LTC1265 LINER [Linear Technology], LTC1265 Datasheet - Page 7

• Manufacturer Part Number: LTC1265
• Description: 1.2A, High Efficiency Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
Under short-circuit condition, the peak inductor current is
determined by:
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
C
The LTC1265 uses a constant off-time architecture with
t
P-channel MOSFET turns on, the voltage on C
approximately 3.3V. During the off time, C
by a current that is proportional to V
is analogous to the current in inductor L, which likewise,
decays at a rate proportional to V
value must track the timing capacitor value.
The value of C
mode operating frequency:
where V
As the operating frequency is increased, the gate charge
losses will reduce efficiency. The complete expression for
operating frequency is given by:
where:
V
the measured output voltage. Thus V
in regulation.
Note that as V
When the input-to-output voltage differential drops below
OFF
T
REG
I
C
f
t
and L Selection for Operating Frequency
SC(PK)
OFF
determined by an external capacitor C
T
is the desired output voltage (i.e. 5V, 3.3V). V
=
t
OFF
= 1.3(10
D
1
1.3(10
is the drop across the Schottky diode.
= 150mV
1
R
V
T
V
SENSE
IN
is calculated from the desired continuous
4
IN
IN
4
)f
– V
)C
+ V
decreases, the frequency decreases.
U
T
OUT
V
D
(Amps)
V
IN
V
V
IN
OUT
REG
INFORMATION
– V
U
(Hz)
+ V
OUT
D
(sec)
OUT
OUT
W
(Farads)
OUT(MAX)
. Thus the inductor
. The voltage on C
T
T
REG
. Each time the
is discharged
.
T
/V
U
is reset to
OUT
OUT
= 1
is
T
LTC1265/LTC1265-3.3/LTC1265-5
2V, the LTC1265 reduces t
current in C
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
following expression for L:
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 M
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
Kool M is a registered trademark of Magnetics, Inc.
SENSE
L 5.2(10
of peak-to-peak ripple current. This results in the
T
5
. This prevents audible operation prior to
)R
SENSE
(C
T
)V
OFF
REG
by increasing the discharge
®
7
will