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| 型号： | LTC1265IS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 7/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.2A 14SOIC |
| 标准包装： | 55 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 1.25 V ~ 13 V |
| 输入电压： | 3.5 V ~ 12 V |
| PWM 型： | 电流模式，混合 |
| 频率 - 开关： | 最高 700kHz |
| 电流 - 输出： | 1.2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 包装： | 管件 |
| 供应商设备封装： | 14-SOIC |
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIO S I FOR ATIO
) )
C T =
1 V IN – V OUT
) )
f ≈
1 V IN – V OUT
Undershort-circuitcondition,thepeakinductorcurrentis
determined by:
I SC(PK) = 150mV (Amps)
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-
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:
(Farads)
1.3(10 4 )f V IN + V D
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:
(Hz)
t OFF V IN + V D
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 M μ ? 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
) )
where:
t OFF = 1.3(10 4 )C T
V REG
V OUT
(sec)
heat dissipation.
CATCH DIODE SELECTION
Losses in the catch diode depend on forward drop and
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
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 μ is a registered trademark of Magnetics, Inc.
7
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