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| 型号： | LM22674QMRX-ADJ/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 8/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 0.5A 8PSOP |
| 标准包装： | 2,500 |
| 系列： | SIMPLE SWITCHER® |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调至 1.285V |
| 输入电压： | 4.5 V ~ 42 V |
| PWM 型： | 电压模式 |
| 频率 - 开关： | 500kHz |
| 电流 - 输出： | 500mA |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm Width）裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-PSOP |
SNVS590L – SEPTEMBER 2008 – REVISED APRIL 2013
The second limitation is the maximum duty cycle before the output voltage will "dropout" of regulation. The
following equation can be used to approximate the minimum input voltage before dropout occurs:
(4)
The values of T OFF and R DS(ON) are found in the Electrical Characteristics table. The worst case here occurs at
the highest load. In this equation, R L is the D.C. inductor resistance. Of course, the lowest input voltage to the
regulator must not be less than 4.5V (typ.).
Current Limit
The LM22674 has current limiting to prevent the switch current from exceeding safe values during an accidental
overload on the output. This peak current limit is found in the Electrical Characteristics table under the heading of
I CL . The maximum load current that can be provided, before current limit is reached, is determined from the
following equation:
where
?
L is the value of the power inductor.
(5)
When the LM22674 enters current limit, the output voltage will drop and the peak inductor current will be fixed at
I CL at the end of each cycle. The switching frequency will remain constant while the duty cycle drops. The load
current will not remain constant, but will depend on the severity of the overload and the output voltage.
For very severe overloads ("short-circuit"), the regulator changes to a low frequency current foldback mode of
operation. The frequency foldback is about 1/5 of the nominal switching frequency. This will occur when the
current limit trips before the minimum on-time has elapsed. This mode of operation is used to prevent inductor
current "run-away", and is associated with very low output voltages when in overload. The following equation can
be used to determine what level of output voltage will cause the part to change to low frequency current foldback:
where
?
F sw is the normal switching frequency and V in is the maximum for the application.
(6)
If the overload drives the output voltage to less than or equal to V x , the part will enter current foldback mode. If a
given application can drive the output voltage to ≤ V x , during an overload, then a second criterion must be
checked. The next equation gives the maximum input voltage, when in this mode, before damage occurs:
where
?
V sc is the value of output voltage during the overload and F sw is the normal switching frequency.
(7)
If the input voltage should exceed this value, while in foldback mode, the regulator and/or the diode may
be damaged. It is important to note that the voltages in these equations are measured at the inductor. Normal
trace and wiring resistance will cause the voltage at the inductor to be higher than that at a remote load.
Therefore, even if the load is shorted with zero volts across its terminals, the inductor will still see a finite voltage.
It is this value that should be used for V x and V sc in the calculations. In order to return from foldback mode, the
load must be reduced to a value much lower than that required to initiate foldback. This load "hysteresis" is a
normal aspect of any type of current limit foldback associated with voltage regulators.
The safe operating area, when in short circuit mode, is shown in Figure 13 . Operating points below and to the
right of the curve represent safe operation.
8
Product Folder Links: LM22674
Copyright ? 2008–2013, Texas Instruments Incorporated
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