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參數(shù)資料
| 型號： | LM5027AMHX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | TSSOP-20 |
| 文件頁數(shù)： | 10/28頁 |
| 文件大小： | 561K |
| 代理商： | LM5027AMHX |
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30111910
FIGURE 8. Feed-Forward Voltage Mode Configuration
Volt-Second Clamp
An external resistor (R
FF) and a capacitor (CFF) connected
between the VIN, RAMP and AGND pins is required to create
a saw-tooth modulation ramp signal as shown in Figure 8. The
slope of the RAMP will vary in proportion to the input line volt-
age. Varying the PWM ramp slope inversely with the input
voltage provides line feed-forward information necessary to
improve line transient response with voltage mode control.
With a constant error signal, the on time (t
ON) varies inversely
with the input voltage (VIN) to stabilize the volt-second prod-
uct of the transformer primary.
The volt-second clamp compares the ramp signal (RAMP) to
a fixed 2.5V reference. By proper selection of R
FF and CFF,
the maximum on-time of the main switch can be set to the
desired duration. An example will illustrate the use of the volt-
second clamp comparator to achieve a 70% duty cycle limit
at 200 kHz and 18V line input: A 70% duty cycle at 200 kHz
requires a 3.5 s on-time. At 18V input the volt-second prod-
uct is 63s (18V x 3.5 s). To achieve this clamp level:
R
FF x CFF = VIN x tON / 2.5V
18V x 3.5 s/ 2.5V = 25.2
Select C
FF = 470 pF
R
FF = 53.6 k
The recommended capacitor value range for CFF is 100 pF
to 1000 pF. The C
FF ramp capacitor is discharged at the con-
clusion of every cycle by an internal discharge switch con-
trolled by either the PWM comparator, the CS comparator or
by the volt-second clamp comparator, which ever occurs first.
Oscillator and Sync Capability
The LM5027A oscillator frequency is set by the external re-
sistor connected between the RT pin and the ground (AGND).
To set a desired oscillator frequency, the necessary R
T re-
sistor is calculated from:
For example, if the desired oscillator frequency is 200 kHz, a
59.7K resistor would be the nearest standard one percent
value. The R
T resistor should be located as close as possible
to the IC and connected directly to the pins (RT and AGND).
The tolerance of the external resistor and the frequency tol-
erance indicated in the Electrical Characteristics must be
taken into account when determining the worst case operat-
ing frequency. The LM5027A can be synchronized to an
external clock by applying a narrow pulse to the RT pin. The
external clock must be at least 10% higher than the free-run-
ning oscillator frequency set by the R
T resistor. If the external
clock frequency is less than the R
T resistor programming fre-
quency, the LM5027A will ignore the synchronizing pulses.
The synchronization pulse should be coupled to the RT pin
through a 100 pF capacitor with a pulse width of 15 ns to 150
ns. When a synchronizing pulse transitions from low-to-high
(rising edge), the voltage at the RT pin must be driven to ex-
ceed 3.2V from its nominal 2.85V level. During the clock
signal low time, the voltage at the RT pin will be clamped at
2V by an internal regulator. The output impedance of the RT
regulator is approximately 100
. The RT resistor is always
required, whether the oscillator is free running or externally
synchronized.
www.national.com
18
LM5027A
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM5027AMHX/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓撲結(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM5027ASQ/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓撲結(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM5027ASQX/NOPB | 功能描述:電壓模式 PWM 控制器 RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓撲結(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
| LM5027MH | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Voltage Mode Active Clamp Controller |
| LM5027MH/NOPB | 功能描述:電壓模式 PWM 控制器 Voltage Mode Active Clamp Controller RoHS:否 制造商:Texas Instruments 輸出端數(shù)量:1 拓撲結(jié)構(gòu):Buck 輸出電壓:34 V 輸出電流: 開關(guān)頻率: 工作電源電壓:4.5 V to 5.5 V 電源電流:600 uA 最大工作溫度:+ 125 C 最小工作溫度:- 40 C 封裝 / 箱體:WSON-8 封裝:Reel |
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