LTC1265/LTC1265-3.3/LTC1265-5
U
W U U
APPLICATIONS INFORMATION
Under short-circuit condition, the peak inductor current is
determined by:
2V, the LTC1265 reduces tOFF by increasing the discharge
current in CT. This prevents audible operation prior to
dropout. (See shelving effect shown in the Operating
Frequency curve under Typical Performance Character-
istics.)
150mV
I
=
(Amps)
SC(PK)
R
SENSE
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-
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
RSENSE of peak-to-peak ripple current. This results in the
following expression for L:
circuit current to be approximately IOUT(MAX)
.
L ≥ 5.2(105)RSENSE(CT)VREG
CT and L Selection for Operating Frequency
Using an inductance smaller than the above value will
result in the inductor current being discontinuous. A
consequenceofthisisthattheLTC1265willdelayentering
Burst Mode operation and efficiency will be degraded at
low currents.
The LTC1265 uses a constant off-time architecture with
tOFF determined by an external capacitor CT. Each time the
P-channel MOSFET turns on, the voltage on CT is reset to
approximately 3.3V. During the off time, CT is discharged
by a current that is proportional to VOUT. The voltage on CT
is analogous to the current in inductor L, which likewise,
decays at a rate proportional to VOUT. Thus the inductor
value must track the timing capacitor value.
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.
The value of CT is calculated from the desired continuous
mode operating frequency:
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.
V – V
V + V
IN
1
IN
OUT
D
C =
T
(Farads)
)
)
4
1.3(10 )f
where VD 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:
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.
V – V
V + V
IN
1
OFF
IN
OUT
D
(Hz)
f ≈
)
)
t
where:
V
V
REG
OUT
CATCH DIODE SELECTION
4
(sec)
t
= 1.3(10 )C
OFF
T
)
)
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.
VREG is the desired output voltage (i.e. 5V, 3.3V). VOUT is
the measured output voltage. Thus VREG/VOUT = 1
in regulation.
The catch diode carries load current during the off time.
The average diode current is therefore dependent on the
Note that as VIN decreases, the frequency decreases.
When the input-to-output voltage differential drops below
Kool Mµ is a registered trademark of Magnetics, Inc.
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