U-100A
应用笔记
U-100A
U-100A
APPLICATION NOTE
UC3842/3/4/5 PROVIDES LOW-COST
CURRENT-MODE CONTROL
INTRODUCTION
CURRENT-MODE CONTROL
The fundamental challenge of power supply design is to
simultaneously realize two conflicting objectives: good
electrical performance and low cost. The
is an integrated pulse width modulator
designed
with both these objectives in mind. This
provides de-
signers an inexpensive controller with which they can ob-
tain all the performance advantages of current mode op-
eration. In addition, the UC3842 series is optimized for ef-
ficient power sequencing of off-line converters, DC to DC
regulators and for driving power MOSFETs or transistors.
This application note provides a functional description of
the UC3842 family and highlights the features of each in-
dividual member, the UC3842, UC3843, UC3844 and
UC3845 Throughout the text, the UC3842 part number
will be referenced, however the generalized circuits and
performance characteristics apply to each member of the
UC3842 series unless otherwise noted. A review of cur-
rent mode control and its benefits is included and meth-
ods of avoiding common pitfalls are mentioned. The final
section presents designs of power supplies utilizing
UC3842 control.
Figure 1 shows the two-loop current-mode control system
in a typical buck regulator application. A clock signal initi-
ates power pulses at a fixed frequency. The termination of
each pulse occurs when an analog of the inductor current
reaches a threshold established by the error signal. In this
way the error signal actually controls peak inductor cur-
rent. This contrasts with conventional schemes in which
the error signal directly controls pulse width without regard
to inductor current.
Several performance advantages result from the use of
current-mode control. First, an input voltage feed-forward
characteristic is achieved; i.e., the control circuit instanta-
neously corrects for input voltage variations without using
up any of the error amplifier’s dynamic range. Therefore,
line regulation is excellent and the error amplifier can be
dedicated to correcting for load variations exclusively.
For converters in which inductor current is continuous,
controlling peak current is nearly equivalent to controlling
average current. Therefore, when such converters employ
current-mode control, the inductor can be treated as an
Figure 1. Two-Loop Current-Mode Control System
3-53
UC3842/3/4/5
提供了低成本的
电流模式控制
引言
电源设计的主要难题是需要同时实现两个相互矛盾的目标，
即：上佳的电性能和低成本。
UC3842/3/4/5
是一款集成脉宽
调制器
(PWM)
，它在设计时兼顾了上述的两个目标。该
IC
为设计师提供了一款廉价的控制器，他们借助该控制器能够
获得电流模式操作的所有性能优势。此外，
UC3842
系列还
专为隔离式转换器和
DC-DC
稳压器的高效电源排序以及功率
MOSFET
或晶体管的驱动进行了优化。
本应用笔记提供了
UC3842
系列的功能描述，并突出介绍了其
每个成员
(UC3842
、
UC3843
、
UC3844
和
UC3845)
的特点。
文章通篇以型号为
UC3842
的器件为参考，不过，除非特别注
明，否则一般化的电路和性能特征将适用于
UC3842
系列的所
有成员。本文评述了电流模式控制及其好处，并提及了避免
常见易犯错误的方法。最后的章节给出了运用
UC3842
控制器
的电源设计方案。
电流模式控制
图
1
示出了双环路电流模式控制系统在典型降压型稳压器中的
应用。时钟信号以一个固定频率来启动电源脉冲。当电感器
电流的模拟量达到由误差信号所确定的门限时，脉冲将被终
止。误差信号以这种方式实际上起到了控制峰值电感器电流
的作用。这与传统方案截然不同，后者是由误差信号直接控
制脉冲宽度，而不考虑电感器电流。
通过使用电流模式控制获得了的一些性能优势。首先，实现
了一种输入电压前馈特性；即：控制电路能够即刻校正输入
电压偏差，而不会耗尽任何误差放大器的动态范围。因此，
线路输入电压调节性能非常出色，而且误差放大器可被指定
专门用于校正负载偏差。
对于那些具有连续电感器电流的转换器而言，控制峰值电流
几乎等同于控制平均电流。于是，当此类转换器使用电流模
式控制时，可将电感器视作一个误差电压控制型电流源，以
便进行小信号分析。（接下页）
U-100A
APPLICATION NOTE
UC3842/3/4/5 PROVIDES LOW-COST
CURRENT-MODE CONTROL
INTRODUCTION
CURRENT-MODE CONTROL
The fundamental challenge of power supply design is to
simultaneously realize two conflicting objectives: good
electrical performance and low cost. The
is an integrated pulse width modulator
designed
with both these objectives in mind. This
provides de-
signers an inexpensive controller with which they can ob-
tain all the performance advantages of current mode op-
eration. In addition, the UC3842 series is optimized for ef-
ficient power sequencing of off-line converters, DC to DC
regulators and for driving power MOSFETs or transistors.
This application note provides a functional description of
the UC3842 family and highlights the features of each in-
dividual member, the UC3842, UC3843, UC3844 and
UC3845 Throughout the text, the UC3842 part number
will be referenced, however the generalized circuits and
performance characteristics apply to each member of the
UC3842 series unless otherwise noted. A review of cur-
rent mode control and its benefits is included and meth-
ods of avoiding common pitfalls are mentioned. The final
section presents designs of power supplies utilizing
UC3842 control.
Figure 1 shows the two-loop current-mode control system
in a typical buck regulator application. A clock signal initi-
ates power pulses at a fixed frequency. The termination of
each pulse occurs when an analog of the inductor current
reaches a threshold established by the error signal. In this
way the error signal actually controls peak inductor cur-
rent. This contrasts with conventional schemes in which
the error signal directly controls pulse width without regard
to inductor current.
Several performance advantages result from the use of
current-mode control. First, an input voltage feed-forward
characteristic is achieved; i.e., the control circuit instanta-
neously corrects for input voltage variations without using
up any of the error amplifier’s dynamic range. Therefore,
line regulation is excellent and the error amplifier can be
dedicated to correcting for load variations exclusively.
For converters in which inductor current is continuous,
controlling peak current is nearly equivalent to controlling
average current. Therefore, when such converters employ
current-mode control, the inductor can be treated as an
Figure 1. Two-Loop Current-Mode Control System
3-53
图
1
：双环路电流模式控制系统
