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参数资料
| 型号： | LM2742 |
| 厂商： | National Semiconductor Corporation |
| 英文描述： | N-Channel FET Synchronous Buck Regulator Controller for Low Output Voltages |
| 中文描述： | N沟道场效应管同步降压稳压控制器输出电压低 |
| 文件页数： | 22/22页 |
| 文件大小： | 619K |
| 代理商： | LM2742 |
Block Diagram
20087501
Application Information
THEORY OF OPERATION
The LM2742 is a voltage-mode, high-speed synchronous
buck regulator with a PWM control scheme. It is designed for
use in set-top boxes, thin clients, DSL/Cable modems, and
other applications that require high efficiency buck convert-
ers. It has power good (PWRGD), and output shutdown
(SD). Current limit is achieved by sensing the voltage V
DS
across the low side FET. During current limit the high side
gate is turned off and the low side gate turned on. The soft
start capacitor is discharged by a 95A source (reducing the
maximum duty cycle) until the current is under control.
START UP
When V
CC exceeds 4.2V and the shutdown pin SD sees a
logic high the soft start capacitor begins charging through an
internal fixed 10A source. During this time the output of the
error amplifier is allowed to rise with the voltage of the soft
start capacitor. This capacitor, C
SS, determines soft start
time, and can be determined approximately by:
An application for a microprocessor might need a delay of
3ms, in which case C
SS would be 12nF. For a different
device, a 100ms delay might be more appropriate, in which
case C
SS would be 400nF. (390 10%) During soft start the
PWRGD flag is forced low and is released when the voltage
reaches a set value. At this point this chip enters normal
operation mode and the Power Good flag is released.
Since the output is floating when the LM2742 is turned off, it
is possible that the output capacitor may be precharged to
some positive value. During start-up, the LM2742 operates
fully synchronous and will discharge the output capacitor to
some extent depending on the output voltage, soft start
capacitance, and the size of the output capacitor.
NORMAL OPERATION
While in normal operation mode, the LM2742 regulates the
output voltage by controlling the duty cycle of the high side
and low side FETs. The equation governing output voltage
is:
V
O =0.6x(RFB1 +RFB2)/RFB1
The PWM frequency is adjustable between 50kHz and
2MHz and is set by an external resistor, R
FADJ, between the
FREQ pin and ground. The resistance needed for a desired
frequency is approximately:
LM2742
www.national.com
9
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| LM2742MTC | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM2742MTC NOPB | 制造商:Texas Instruments 功能描述:Bulk |
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| LM2742MTC/NOPB | 制造商:Texas Instruments 功能描述:DC/DC Converter IC |
| LM2742MTCX | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
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