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參數(shù)資料
| 型號： | L7981TR |
| 廠商： | STMICROELECTRONICS |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, DSO8 |
| 封裝： | 3 X 3 MM, 1.08 MM HEIGHT, ROHS COMPLIANT, VFQFP-8 |
| 文件頁數(shù)： | 44/44頁 |
| 文件大?。?/td> | 1315K |
| 代理商： | L7981TR |
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L7981
Functional description
Doc ID 15182 Rev 3
9/44
5.1
Oscillator and synchronization
Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides a
constant frequency clock. Its frequency depends on the resistor externally connect to FSW
pin. In case the FSW pin is left floating the frequency is 250 kHz; it can be increased as
shown in Figure 6 by external resistor connected to ground.
To improve the line transient performance, keeping the PWM gain constant versus the input
voltage, the voltage feed forward is implemented by changing the slope of the sawtooth
according to the input voltage change (see Figure 5.a).
The slope of the sawtooth also changes if the oscillator frequency is increased by the
external resistor. In this way a frequency feed forward is implemented (Figure 5.b) in order to
keep the PWM gain constant versus the switching frequency (see Section 6.4 for PWM gain
expression).
On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of
180° with respect to the clock. This delay is useful when two devices are synchronized
connecting the SYNCH pin together. When SYNCH pins are connected, the device with
higher oscillator frequency works as Master, so the Slave device switches at the frequency
of the Master but with a delay of half a period. This minimizes the RMS current flowing
through the input capacitor [see L5988D Data sheet].
Figure 4.
Oscillator circuit block diagram
The device can be synchronized to work at higher frequency feeding an external clock
signal. The synchronization changes the sawtooth amplitude, changing the PWM gain
(Figure 5.c). This changing has to be taken into account when the loop stability is studied.
To minimize the change of the PWM gain, the free running frequency should be set (with a
resistor on FSW pin) only slightly lower than the external clock frequency. This pre-adjusting
of the frequency will change the sawtooth slope in order to get negligible the truncation of
sawtooth, due to the external synchronization.
Clock
Generator
Ramp
Generator
FSW
Sawtooth
Clock
Synchronization
SYNCH
Clock
Generator
Ramp
Generator
FSW
Sawtooth
Clock
Synchronization
SYNCH
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| L7983 | 制造商:Leviton Manufacturing Co 功能描述: |
| L7985 | 功能描述:線性穩(wěn)壓器 - 標準 2A Step Down REG 4.5V to 38V 250 KHz RoHS:否 制造商:STMicroelectronics 輸出類型: 極性: 輸出電壓:1.8 V 輸出電流:150 mA 負載調(diào)節(jié): 最大輸入電壓:5.5 V 線路調(diào)整率: 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOT-323-5L |
| L7985A | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 2A DC 4.5V to 38V 250 KHz Step-Down RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
| L7985ATR | 功能描述:線性穩(wěn)壓器 - 標準 2A Step-Down REG 2A DC 4.5 to 38V RoHS:否 制造商:STMicroelectronics 輸出類型: 極性: 輸出電壓:1.8 V 輸出電流:150 mA 負載調(diào)節(jié): 最大輸入電壓:5.5 V 線路調(diào)整率: 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOT-323-5L |
| L7985TR | 功能描述:線性穩(wěn)壓器 - 標準 2A Step-Down Reg 4.5V to 38V 250 KHz RoHS:否 制造商:STMicroelectronics 輸出類型: 極性: 輸出電壓:1.8 V 輸出電流:150 mA 負載調(diào)節(jié): 最大輸入電壓:5.5 V 線路調(diào)整率: 最大工作溫度:+ 125 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOT-323-5L |
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