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| 型号： | LTC1265CS-3.3 |
| 厂商： | Linear Technology |
| 文件页数： | 6/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK 3.3V 1.2A 14SOIC |
| 标准包装： | 55 |
| 类型： | 降压（降压） |
| 输出类型： | 固定 |
| 输出数： | 1 |
| 输出电压： | 3.3V |
| 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： | 0°C ~ 70°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
OPERATIO
(Refer to Functional Diagram)
capacitor discharges past V TH1 until its voltage drops
below V TH2 . Comparator S then trips and a sleep signal is
generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1265 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 2mA to 160 μ A. When the output capaci-
tor discharges by the amount of the hysteresis of the
comparator V, the P-channel switch turns on again and the
process repeats itself. During Burst Mode operation the
peak inductor current is set at 25mV/R SENSE .
APPLICATIO S I FOR ATIO
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V OS is incorporated
in the gain stage. This prevents the current from increas-
ing until the output voltage has dropped below a minimum
threshold.
Using constant off-time architecture, the operating fre-
quency is a function of the voltage. To minimize the
frequency variation as dropout is approached, the off-time
controller increases the discharge current as V IN drops
below V OUT + 2V. In dropout the P-channel MOSFET is
turned on continuously (100% duty cycle) providing low
dropout operation with V OUT ? V IN .
The basic LTC1265 application circuit is shown in
Figure 1. External component selection is driven by the
I OUT(MAX) = 150mV –
R SENSE
25mV
2 ? R SENSE
(Amps)
load requirement, and begins with the selection of R SENSE .
Once R SENSE is known, C T and L can be chosen. Next, the
Schottky diode D1 is selected followed by C IN and C OUT .
R SENSE Selection for Output Current
R SENSE is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R SENSE , the threshold of the comparator
determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
lies between 25mV/R SENSE and 150mV/R SENSE . The maxi-
mum output current of the LTC1265 is:
I OUT(MAX) = 150mV – I RIPPLE (Amps)
R SENSE 2
where I RIPPLE is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1265 is in Burst Mode
operation. In this mode the peak inductor current is set at
25mV/R SENSE . To fully benefit from Burst Mode operation,
the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/R SENSE .
= 137.5mV (Amps)
R SENSE
Solving for R SENSE and allowing a margin of variations in
the LTC1265 and extended component values yields:
R SENSE = 100mV ( ? )
I OUT(MAX)
The LTC1265 is rated with a capability to supply a maximum
of 1.2A of output current. Therefore, the minimum value of
R SENSE that can be used is 0.083 ? . A graph for selecting
R SENSE versus maximum output is given in Figure 2.
0.5
0.4
0.3
0.2
0.1
To account for light and heavy load conditions, the I OUT(MAX)
0
0
0.2
0.4
0.6
0.8
1
is then given by:
MAXIMUM OUTPUT CURRENT (A)
1265 G10
Figure 2. Selecting R SENSE
6
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