFF
V
IN
– V
OUT
V
IN
+ V
D
(Hz)
)
)
where:
t
OFF
= 1.3(10
4
)C
T
V
REG
V
OUT
(sec)
)
)
V
REG
is the desired output voltage (i.e. 5V, 3.3V). V
OUT
is
the measured output voltage. Thus V
REG
/V
OUT
= 1
in regulation.
Note that as V
IN
decreases, the frequency decreases.
When the input-to-output voltage differential drops below
2V, the LTC1265 reduces t
OFF
by increasing the discharge
current in C
T
.
This prevents audible operation prior to
dropout. (See shelving effect shown in the Operating
Frequency curve under Typical Performance Character-
istics.)
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
R
SENSE
of peak-to-peak ripple current. This results in the
following expression for L:
L ≥ 5.2(10
5
)R
SENSE
(C
T
)V
REG
Using an inductance smaller than the above value will
result in the inductor current being discontinuous. A
consequence of this is that the LTC1265 will delay entering
Burst Mode operation and efficiency will be degraded at
low currents.
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor; core and copper losses.
Core losses are dependent on the peak-to-peak ripple
current and core material. However it is independent of
the physical size of the core. By increasing the induc-
tance, the peak-to-peak inductor ripple current will de-
crease, therefore reducing core loss. Utilizing low core
loss material, such as molypermalloy or Kool Mµ
®
will
allow user to concentrate on reducing copper loss and
preventing saturation.
Although higher inductance reduces core loss, it in-
creases copper loss as it requires more windings. When
space is not at a premium, larger wire can be used to
reduce the wire resistance. This also prevents excessive
heat dissipation.
CATCH DIODE SELECTION
Losses in the catch diode depend on forward drop and
switching times. Therefore Schottky diodes are a good
choice for low drop and fast switching times.
The catch diode carries load current during the off time.
The average diode current is therefore dependent on the
APPLICATIO S I FOR ATIO
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